Phytophthora capsici on Vegetable Crops: Research Progress and Management Challenges

Transcription

1 Mary K. Hausbeck Michigan State University, East Lansing Kurt H. Lamour The University of Tennessee, Knoxville Phytophthora capsici on Vegetable Crops: Research Progress and Management Challenges Corresponding author: M. K. Hausbeck Publication no. D F 2004 The American Phytopathological Society 1292 Plant Disease / Vol. 88 No. 12 Phytophthora capsici was first described by Leon H. Leonian at the New Mexico Agricultural Research station in Las Cruces in 1922 (65). In his report, he described a novel species of Phytophthora that caused considerable damage to chili pepper plants in the fall of A year later, the disease reappeared at the same site and also affected surrounding farms. During the late 1930s and early 1940s, recurrent problems with P. capsici in the Arkansas River Valley of Colorado were described on several vegetable hosts (51 55,103). The first reported occurrence of P. capsici on a cucurbit crop occurred in 1937, when a 3.2-ha field of cucumbers became diseased resulting in 100% of the fruit rotting (51). By 1940, P. capsici had also been described on eggplant, honeydew melon fruit, summer squash, and tomato fruit (52,103). The disease on tomatoes was reportedly so severe that the viability of the processing tomato industry in the region was threatened. These early reports mirror the situation with P. capsici today on many modern vegetable production farms, especially those in the eastern United States (4,72,84,94). Our research was initiated in 1997, when crop losses caused by P. capsici threatened to bankrupt a number of vegetable producers in Michigan. Growers wanted to know why crop rotation and the use of fungicides in well-drained fields had not provided adequate protection against full-scale epidemics. At that time, there were fundamental gaps in our understanding of P. capsici s epidemiology in Michigan, and it was difficult to answer these questions with any degree of certainty. We did not recognize the extent to which sexual recombination and genetic diversity could influence management options and success. In particular, the fungicide mefenoxam was being applied by some growers, and the sensitivity of natural populations of P. capsici in Michigan to mefenoxam was unknown at that time. Here we review recent advances in our understanding of P. capsici s biology, in particular the role of sexual reproduction, and provide an overview of some of the management challenges presented by this information. Host Range and Disease Symptoms In Michigan, there are 32,356 ha of vegetables (currently valued at approximately $134 million) that are highly susceptible to crown, root, and fruit rot caused by P. capsici (Table 1). It is estimated that when weather favors P. capsici, up to 25% of the state s value of these vulnerable vegetables has been lost to disease. Individual producers have experienced devastating losses. When a farm in southern Michigan was unable to harvest ha of diseased pickling cucumbers, an estimated $300,000 was lost, along with a $40,000 loss on approximately 40.5 ha of processing tomatoes. Due to the impact of P. capsici on this farm s ability to meet contractual obligations for cucumbers, production of this crop was discontinued (57). While ranked nationally as the number one producer and processor of cucumbers for pickling, Michigan also is a major midwestern supplier of several vegetables for fresh consumption and for processing (49). In the north-central region of the United States, P. capsici also is a reported problem on cucumber in Wisconsin (95,96), on pumpkin in Illinois (5), and on pepper and cucurbit crops in Ohio (72). The occurrence of P. capsici throughout many vegetable growing regions in the United States has prompted recent research in Virginia (100), New York (70), Florida (69), Arizona (68), North Carolina (66), and Georgia (91). P. capsici affects a wide range of solanaceous and cucurbit hosts worldwide (17,27,43). In 1967, Satour and Butler (87) reported that 45 species of cultivated plants and weeds, representing 14 families of flowering plants, were susceptible to P. capsici. They found 19 species in 8 families that were highly susceptible, with the roots and crowns completely rotting 7 to 10 days after inoculation. This was the widest host range study conducted to date. Beans, lima beans, and soybeans were reported (87) to be immune to P. capsici infection under greenhouse conditions highly favorable to infection. It is significant, therefore, that in the summers of 2000 and 2001, P. capsici was isolated from five commercial cultivars of lima bean in Delaware, Maryland, and New Jersey (21). Also, P. capsici has recently been isolated from commercial snap bean fields in northern Michigan, adding this crop to the long list of susceptible crops (35). These snap bean fields had a history of zucchini cropping and P. capsici infestation. All isolates from snap bean were pathogenic to cucumber fruit, and select isolates were pathogenic to soybean plants under laboratory conditions (36). Disease caused by P. capsici may initially occur in the low areas of a field where water accumulates. Growers often assume that stunting or death of plants in such areas is due to the waterlogging of the roots, but infection by P. capsici may be to blame. Under warm (25 to 30 C), Table 1. Crops susceptible to Phytophthora capsici under field conditions Cucurbitaceae Solanaceae Leguminosae Cantaloupe Bell pepper Snap bean Cucumber Hot pepper Lima bean Gourd Eggplant Honeydew Tomato melon Pumpkin Muskmelon Summer squash Watermelon Winter squash Zucchini

2 wet conditions, root and crown infection of pepper, zucchini, squash, and pumpkin typically causes permanent wilt and plant death (Fig. 1D F,L). Plants often have brown to black discolored roots and/or crowns. In contrast, infected cucumber and tomato plants may be relatively asymptomatic or exhibit limited root rot and plant stunting (Fig. 1A,B,M,N). However, when a rainstorm splashed P. capsici infested soil onto the cotyledons of emerging cucumbers, the entire 24.3-ha planting was killed. Similarly, extremely rainy weather that saturates soil for extended periods can prompt a severe root and crown rot that kills even established tomato plants. Disease symptoms on snap beans include water-soaking on the leaves, stem necrosis (Fig. 1O), and overall decline. Disease symptoms were most severe on bean plants located along the surface water drainage pattern. P. capsici was recovered from crown, stem, and leaf tissue (35). While plant death is always a concern for vegetable producers, fruit rot seems to be especially insidious on cucurbits. In general, infected cucurbit fruit initially exhibit dark, water-soaked lesions (Fig. 1C,I), followed by a distinctive white powdered-sugar layer of spores on the Fig. 1. Symptoms of disease caused by Phytophthora capsici on: A to C, cucumber; D and E, yellow squash; F, hard squash; G, zucchini; H, immature pumpkin; I, spaghetti squash; J, bell pepper; K and L, banana pepper; M and N, tomato; and O, snap bean. Plant Disease / December

3 surface of the fruit 2 to 3 days later (Fig. 1A,G,H). While P. capsici regularly causes a blight of pepper fruit in other growing regions (84), this is not a common occurrence in Michigan and has been observed only occasionally in the last several years (Fig. 1J,K). Cucumber plants appear to tolerate root infection by P. capsici, yet the fruit are especially susceptible. In Michigan, fields of healthy-appearing cucumber vines with mature fruit have been abandoned in the field at harvest, or semi-truck loads of fruit rejected at the processing facility, due to rot. In our studies, we routinely observe a delay of at least 48 hours in symptom expression in cucumber following successful penetration by P. capsici (K. H. Lamour and M. K. Hausbeck, unpublished results). A similar 3- to 6-day lag prior to symptom expression for P. capsici infecting peppers has been described previously by Schlub (89). This delay explains why producers in Michigan who harvest seemingly healthy fruit have had entire loads rejected; fruit become infected while in the field but the disease progresses during storage and transit, with symptoms and/or signs becoming evident after delivery to the processor or retailer. The increased temperatures during harvest, storage, and transit may be an important factor. The Pathogen Early investigators recognized that the genus Phytophthora exhibited striking dissimilarities to many other fungal organisms, but a full resolution of its taxonomic and evolutionary standing would not be made until DNA sequence analysis was completed by Forster et al. in They found that oomycetes are more closely related to heterokont photosynthetic algae than to members of the kingdom Fungi (29). The modern description of P. capsici as a species falls into Waterhouse s Group II (101) and is characterized by sporangia that are conspicuously papillate with amphigynous oospores generally forming only when A1 and A2 mating types are paired. Information concerning the different spore types produced by members of the genus Phytophthora accumulated slowly between 1940 and In 1970, Waterhouse (101) provided a useful, and still used, key for identifying isolates to species based on the morphology of sporangia and oospores and whether or not an isolate could produce oospores in single culture. Research with other Phytophthora species established much of what is known about the three dominant spore types produced by P. capsici (27). The thallus is composed of coenocytic mycelium which may give rise to lemon-shaped sporangia borne on long caducous pedicels (1). When sporangia are immersed in free water, they differentiate to produce 20 to 40 bi-motile swimming zoospores (Fig. 2) (8). Longterm survival outside of host tissue is accomplished by the oospore (2,3,10,42,58 60), which has a thick, multilayered wall containing β-glucan and cellulose (27). Oospores require a dormancy period of at least a month (27,88) before germinating directly or by forming sporangia (Fig. 2). Sexual Reproduction and Oospores Approximately half of the 60 recognized species in the genus Phytophthora are homothallic (self-fertile), and for these species, a single isolate is able to complete the sexual stage and form oospores (27). The remaining species, including P. capsici, are heterothallic and require two compatibility types (=mating types), designated A1 and A2, to complete the sexual stage (27). Oospores are formed when A1 and A2 compatibility types come into close association (Fig. 2) (50). Each of the parent isolates makes both male (antheridium) and female (oogonium) gametangia once Fig. 2. Disease cycle of Phytophthora capsici on cucumber. A, Dormant oospores germinate during wet conditions to produce lemon-shaped sporangia, which may germinate directly or release swimming zoospores. Sporangia are produced on the roots, crowns, and fruit of infected plants. B, In a cucumber field, sporangia and zoospores are disseminated by rain, irrigation, and drainage water, which can saturate soils and contribute to multiple cycles of inoculum that drive the disease during a single growing season. C, Oospores are formed when A1 and A2 compatibility types come into close proximity; oospores are able to survive for years in the soil Plant Disease / Vol. 88 No. 12

4 the sexual stage has been initiated, and self-fertilization is possible in obligate outcrossing species (50). To our knowledge, P. capsici is the only heterothallic Phytophthora species that has been shown to regularly complete the sexual stage (outcross) in the United States (37,56,58 62). The A1 and A2 mating types both occur within natural field populations of P. capsici. The presence of A1 and A2 isolates of P. capsici in single fields was reported in New Jersey in 1981 (76) and in North Carolina in 1990 (81). Both mating types have been recovered from farms surveyed in other states when at least 15 isolates were collected from diverse locations within a field (Table 2). During 1997 and 1998, 14 Michigan farms were sampled, with 473 isolates recovered from cucurbit hosts and 30 from bell pepper (58). The A1 and A2 compatibility types were recovered in roughly a 1:1 ratio for eight farms. In 2001, we collected isolates of P. capsici from fields in New York, Connecticut, Pennsylvania, Ohio, North Carolina, and California; similar trends were revealed. When 429 isolates from these states were screened for mating type, 53% (227) were A1 and 47% (202) were A2 mating types. Both mating types were recovered from every location, and the A1/A2 ratio was close to 1:1 within locations (Table 2). To determine if both mating types are present in a field, the timing and spatial scale of sampling are important. Multiple cycles of infection and spore production allow P. capsici to spread rapidly throughout fields during warm, wet weather, and samples collected from a few plants at the height of an epidemic may erroneously suggest that only a single mating type is present (76,82). Samples collected every 2 weeks over a 3-month period from a single field of squash in Michigan illustrated how the percentage of unique genotypes fell from 100% at the beginning of the epidemic to less than 30% by the end of the growing season (59). Papavizas et al. (76) provided the first report of naturally occurring P. capsici oospores in diseased host tissue in North America. In Michigan, amphigynous oospores typical of P. capsici have been found in infected pumpkin, cucumber (Fig. 3C), and butternut squash fruit and in the stems of P. capsici infected yellow squash seedlings. Interestingly, fungal gnat larvae (Sciaridae) feeding on pumpkin fruit infected with P. capsici had numerous oospores in the digestive tracts of three specimens (Fig. 3A,B). No attempt to determine the viability of the excreted oospores was made, but a study conducted with oospores of Pythium spp. and fungal gnat larvae indicates that oospores remain viable and suggests that the gnat s larval stage may serve as a vector (33). Although oospores have been considered the primary source of inoculum in the field, little is known about the influence of soil physical factors on infection of host crops in oospore-infested soils. In vitro treatment with chemicals and physical factors that may interact with oospores in the soil can provide information on germination and viability (44,48). Although information about oospore germination in situ is limited and reportedly difficult to observe and definitively assay (44,64), it is important to monitor oospore germination in a simulated, complex soil setting (76). Oospore survival has been successfully studied in situ with P. infestans (23). Thus, an important precedent for research on P. capsici is in place. The main impediment to detailed studies of oospores and the inherent genetics therein was primarily the difficulty in separating and germinating oospores (65,92). In 1968, Satour and Butler provided crucial information concerning the generation and germination of P. capsici oospores (88). They reported that relatively young oospores produced in paired cultures of P. capsici germinated to produce recombinant progeny after 30 days incubation. Prior to this, it was generally thought that 6- to 9- month incubation periods were necessary for oospore germination. The progeny from their crosses were shown to differ from the parental types in both morphology and pathogenicity. For example, one progeny isolate exhibited increased virulence on pepper compared with either of the parents, which suggests that sexual reproduction could lead to increased virulence in the field. A number of important milestones were reached in this investigation. A simple method for the production, germination, and harvesting of oospore progeny for P. capsici was formally presented, and the authors convincingly argued that proper media containing ample nutrients as well as genetically compatible parent isolates are required for successful crosses. In addition, this work provided convincing evidence for the potential role of oospores in generating genetic variation (88). In 1971, Polach and Webster (80) corroborated this finding using the oospore incubation and germination techniques of Satour and Butler (88). Polach and Webster (80) investigated 391 single oospore Table 2. Phenotypic diversity of Phytophthora capsici isolates recovered from cucurbit and solanaceous hosts at diverse locations in the United States during 2001 No. of Compatibility type/mefenoxam sensitivity b Location isolates a A1/S A1/IS A1/I A2/S A2/IS A2/I Connecticut Pennsylvania California Ohio New York (upstate) New York (Long Island) North Carolina North Carolina North Carolina North Carolina Total a Isolates originated from single fields within a state except for New York and North Carolina, which had 2 and 4 fields sampled, respectively. b Mefenoxam sensitivity determined by in vitro screening on 100 ppm AI amended media, with S (sensitive) = <30% growth of control (GC), IS (intermediately sensitive) = between 30 and 90% GC, and I (insensitive) = >90% GC. Fig. 3. Typical amphigynous oospores of Phytophthora capsici, A and B, in the gut of a fungal gnat that was feeding on a P. capsici infected pumpkin; and C, on the fruit of a naturally infected cucumber. Plant Disease / December

5 progeny from four mating reactions and reported that the parent isolates differed in their pathogenicity to cucurbit and solanaceous hosts and that segregation and recombination were observed for all the characters studied. Role of Sporangia and Zoospores in Field Epidemics Like many species in the genus Phytophthora, P. capsici has the potential for rapid polycyclic disease development from a limited amount of inoculum (82). The asexual sporangia and zoospores proved to be much easier to manipulate and study than the oospore, and it is not surprising that the salient features of these spore types were outlined relatively early (40,73). P. capsici grows optimally between 25 and 28 C and can produce copious amounts of deciduous sporangia on the surface of infected tissue (1,17,99,102). When cucumber fruit were inoculated and incubated at 60, 80, and 98% relative humidity (RH) for 5 days, more sporangia were produced at 60 and 80% RH than at >90% RH (Fig. 4) (K. H. Lamour and M. K. Hausbeck, unpublished results). Mature sporangia are easily dislodged by rain and irrigation and can directly germinate or, when immersed in water, release 20 to 40 motile zoospores (40) that travel with water in fields (89). Zoospores exhibit negative geotropism and chemotactically follow nutrient gradients while swimming (27). Once zoospores contact the plant surface, they encyst and germinate to produce germ tubes (40). Scanning electron microscopy illustrates that zoospores are able to directly penetrate the intact cuticle within an hour (K. H. Lamour and M. K. Hausbeck, unpublished data). Penetration of leaf surfaces by P. capsici occurs directly and through natural openings such as stomata (47). P. capsici produces an extra-cellular macerating enzyme that likely plays a significant role in breaching the host epidermis and ramifying through susceptible host tissue (104). Fig. 4. Average number of sporangia recovered from cucumber fruit inoculated with Phytophthora capsici and incubated at 60, 80, and 98% relative humidity (RH) for 5 days. Error bars indicate standard error of the means Plant Disease / Vol. 88 No. 12 In general, sporangia and zoospores are thought to be relatively ephemeral structures contributing to the spread of P. capsici within a single growing season but unlikely to survive the harsh conditions typical of nonhost periods in North America (2,3,11,58,59,61,62). Results from investigations with P. capsici in Michigan suggest that overwintering of clonal inoculum is rare but that reproduction of clonal populations within a single season is significant (58). Tracking a single population of P. capsici over the course of the growing season in 1999 using molecular markers indicated that asexual spread increased dramatically as the season progressed and that a single clone accounted for approximately 50% of the isolates recovered in the final one-third of the growing season (59). Thus, the infection and subsequent sporulation on host tissue and fruit likely play a key role in driving the polycyclic phase of disease development in the field. The number of sporangia on a single naturally infected spaghetti squash fruit was estimated to be 44 million with the potential to release 840 million zoospores (K. H. Lamour and M. K. Hausbeck, unpublished results). In addition to the epidemiological advantage provided by a large aboveground reservoir of inoculum, there may be an additional evolutionary advantage conferred by the large number of hyaline sporangia exposed to UV irradiation on the surface of infected fruit. Fungicide insensitivity was easily induced in P. capsici using UV irradiation (14), and it seems reasonable that the thousands of sporangia present on an infected cucurbit fruit represent a significant substrate for the effects of UV-mediated mutation. Dekker (22) states that the buildup of a chemical-resistant pathogen population will occur faster in a heavily sporulating pathogen on aerial plant parts than in a slowly spreading, soilborne pathogen, and cites as an example that the buildup of metalaxyl resistance in the aerially sporulating P. infestans was much faster than occurred with P. cinnamomi causing avocado root disease. Sexual Reproduction and Adaptation to PAFs Historically, growers have relied on a limited number of fungicides for control of Phytophthora root, crown, and fruit rot. The phenylamide class of fungicides (PAF), specifically metalaxyl and the newest fungicide mefenoxam (Ridomil Gold EC), has been used by many growers to combat P. capsici. Mefenoxam is the active enantiomer contained in the racemic fungicide metalaxyl (77,78). Both compounds are strongly fungicidal to sensitive isolates (20,75), and isolates recovered from farms without a history of PAF use are highly sensitive to both mefenoxam and metalaxyl (58,78). Metalaxyl has been shown to specifically inhibit the incorporation of uridine into RNA in sensitive oomycetes (20). The mode of action of metalaxyl is postulated to be site specific, and it was not surprising when resistance surfaced in populations of susceptible plant pathogens after PAFs were introduced during the late 1970s (20). As early as 1981, researchers working with P. capsici demonstrated that insensitivity to metalaxyl was readily selected for by using sublethally amended media (12,13). Insensitivity soon developed in natural populations of oomycetous organisms where metalaxyl was heavily relied upon (18,34,46). Adaptation to PAFs is common throughout the oomycetes (19,34) and is generally accepted as inevitable due to the specificity of this group of fungicides (20). Studies characterizing the inheritance of mefenoxam insensitivity in P. capsici suggest that insensitivity is conferred by a single incompletely dominant locus (58). Recovering insensitive P. capsici isolates from farms with a history of PAF use is increasingly common in the United States. Data from North Carolina, Michigan, and New Jersey indicate that a significant proportion of P. capsici populations under PAF selection pressure may be intermediately or fully insensitive to mefenoxam (28,58,77,78). Insensitivity to mefenoxam, which also conferred insensitivity to metalaxyl, was reported from field populations of P. capsici on bell pepper (77). The inheritance of mefenoxam sensitivity was assessed in naturally occurring populations of P. capsici in Michigan. In Michigan, greater than half (55%) of the 498 isolates sampled were sensitive, 32% were intermediate, and 13% were fully insensitive to mefenoxam (58). Three farms, two in North Carolina and one in New York, had a history of mefenoxam use, and insensitive isolates were recovered from each (Table 2). Overall, 70% (301) of the isolates were fully sensitive, 17% (75) were intermediately sensitive, and 13% (53) were insensitive to mefenoxam. The majority (40) of the fully insensitive isolates were recovered from a single bell pepper field in North Carolina with a history of mefenoxam use. In North Carolina, the process of adaptation to mefenoxam appears to have occurred rapidly (78). Because only sensitive isolates of P. capsici are controlled by the mefenoxam fungicide (63), the observed control failure in some Michigan fields during the last few years is likely due to the development and increasing incidence of P. capsici isolates insensitive to this fungicide. Sexual recombination appears to play an important role in adaptation by generating fully insensitive isolates (e.g., mating between intermediately sensitive isolates) (Fig. 5). A Michigan population of P. capsici comprised of intermediate and fully insensitive isolates tracked for 3 years (1999 to 2001) in the absence of PAF use showed no evi-

6 dence of reversion back to the wild-type, PAF-sensitive, state (62). Effective fungicides that act on a single enzyme or molecular pathway exert significant selection pressure favoring isolates able to withstand the activity of the fungicide. In the case of mefenoxam, there appears to be a low level of isolates harboring a mutation responsible for insensitivity to mefenoxam. Application of mefenoxam favors these isolates, and sexual reproduction results in numerous genetically unique progeny carrying what was previously a rare trait. Because of sexual reproduction, the process of incorporating a novel advantageous trait into numerous genetic backgrounds makes it less likely that insensitive isolates will be less fit than their fungicidesensitive wild-type counterparts. It is reasonable to suspect that sexual recombination may play a similar role in the adaptation of P. capsici to other fungicidal compounds whether they are applied manually or generated by resistant varieties of plants. Genetic Diversity Significant molecular investigations into the genetics of P. capsici do not appear in the literature until the late 1980s and early 1990s, when isozyme and restriction fragment length polymorphism (RFLP) analysis of both mitochondrial and nuclear DNA were conducted on isolates from widely different geographical locations, years, and hosts located in a worldwide Phytophthora culture collection at the University of California at Riverside (30,71,74). Results Fig. 5. An illustration of how selection for mefenoxam resistance occurs in the field. A, Sensitive Phytophthora capsici individuals are unable to infect when mefenoxam is applied. B, Only the rare intermediately sensitive isolates produce oospores. C, The process of only the resistant isolates mating and producing oospores is continued when mefenoxam is applied in subsequent years. from an isozyme study involving 113 P. capsici isolates were interpreted as revealing two subgroups within the P. capsici species (71). Subgroups are defined as being significantly different based on sporangial morphology and ontogeny. RFLP investigation of mitochondrial DNA revealed no patterns of similarity based on host or geographical location (30). RFLP analysis of nuclear DNA using low copy number probes of 15 P. capsici isolates indicated nuclear DNA diversity was high (30). These early studies highlighted the diversity of P. capsici on a worldwide scale. In the United States, this genetic diversity has been exploited to better understand how natural populations of P. capsici are distributed in space and time. Almost 15 years ago, J. B. Ristaino (81) showed that morphological characters varied widely in natural populations and that variation in pathogenicity among solanaceous and cucurbit hosts existed in field populations. This work corroborated earlier laboratory studies showing that pathogenicity and virulence to tomato and pepper segregate during sexual recombination and that sex can generate strains more virulent than either parent (88). Mating type and sensitivity to mefenoxam provide a limited level of resolution, and choosing among the many techniques available for measuring variation at the DNA level can be difficult due to the advantages and limitations inherent in each. Because P. capsici has the potential for significant polycyclic reproduction, one of our primary goals was to differentiate uniparental (clonal) lineages. This is an important consideration to accurately determine how far P. capsici is dispersed and if clonal lineages are able to survive outside of hosts. The amplified fragment length polymorphism (AFLP) technique is useful for this type of differentiation because it allows numerous markers to be resolved simultaneously and provides a robust sample across individual genomes. The AFLP technique results in the selective amplification of restriction fragments from a digest of total genomic DNA using the polymerase chain reaction (PCR). The DNA fragments, called AFLP markers, are resolved using a polyacrylamide gel or, if the PCR primers are labeled with a fluorescent dye, a DNA sequencing machine. An example of AFLP analysis by automated DNA sequencing is shown in Figure 6. The advantages to this technique are its reproducibility and sensitivity (e.g., between 50 and 70 AFLP markers are resolved per reaction per P. capsici isolate) (9). Characterization of 107 oospore progeny from a laboratory cross between parents with differing AFLP genotypes indicated that the progeny were all recombinant and that the AFLP markers segregated as Mendelian characters (59). A key expectation when studying outcrossing populations is the recovery of unique combinations of phenotypic and molecular characters. If outcrossing is occurring in natural populations of P. capsici, then multiple combinations of mating type, mefenoxam sensitivity, and AFLP markers should be present (Tables 2 and 3) (98). In Michigan, 70% (454) of the 646 isolates analyzed had unique AFLP profiles. In total, 94 AFLP markers were resolved but no single population had all 94 markers. Individual populations had between 68 and 80 AFLP markers, and isolates were clearly more similar based on geographic locations (60). The high number of unique AFLP profiles and high proportion of polymorphic markers suggests that populations residing at all monitored locations are sexually active (Table 3). Studies of individual populations over multiple years indicated that the pools of genetic diversity remained stable and that outcrossing among locations was limited (60 62). As expected for an organism with the potential for significant polycyclic disease development, clonal lineages were detected and were shown to play an important role in epidemic development (59 61). But unlike oomycetes such as P. infestans, where clonal lineages have made their way around the world and have persisted for many years (32), the clonal lineages of P. capsici were confined in space to single fields and in time to single years (61). Management Strategies and Challenges As P. capsici has spread to more acreage devoted to vegetables, producing vulnerable crops has become a significant, and Plant Disease / December

7 Fig. 6. Segments (approximately 90 of 500 bases) of electropherograms from amplified fragment length polymorphism (AFLP) profiles of genomic DNA from three Phytophthora capsici isolates recovered from water sources in Michigan. The AFLP profiles were produced using a Beckman CEQ 8000 capillary genetic analysis system and visualized using the CEQ fragment analysis software. Polymorphic markers of 325, 335, 349, 358, and 390 nucleotides are clearly visible. Table 3. Genetic diversity of Phytophthora capsici isolates recovered from locations in the United States Location Isolates analyzed Unique isolates a AFLP markers resolved Polymorphic markers (%) Connecticut (51) Pennsylvania (57) California (52) Ohio (52) New York (upstate) (58) New York (Long Island) (64) North Carolina (56) a All isolates have unique multilocus amplified fragment length polymorphism (AFLP) profiles. for some producers, an overwhelming challenge. The future of the vegetable industry in Michigan and other regions of the United States plagued by P. capsici is at risk without long-term sustainable approaches such as genetic resistance and remediation of infested sites. In the short term, the economic risk of growing P. capsici susceptible crops may be reduced by using several management tools. Ristaino and Johnston (84) previously provided a summary of management of this disease in bell pepper. Crop rotation. While crop rotation is an important foundation of disease management, the long-term survival of oospores in absence of a host limits the effectiveness of this strategy as a stand-alone tool. The survivability of oospores has been clearly demonstrated with a number of Phytophthora spp., including P. capsici (3,11). Growers practicing even lengthy rotations (>5 years) to nonsusceptible hosts have experienced significant crop loss to P. capsici. A spatiotemporal study conducted on a Michigan farm used molecular tools to identify P. capsici isolates. The data suggest that a P. capsici epidemic on squash in 1999 was initiated by dormant oospores generated 5 years previously, despite rotation to corn and soybeans (59). Although a minimum 3- to 4-year rotation to nonsusceptible hosts is recommended to limit the buildup of P. capsici (84), the availability of noninfested land is becoming increasingly scarce. The development of agriculture land for urban use and the relatively low value of some field crops have forced many vegetable producers to reduce their crop rotation to only 1 or 2 years, thus contributing to the disease problem. Some weeds also may play an important role in the survival of P. capsici from one growing season to another (31,79). Today, many vegetable producers in the United States recognize that cucurbit and solanaceous crops are at risk for P. capsici infection and rotate these crops with other vegetables (i.e., carrots, beans, onions, and asparagus are examples from Michigan) or agronomic crops (soybeans, alfalfa, small grains). However, recent reports of commercial losses in lima beans (21) and snap beans (35,36) from this pathogen and susceptibility of soybeans (35,36) and other commonly grown vegetables (97) under laboratory conditions highlight the gaps in our knowledge and standard management recommendations and suggest that guidelines for crop rotation should be re-evaluated. Exclusion. In Michigan, it does not appear that P. capsici is dispersed over long distances, and excluding the pathogen from noninfested growing areas is emphasized to producers during extension programs and farm visits. Increased attention to the routes by which P. capsici may be introduced is warranted. Movement of other Phytophthora spp. via irrigation water has been documented (27), and aboveground water sources may play a role in the long-distance movement of P. capsici. Runoff water from infested fields can transport the pathogen from diseased 1298 Plant Disease / Vol. 88 No. 12

8 plants to nearby water sources used for irrigation. We began testing aboveground water sources in Michigan for contamination with P. capsici during 2001 and recovered the pathogen from irrigation ponds on two farms (K. H. Lamour and M. K. Hausbeck, unpublished data). Additional irrigation water sources were monitored for P. capsici in 2002 and 2003, and the pathogen was frequently detected in a river, creek, and a naturally fed pond (35,36). All of these water sources were located near crops infected with P. capsici. Prior to this research, the presence of P. capsici in Michigan irrigation sources had not been reported. Another potential source of P. capsici contaminated water may be from vegetable processing facilities that apply their waste water to nearby vegetable production sites. Using water that may be contaminated with P. capsici to irrigate healthy crops must be avoided to limit pathogen spread. Identifying factors contributing to the spread of P. capsici to new locations can be challenging. Producers are warned against dumping P. capsici infected produce on or near their farms. However, a survey of cultural practices in Michigan indicated that in some cases producers were spreading over- and under-size cull and diseased fruit onto fields after returning from processing stations. Historically, some processors mandated that producers haul culls and diseased fruit from the processing station for disposal in their fields even if the fruit were from other farms. A single fruit infected with both A1 and A2 mating types may contain thousands of genetically unique oospores that can establish a resident population of P. capsici in a field with no history of P. capsici problems. Once P. capsici is established in a field, tillage and cultivation distribute diseased plant material and spread oospores throughout the field and soil profile. It is possible that P. capsici may be disseminated to new fields via equipment even when no remnants of diseased plant material are visible. Cultural control. Commonly recommended cultural control strategies reflect our understanding of the importance of water in the epidemiology of P. capsici and include planting into well-drained fields and into raised beds whenever possible (84). Excess moisture is the single most important component to the initial infection and subsequent spread of P. capsici (10,11,82,83,85,89,94). Similar findings exist for many species in the genus Phytophthora and are not surprising in light of these organisms evolutionary ties to the algae (25,26). Since water plays a key role in disease development (82,89), water is managed based on the crop and the water dynamics of the region. A significant problem in the eastern United States is that heavy rainstorms typically occur and provide a strong, uncontrollable force for driving disease development. In growing areas where rainfall is prevalent, growers are encouraged to choose well-drained sites and plant into raised beds and/or mowed cover crops (84,86). However, plants growing in well-drained fields on raised beds may become diseased if the rainfall is heavy ( 2.5 cm), because even a welldrained field may hold standing water long enough for zoospores to be released. Driving rain likely assists in disseminating sporangia. Strategies to limit splash dispersal such as planting into mowed cover crops and trellising of cucurbits appear promising as the fruit are kept off the ground and out of standing water (86). Unfortunately, trellising may not be an option for large cucurbit fruit such as pumpkins. In Michigan, the dependence of many large-acreage cucumber and winter squash producers on mechanical harvesters limits the range of cultural modifications available. In many areas of the southwestern United States, P. capsici has plagued vegetable growers since being described more than 80 years ago. Although water-poor farmers may not see it as such, a major advantage in these arid areas is low annual rainfall. Growers can control the amount and frequency of irrigation and thus can significantly impact the severity of disease in fields known to harbor P. capsici (15,16). For example, in California where rainfall is low, placing drip emitters away from the stems of pepper plants can reduce incidence of Phytophthora crown rot of peppers (15). Café-Filho et al. (16) showed that the incidence of root and fruit rot of squash caused by P. capsici in California increased with increased frequency of irrigation. They recorded almost total crop loss with an irrigation frequency of 7 days, compared with almost no disease when the field was furrow irrigated every 21 days. In the absence of the disease, irrigation intervals of 21 days did not negatively affect fruit yield compared with more frequent irrigations (16). Similar observations were reported for Phytophthora root and crown rot of bell peppers in North Carolina, where disease incidence increased with increased frequency of drip irrigation (82,83). Heavy rainfall (>2.0 cm) was also directly implicated with increased disease (82). In addition to splash dispersal, a heavy rainfall causes mass flow of water on the soil surface and inoculum redistribution in the field. Reducing field wetness periods may be a useful tool in managing fruit rot. Most irrigation systems in Michigan use a traveler that produces relatively large water droplets, thereby increasing the risk of contaminating fruit with soil that is splashed via water (67). Irrigation may be reduced to a minimum after fruit set and even completely eliminated prior to crop harvest with no yield reduction (16) and may reduce fruit rot not only in the field but also after harvest. When a field is infested with P. capsici, narrow spacing enhances disease spread and development by increasing relative humidity in the microclimate and lengthening the duration of soil surface and fruit wetness after a rain or irrigation episode (16). Growers of pickling cucumbers in Michigan have historically used a narrow (27.9 cm) row spacing in a production system that was developed over 15 years ago by university and industry professionals to maximize yield through high plant densities and suppress weeds through early canopy closure. Most growers have been reluctant to alter their current production system because they anticipate a reduced yield with increased row spacing. However, Schultheis and Wehner showed that the density of cucumber plants could be reduced without significantly reducing yield (90). They evaluated densities ranging from about 34,500 to 556,000 plants per ha and observed more culls with high plant densities. Preliminary studies have been conducted at Michigan State University to integrate cultural control methods of controlling P. capsici on zucchini, methods including soil amendments, protective mulches, and water management. Raised beds, flat beds, and raised beds with black plastic cm straw and/or 4,483.3 kg/ha compost were compared (Fig. 7B,C). Significant differences in P. capsici incidence occurred each year the trial was conducted (Fig. 7A) (M. K. Hausbeck and B. Cortright, unpublished data). Although the treatments with raised beds in combination with plastic, straw, and/or compost were significantly better than flat beds for stand count, numbers, and weight of healthy fruit both years (Fig. 7A), disease still occurred in these treatments. While cultural strategies offer reasonably effective protection for fresh-market zucchini or similar bush-type cucurbit varieties, these management tools are too costly and impractical for growers of cucurbits for the processing industry where the profit margin is relatively small. Fungicides. While fungicides cannot be relied upon alone to prevent disease, they have provided Michigan growers with an extra degree of protection, especially when used in combination with other management practices, such as crop rotation, raised beds, and water management. A limited number of fungicides are available for combating P. capsici, especially when the pathogen is resistant to mefenoxam, but none have proven wholly efficacious under optimal conditions for disease (5,41,93). When resistance of P. capsici to mefenoxam was discovered in Michigan, we obtained a Specific Exemption in 1998 for the use of the fungicide Acrobat (dimethomorph). This product now has a full label, and its efficacy has been demonstrated in controlled, replicated large-scale Plant Disease / December

9 pickling cucumber field studies (Fig. 8A C) (38,39). In 2002, the fungicide Gavel (zoxamide + mancozeb) was registered for use against P. capsici and has also proven to be helpful (Fig. 8A). Studies have indicated that mixing a full rate of copper hydroxide with Acrobat 50WP or Gavel 75DF may be helpful, and is recommended (Fig. 8A) (38,39). Seed treatment with either Apron XL LS (mefenoxam) or Allegiance FL (metalaxyl) may be helpful during seed germination to limit pre- and post-damping off caused by P. capsici (7). Growers are encouraged to alternate fungicides and avoid relying on a single fungicide to delay development of fungicide resistance in P. capsici. Good coverage of the plant and fruit with fungicide is essential for maximum protection, but can be difficult to achieve when fruit are shielded by a dense foliar canopy. Plant spacing within the field has been increased by some growers to facilitate improved fungicide coverage. Early and frequent fungicide applications are required for maximum disease control, but increase the cost of production. In Michigan, a fungicide spray may be needed every 5 to 7 days when the weather is wet and rainy. However, the preharvest interval required for Gavel ( 4 days) makes it difficult to use this fungicide in some production systems. Also, mancozeb (a component of Gavel) is a B2 carcinogen, and may be impacted by the Food Quality Protection Act. Fumigation. The long-term persistence of the oospore in agricultural soils poses a continual threat to the successful commercial production of host crops (11,64,89). Oospores germinate asynchronously, and detecting P. capsici oospores in the soil prior to an epidemic is notoriously difficult and the likelihood of obtaining a false negative is high (27,64). To reduce the risk and uncertainty of growing P. capsici susceptible crops, producers of solanaceous and cucurbit crops for the fresh market rely on methyl bromide fumigation as the primary means of ensuring fruit yield and quality. Methyl bromide is used in Fig. 7. A, A replicated demonstration trial with a commercial grower to highlight cultural tools to manage Phytophthora capsici, including raised planting beds, black plastic mulch, composted chicken manure, and straw mulch. B, Zucchini grown on raised planting beds (right) were healthier than those raised on flat beds (left). C, Using a combination of cultural practices, including a raised planting bed, plastic mulch, and straw mulch over the plastic (right) kept zucchini healthy compared with growing zucchini on a flat bed (left). Fig. 8. A, Efficacy of fungicides in reducing fruit rot incidence compared with untreated fruit. B, Application of fungicide in a large-scale, replicated trial. C, Fungicides were applied when fruit were approximately 2.5, 7.6, and 12.7 cm in length Plant Disease / Vol. 88 No. 12

10 conjunction with raised beds, black plastic, and fungicide applications. Because of the short plant-back interval of methyl bromide, crops can be transplanted as soon as the soil reaches an appropriate temperature in the spring, allowing access to early marketing opportunities. Critical Use Exemptions have been submitted and accepted by EPA on behalf of Michigan s solanaceous and cucurbit producers for the extended use of methyl bromide on these crops. Given the scheduled phaseout of methyl bromide in the very near future, it is imperative that effective and cost efficient replacements be identified and implemented. Both registered and experimental fumigants have been tested in Michigan in conjunction with commercial producers at known P. capsici infested sites. A study conducted by the authors in 2003 at a site infested with P. capsici showed that metam sodium (Vapam), 66% methyl bromide, 33% chloropicrin (methyl bromide/chloropicrin), and 61% 1,3-dichloropropene, 35% chloropicrin (Telone C-35) all effectively limited disease when used in a raised bed, plastic mulch system. Genetic resistance. Genetic resistance or tolerance is often at the core of integrated management programs and would be especially helpful in managing P. capsici. Screening cucurbit germ plasm for resistance to P. capsici has been an ongoing effort at Michigan State University. To date, the fruit of over 300 cucumber varieties have been screened for resistance to this pathogen, including pickling varieties, slicing cucumber varieties, and plant introduction accessions. Although complete fruit disease resistance has not been observed, varieties that appear to have limited lesion development and sporulation have been identified (A. Gevens and M. K. Hausbeck, unpublished data). Babadoost and Islam, Johnston et al., and Driver and Louws evaluated commercial varieties and experimental breeding lines of pepper for resistance to P. capsici (6,24,45). Paladin, a commercially available pepper cultivar with resistance to Phytophthora crown rot, appeared promising in these studies. In Michigan, Paladin has been commercially grown in P. capsici infested sites, although the plants have been observed to eventually succumb to disease when environmental conditions are favorable. Since neither genetic resistance nor fungicide management appears to be perfect, combining the two may provide significant control advances. Information dissemination. While preventing the introduction of the pathogen is optimal, once P. capsici is introduced, several control measures need to be used in a comprehensive management program to reduce losses from disease (Sidebar). As techniques and tools are developed to ease the severity of crop loss due to P. capsici, on-farm research trials and educational workshops are emphasized to enhance grower implementation. Further, education of other crop specialists, extension personnel, and consultants is ongoing to ensure that growers receive accurate and consistent information and recommendations. Acknowledgments Special thanks to M. McGrath (Cornell University, Riverhead, NY), G. Holmes (North Carolina State University, Raleigh), M. Davis (University of California, Davis), and W. Elmer (Connecticut Agric. Exp. Station, New Haven) for their assistance in collecting P. capsici isolates. Research regarding fungicide and fumigation evaluation was designed and conducted with the assistance of B. Cortright (Michigan State University). We thank S. Linderman (Michigan State University) for valuable assistance in manuscript formatting and preparation of the figures and tables. Portions of the research discussed have been funded by the Pickle and Pepper Research Committee of Michigan State Recommended Control Strategies for Blight Caused by Phytophthora capsici Preplant Use a seed treatment that is effective against oomycetes. Consider a preplant banded fungicide application for infested fields. Plant susceptible hosts in well-drained fields. Utilize raised beds (15 to 20 cm minimum) whenever possible. Do not plant in low-lying areas of the field. Production Monitor fields for disease, including damping-off, plant stunting, root and crown rot. Do not irrigate a field with water that contains runoff from fields with a history of Phytophthora disease. Irrigate conservatively, and if possible, do not irrigate close to harvest time. Plow under portions of the field with diseased plants, including healthy plants that border diseased areas. Remove diseased fruit from the field. Never dump culls or diseased fruit from other fields or farms into production fields. Once P. capsici is introduced, it may remain indefinitely. Apply fungicide preventively and frequently, especially for known problem fields. Rotate the types of fungicides used. Postharvest Harvest fruit as soon as possible from problem fields and plow under crop residue immediately. Keep harvested fruit dry and cool. University (Pickle Packers International, Inc.), Pickle Seed Research Fund (Pickle Packers International, Inc.), Project GREEEN (a cooperative effort by plant-based commodities and businesses with Michigan State University Extension, the Michigan Agricultural Experiment Station, and the Michigan Department of Agriculture), Michigan Department of Agriculture Specialty Crop Block Grant, and Michigan Agriculture Experiment Station. Literature Cited 1. Alconero, R., and Santiago, A Characteristics of asexual sporulation in Phytophthora palmivora and Phytophthora parasitica nicotianae. Phytopathology 62: Ansani, C. V., and Matsuko, K Infectividade e viabilidade de Phytophthora capsici no solo. Fitopatol. Bras. 8: Ansani, C. V., and Matsuko, K Sobevivencia de Phytophthora capsici no solo. Fitopatol. Bras. 8: Babadoost, M Phytophthora blight: A serious threat to cucurbit industries. APSnet feature, Apr.-May. Online publication. American Phytopathological Society, St. Paul, MN. 5. Babadoost, M., and Islam, S. Z Evaluation of fungicides for control of Phytophthora blight of processing pumpkin, Fungic. Nematicide Tests 56:V65. Online publication. 6. Babadoost, M., and Islam, S. Z Bell peppers resistant to Phytophthora blight. (Abstr.) Phytopathology 92:S5. 7. Babadoost, M., and Islam, S. Z Fungicide seed treatment effects on seedling damping-off of pumpkin caused by Phytophthora capsici. Plant Dis. 87: Bernhardt, E. A., and Grogan, R. G Effect of soil matric potential on the formation and indirect germination of sporangia of Phytophthora parasitica, Phytophthora capsici, and Phytophthora cryptogea rots of tomatoes, Lycopersicon esculentum. Phytopathology 72: Blears, M. J., De Grandis, S. A., Lee, H., and Trevors, J. T Amplified fragment length polymorphism (AFLP): A review of the procedure and its applications. J. Ind. Microbiol. Biotech. 21: Bowers, J. H., and Mitchell, D. J Effect of soil-water matric potential and periodic flooding on mortality of pepper caused by Phytophthora capsici. Phytopathology 80: Bowers, J. H., Papavizas, G. C., and Johnston, S. A Effect of soil temperature and soil-water matric potential on the survival of Phytophthora capsici in natural soil. Plant Dis. 74: Bruin, G. C., and Edgington, L. V Induced resistance to ridomil of some oomycetes. (Abstr.) Phytopathology 70: Bruin, G. C. A Adaptive resistance in Peronosporales to metalaxyl. Can. J. Plant Pathol. 3: Bruin, G. C. A., and Edgington, L. V Induction of fungal resistance to metalaxyl by ultraviolet irradiation. Phytopathology 72: Café-Filho, A. C., and Duniway, J. M Effect of location of drip irrigation emitters and position of Phytophthora capsici infections in roots on Phytophthora root rot of pepper. Phytopathology 86: Café-Filho, A. C., Duniway, J. M., and Davis, R. M Effects of the frequency of furrow irrigation on root and fruit rots of squash caused by Phytophthora capsici. Plant Dis. 79: Crossan, D. F., Haasis, F. A., and Ellis, D. E Phytophthora blight of summer squash. Plant Dis. Rep. 38: Crute, I. R The occurrence, character- Plant Disease / December

11 istics, distribution, genetics, and control of a metalaxyl-resistant pathotype of Bremia lactucae in the United Kingdom. Plant Dis. 71: Crute, I. R., and Harrison, J. M Studies on the inheritance of resistance to metalaxyl in Bremia lactucae and on the stability and fitness of field isolates. Plant Pathol. 37: Davidse, L. C., van den Berg-Velthuis, G. C. M., Mantel, B. C., and Jespers, A. B. K Phenylamides and Phytophthora. Pages in: Phytophthora. J. A. Lucas, R. C. Shattock, D. S. Shaw, and L. R. Cooke, eds. British Mycol. Soc., Cambridge. 21. Davidson, C. R., Carroll, R. B., Evans, T. A., Mulrooney, R. P., and Kim, S. H First report of Phytophthora capsici infecting lima bean (Phaseolus lunatus) in the Mid-Atlantic Region. Plant Dis. 86: Dekker, J The fungicide resistance problem: Current status and the role of systemics. Pages in: Pesticide Interactions in Crop Production. CRC Press, Inc., Boca Raton, FL. 23. Drenth, A., Janssen, E. M., and Govers, F Formation and survival of oospores of Phytophthora infestans under natural conditions. Plant Pathol. 44: Driver, J. G., and Louws, F. J Management of Phytophthora crown and root rot in peppers. (Abstr.) Phytopathology 93:S Duniway, J. M Water relations of water molds. Annu. Rev. Phytopathol. 17: Duniway, J. M Role of physical factors in the development of Phytophthora diseases. Pages in: Phytophthora: Its Biology, Taxonomy, Ecology, and Pathology. D. C. Erwin, S. Bartnicki-Garcia, and P. H. Tsao, eds. American Phytopathological Society, St. Paul, MN. 27. Erwin, D. C., and Ribeiro, O. K Phytophthora Diseases Worldwide. American Phytopathological Society, St. Paul, MN. 28. Fogg, M. L., and Johnston, S. A Mefenoxam sensitivity of Phytophthora capsici isolates in New Jersey. (Abstr.) Phytopathology 93:S Forster, H., Coffey, M. D., Elwood, H., and Sogin, M. L Sequence analysis of the small subunit ribosomal RNAs of three zoosporic fungi and implications for fungal evolution. Mycologia 82: Forster, H., Oudemans, P., and Coffey, M. D Mitochondrial and nuclear DNA diversity within six species of Phytophthora. Experimental Mycol. 14: French-Monar, R. D., Roberts, P. D., and Jones, J. B Phytophthora capsici associated with weeds in conventional vegetable farms of southeast Florida. (Abstr.) Phytopathology 93:S Fry, W. E., Goodwin, S. B., Matuszak, J. M., Speilman, L. J., and Milgroom, M. G Population genetics and intercontinental migrations of Phytophthora infestans. Annu. Rev. Phytopathol. 30: Gardiner, R. B., Jarvis, W. R., and Shipp, J. L Ingestion of Pythium spp. by larvae of the fungus gnat Bradysia impatiens (Diptera: Sciaridae). Ann. Appl. Biol. 116: Georgopoulos, S. G., and Grigoriu, A. C Metalaxyl-resistant strains of Pseudoperonospora cubensis in cucumber greenhouses of southern Greece. Plant Dis. 65: Gevens, A., and Hausbeck, M. K Phytophthora capsici in irrigation water and isolation of P. capsici from snap beans in Michigan. Mich. State University Ext. Veg. Crop Advisory Team Alert 18: Gevens, A., and Hausbeck, M. K. Phytophthora capsici isolated from snap bean is pathogenic to cucumber fruit and soybean Plant Disease / Vol. 88 No. 12 American Phytopathological Society North Central Division Meeting, St. Paul, MN. In press. 37. Goodwin, S. B The population genetics of Phytophthora. Phytopathology 87: Hausbeck, M. K., and Cortright, B Phytophthora fruit rot: Lessons learned. Proc Great Lakes Fruit, Veg., Farm Market Expo. Grand Rapids, MI. pp Hausbeck, M. K., Cortright, B., and Gevens, A Developments in Phytophthora control. Proc Great Lakes Fruit, Veg., Farm Market Expo. Online, Session Summaries, Pickle. 40. Hickman, C. J Biology of Phytophthora zoospores. Phytopathology 60: Holmes, G. J., Lancaster, M. E., and Louws, F. J Evaluation of fungicides and host resistance for control of Phytophthora crown rot of summer squash, Fungic. Nematicide Tests 55: Hord, M. J., and Ristaino, J. B Effect of the matric component of soil water potential on infection of pepper seedlings in soil infested with oospores of Phytophthora capsici. Phytopathology 82: Hwang, B. K., and Kim, C. H Phytophthora blight of pepper and its control in Korea. Plant Dis. 79: Jiang, J Phytophthora oospore germination in vitro and in situ and β-1, 3- glucanase activity in oospores and mycelium of Phytophthora cactorum. Ph.D. diss. University of California, Riverside. 45. Johnston, S. A., Kline, W. L., Fogg, M. L., and Zimmerman, M. D Varietal resistance evaluation for control of Phytophthora blight of pepper. (Abstr.) Phytopathology 92:S Katan, T., and Bashi, E Resistance to metalaxyl in isolates of Pseudoperonospora cubensis, the downy mildew pathogen of cucurbits. Plant Dis. 65: Katsura, K., and Miyazaki, S Leaf penetration by Phytophthora capsici Leonian. Sci. Rep. Kyoto Prefect. Univ. Agric. 12: Kellam, M. K., and Zentmyer, G. A Comparisons of single-oospore isolates of Phytophthora species from naturally infected cocoa pods in Brazil. Mycologia 78: Kleweno, D., and Matthews, V Michigan Agricultural Statistics: Mich. Dep. Agric., Mich. Agric. Stat. Serv., Lansing; U.S. Dep. Agric., National Agric. Stat. Serv. Online publication. 50. Ko, W Hormonal heterothallism and homothallism in Phytophthora. Annu. Rev. Phytopathol. 26: Kreutzer, W. A A Phytophthora rot of cucumber fruit. Phytopathology 27: Kreutzer, W. A., Bodine, E. W., and Durrell, L. W Cucurbit diseases and rot of tomato fruit caused by Phytophthora capsici. Phytopathology 30: Kreutzer, W. A., Bodine, E. W., and Durrell, L. W A sexual phenomena exhibited by certain isolates of Phytophthora capsici. Phytopathology 30: Kreutzer, W. A., and Bryant, L. R A method of producing an epiphytotic of tomato fruit rot in the field. Phytopathology 34: Kreutzer, W. A., and Bryant, L. R Certain aspects of the epiphytology and control of tomato fruit rot caused by Phytophthora capsici Leonian. Phytopathology 36: Lamour, K. H., Daughtrey, M. L., Benson, D. M., Hwang, J., and Hausbeck, M. K Etiology of Phytophthora drechsleri and P. nicotianae (=P. parasitica) diseases affecting floriculture crops. Plant Dis. 87: Lamour, K. H., and Hausbeck, M. K Fruit rot of tomato caused by Phytophthora capsici. Pages in: Proc. Annu. Tomato Dis. Workshop, 14th. Michigan State University, East Lansing. 58. Lamour, K. H., and Hausbeck, M. K Mefenoxam insensitivity and the sexual stage of Phytophthora capsici in Michigan cucurbit fields. Phytopathology 90: Lamour, K. H., and Hausbeck, M. K The dynamics of mefenoxam insensitivity in a recombining population of Phytophthora capsici characterized with amplified fragment length polymorphism markers. Phytopathology 91: Lamour, K. H., and Hausbeck, M. K Investigating the spatiotemporal genetic structure of Phytophthora capsici in Michigan. Phytopathology 91: Lamour, K. H., and Hausbeck, M. K The spatiotemporal genetic structure of Phytophthora capsici in Michigan and implications for disease management. Phytopathology 92: Lamour, K. H., and Hausbeck, M. K Effect of crop rotation on the survival of Phytophthora capsici in Michigan. Plant Dis. 87: Lamour, K. H., and Hausbeck, M. K Susceptibility of mefenoxam-treated cucurbits to isolates of Phytophthora capsici sensitive and insensitive to mefenoxam. Plant Dis. 87: Larkin, R. P., Ristaino, J. B., and Campbell, C. L Detection and quantification of Phytophthora capsici in soil. Phytopathology 85: Leonian, L. H Stem and fruit blight of peppers caused by Phytophthora capsici sp. nov. Phytopathology 12: Louws, F. J., Lancaster, M. E., Holmes, G. J., and Driver, J. G Evaluation of fungicides and host resistance for control of Phytophthora crown rot of pepper, Fungic. Nematicide Tests 55: Madden, L. V Effects of rain splash dispersal of fungal pathogens. Can. J. Plant Pathol. 19: Matheron, M. E., and Porchas, M Suppression of Phytophthora root and crown rot on pepper plants treated with acibenzolar-smethyl. Plant Dis. 86: McGovern, R. J., Davis, T. A., Myers, D. S., and Seijo, T. E Evaluation of fungicides for control of diseases of tropical pumpkin, Fungic. Nematicide Tests 58:V124. Online publication. 70. McGrath, M. T Evaluation of fungicides for managing Phytophthora blight of squash, Fungic. Nematicide Tests 59:V054. Online publication. 71. Mchau, G. R. A., and Coffey, M. D Evidence for the existence of two subpopulations in Phytophthora capsici and a redescription of the species. Mycol. Res. 99: Miller, S. A., Bhat, R. G., and Schmitthenner, A. F Detection of Phytophthora capsici in pepper and cucurbit crops in Ohio with two commercial immunoassay kits. Plant Dis. 78: Minogue, K. P., and Fry, W. E Effect of temperature, relative humidity, and rehydration rate on germination of dried sporangia of Phytophthora infestans. Phytopathology 71: Oudemans, P., and Coffey, M. D Isozyme comparison within and among worldwide sources of three morphologically distinct species of Phytophthora. Mycol. Res. 95: Papavizas, G. C., and Bowers, J. H Comparative fungitoxicity of captafol and metalaxyl to Phytophthora capsici. Phytopathology 71:

12 76. Papavizas, G. C., Bowers, J. H., and Johnston, S. A Selective isolation of Phytophthora capsici from soils. Phytopathology 71: Parra, G., and Ristaino, J Insensitivity to Ridomil Gold (mefenoxam) found among field isolates of Phytophthora capsici causing Phytophthora blight on bell pepper in North Carolina and New Jersey. Plant Dis. 82: Parra, G., and Ristaino, J. B Resistance to mefenoxam and metalaxyl among field isolates of Phytophthora capsici causing Phytophthora blight of bell pepper. Plant Dis. 85: Ploetz, R. C., Heine, G., Haynes, J., and Watson, M An investigation of biological attributes that may contribute to the importance of Phytophthora capsici as a vegetable pathogen in Florida. Ann. Appl. Biol. 140: Polach, F. J., and Webster, R. K Identification of strains and inheritance of pathogenicity in Phytophthora capsici. Phytopathology 62: Ristaino, J. B Intraspecific variation among isolates of Phytophthora capsici from pepper and cucurbit fields in North Carolina. Phytopathology 80: Ristaino, J. B Influence of rainfall, drip irrigation, and inoculum density on the development of Phytophthora root and crown rot epidemics and yield in bell pepper. Phytopathology 81: Ristaino, J. B., Hord, M. J., and Gumpertz, M. L Population densities of Phytophthora capsici in field soils in relation to drip irrigation, rainfall, and disease incidence. Plant Dis. 76: Ristaino, J. B., and Johnston, S. A Ecologically based approaches to management of Phytophthora blight on bell pepper. Plant Dis. 83: Ristaino, J. B., Larkin, R. P., and Campbell, C. L Spatial and temporal dynamics of Phytophthora epidemics in commercial bell pepper fields. Phytopathology 83: Ristaino, J. B., Parra, G., and Campbell, C. L Suppression of Phytophthora blight in bell pepper by a no-till wheat cover crop. Phytopathology 87: Satour, M. M., and Butler, E. E A root and crown rot of tomato caused by Phytophthora capsici and P. parasitica. Phytopathology 57: Satour, M. M., and Butler, E. E Comparative morphological and physiological studies of the progenies from intraspecific matings of Phytophthora capsici. Phytopathology 58: Schlub, R. L Epidemiology of Phytophthora capsici on bell pepper. J. Agric. Sci., Camb. 100: Schultheis, J. R., and Wehner, T. C Optimum density of determinate and normal pickling cucumbers harvested once-over. (Abstr.) Proc. Pickling Cucumber Improvement Conf., 5 Oct. 1995, Lexington, KY. 91. Seebold, K. W., and Horten, T. B Evaluation of fungicides for control of Phytophthora crown and fruit rot of summer squash, Fungic. Nematicide Tests 58:V098. Online publication. 92. Shaw, D. S A method of obtaining single-oospore cultures of Phytophthora cactorum using live water snails. Phytopathology 57: Shishkoff, N., and McGrath, M. T Mary K. Hausbeck Dr. Hausbeck is a professor and plant pathologist with extension and research responsibilities in the Department of Plant Pathology at Michigan State University. She earned her B.S. and M.S. degrees in horticulture from Michigan State University and her Ph.D. in plant pathology from the Pennsylvania State University. She joined the faculty at Michigan State University in 1990 as a visiting assistant professor and became an assistant professor in Her research interests include the epidemiology and management of diseases of vegetables in the field and flower crops and vegetable transplants in the greenhouse. Dr. Hausbeck received the Michigan Master Farmer Associate Award and is a two-time recipient of the Michigan Extension Specialist Award for research and extension contributions to the vegetable industry. She also received the Society of American Florists 2004 Alex Laurie Award for research and education. Evaluation of fungicides and host resistance for control of Phytophthora crown rot of summer squash, Fungic. Nematicide Tests 55: Springer, J. K., and Johnston, S. A Black polyethylene mulch and Phytophthora blight of pepper. Plant Dis. 66: Stevenson, W. R., James, R. V., and Rand, R. E Evaluation of selected fungicides to control Phytophthora blight and fruit rot of cucumber. Fungic. Nematicide Tests 55: Stevenson, W. R., James, R. V., and Rand, R. E Evaluation of selected fungicides to control Phytophthora blight and fruit rot of cucumber. Fungic. Nematicide Tests 56:V16. Online publication. 97. Tian, D., and Babadoost, M Host range of Phytophthora capsici from pumpkin and pathogenicity of isolates. Plant Dis. 88: Tibayrenc, M., Kjellberg, F., Arnaud, J., Oury, B., Breniere, S., Darde, M., and Ayala, F. Kurt H. Lamour Dr. Lamour finished his Ph.D. in the Botany and Plant Pathology Department at Michigan State University in His research focused on the population biology of Phytophthora capsici particularly the impact of sexual reproduction within naturally occurring populations. He participated in the APS I. E. Melhus graduate symposium during the final year of his doctoral work. Dr. Lamour studied Phytophthora species as a postdoctoral researcher and as a visiting assistant professor at MSU before starting as an assistant professor in the Department of Entomology and Plant Pathology at the University of Tennessee in Knoxville in January of He received a National Science Foundation CAREER award in 2004 to develop reverse-genetic technology for Phytophthora functional genomics, and his research is focused on understanding the molecular machinery underlying Phytophthora s unique biology Are eukaryotic microorganisms clonal or sexual? A population genetics vantage. Proc. Natl. Acad. Sci. USA 88: Tompkins, C. M Phytophthora rot of honeydew melon. J. Agric. Res. 54: Waldenmaier, C. M Evaluation of fungicides for control of pumpkin diseases, Fungic. Nematicide Tests 59:V064. Online publication Waterhouse, G. M Taxonomy of Phytophthora. Phytopathology 60: Weber, G. F Blight of peppers in Florida caused by Phytophthora capsici. Phytopathology 22: Wiant, J. S A rot of winter queen watermelons caused by Phytophthora capsici. J. Agric. Res. 60: Yoshikawa, M., Tsukadaira, T., Masago, H., and Minoura, S A non-pectolytic protein from Phytophthora capsici that macerates plant tissue. Physiol. Plant Pathol. (UK) 11: Plant Disease / December