PHYTOPHTHORA ROOT ROT-RESISTANT AVOCADO ROOTSTOCKS FOR SOUTHERN FLORIDA: SELECTION OF OPEN-POLLINATED SEEDLING PROGENY

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1 had a mean weight of. g (. oz) with a range of 23. to 2. g (.3-. oz). Ripe fruit had a mean of 33.2 g (.7 oz) with a range of 6. to 6.6 g ( oz). ean fruit peel weight was. g (. oz). The mean percentage of fresh weight lost from harvest to ripening was.% with a range of. to 9.2%. On average fruit had 7.9% edible flesh with a wide range from. to.%. ean fruit length was 72.6 mm (2 7/ in) and diameter, 6. mm (22/2 in). The mean Brix was 29. with a range from 2. to 32. The mean number of seeds per fruit was. with only % of the fruit having 2 seeds. ean seed weight was. g (. oz) and ranged from. to.6 g (.3-. oz). Seed length ranged from 33. to. mm (P/ie-P/s in) with a mean of 3. mm (A in). Seed width was 26.9 mm (P/ir, in) on average, but ranged from 22.6 to 2. mm (7/s-l Vs in). ost of the fruit had an ovoid shape. The peel at ripening was very thin, almost translucent, with a light green color and orange blush. Ripe flesh color ranged from deep orange to salmon colored. The fruit flesh was moist and had a pleasant sweet flavor similar to mamey sapote. The peels of all fruit showed some shriveling upon ripening (softening). Effect of relative humidity on fruit ripening. For the fruit col lected on 2 ar., the mean number of days from harvest to ripening was. d and 9.2 d, respectively, for fruit allowed to ripen at LRH and HRH. At harvest, mean fruit weight was.2 g (. oz) and.2 g (.2 oz) for the LRH and HRH fruit, respectively. After ripening, mean fruit weight of the LRH treatment was 27. g (. oz) and 3.2 g (. oz) for the HRH treatment. From harvest to ripening, fruit in LRH lost 3% and HRH lost 9% fresh weight, respectively. Conclusions. At ambient temperatures, green sapote rip ened in about d. ost fruit had only seed, the mean fruit weight was g (. oz), the percent edible flesh was 79%, and the Brix was 29. Fruit lost about % of the fresh weight from harvest to ripening. Exposing fruit to HRH from harvest to rip ening decreased the percentage of fresh weight lost by %. reen sapote has a good potential as an alternative fruit crop to mamey sapote during the winter months when com mercially produced mamey sapote is in short supply. The flesh color, texture, and flavor of green sapote are of high quality and very similar to mamey sapote. Further introduc tions of superior germplasm and more research on postharvest handling are warranted for green sapote. Literature Cited Campbell, C. W., R. J. Knight, Jr., and N. L. Zareski Freeze damage to tropical fruits in southern Florida in 977. Proc. Fla State Hort Soc 9:2-27. artin, F. W., C. W. Campbell, and R.. Ruberte. 97. Perennial edible fruits of the tropics: an inventory. USDA-ARS, Agr. Handbook No. 62, U.S. ov. Printing Office, Washington, D.C. orton, J. F. 97. Fruits of warm climates. Creative Resource Systems Inc Winterville, N.C. owry, H., L. R. Toy, H. S. Wolfe, and. D. Ruehle iscellaneous tropical and subtropical Florida fruits. Bulletin 6A. Agr. Ext. Service, IF- AS, Univ. of Fla., ainesville. Popenoe, W. 97. anual of tropical and subtropical fruits. acmillan New York. Whitman, W. F. 96. The green sapote, a new fruit for south Florida Proc Fla. State Hort. Soc. 7: Whitman, W. F Two decades of tropical fruit introduction. Proc. Fla State Hort. Soc. :2-27. Proc. Fla. State Hort. Soc. :6-.. PHYTOPHTHORA ROOT ROT-RESISTANT AVOCADO ROOTSTOCKS FOR SOUTHERN FLORIDA: SELECTION OF OPEN-POLLINATED SEEDLIN PROENY Randy Ploetz and Jody Haynes University of Florida Tropical Research and Education Center 9 SW2th Street Homestead, FL Raymond J. Schnell U.S. Department of Agriculture Agricultural Research Service 36 Old Cutler Road iami, FL 33 Additional index words. Oomycota; Persea americana vars. ameri cana, drymifolia and guatemalensis. This research was supported by the Florida Agricultural Experiment Sta tion, and approved for publication as Journal Series No. N-9. Abstract. Phytophthora root rot (PRR) is the most important constraint to the production of avocado, Persea americana, worldwide. The pathogen, Phytophthora cinnamomi, does not occur naturally in Florida, but has spread to all produc tion areas in the state. The use of PRR-resistant rootstocks is the most effective long-term management strategy for this disease. In previous work, resistant rootstocks from the program of the University of California at Riverside were tested in Homestead. Their generally poor performance prompted the described research. From 996 to, we screened 2, open-pollinated seedlings from 2 acces sions in the USDA-ARS National Avocado ermplasm Re pository in iami for resistance to PRR. Although most seedlings were highly susceptible (i.e., developed >% root necrosis), tolerance to PRR was observed in several families. In general, the greatest tolerance was found in progeny with a West Indian heritage. Four seedlings sur vived two rounds of screening with minimal PRR damage. These selections will be clonally propagated for screeningunder field conditions. Proc. Fla. State Hort. Soc. :.

2 Table. Annual production for the major avocado-producing countries in / Country exico United States Indonesia South Africa Chile Brazil Dominican Republic Colombia Peru Israel Cameroon Spain Venezuela Haiti El Salvador Philippines Democratic Republic of Congo 'FAO production statistics. etric tons (XI,) Avocado {Persea americana iller) is one of the most im portant fruit crops. lobal production in exceeded 2. million metric tons, most of which were from the Western Hemisphere (Table ). Phytophthora root rot (PRR) is the most serious disease of avocado worldwide (Erwin and Ribiero, 996; Zentmyer, 9). It has decimated commercial production in many ar eas in Latin America and has destroyed germplasm collec tions throughout the region (C. W. Campbell, pers. comm.; R. Knight, pers. comm.). PRR is the major factor limiting pro duction in Australia and South Africa, and in California alone causes annual losses of ca. $3 million (Coffey, 9). PRR is caused by a soilborne organism in the Kingdom Chromista, Phytophthora cinnamomi Rands. The pathogen was thought previously to be a fungus, but is now known to be un related to the true fungi (Eumycota). One of the distinctive features of its life cycle is the production of motile zoospores. These propagules are chemotactically attracted to, infect, and kill root tips of avocado. Since free water is required for zoospore formation and motility, PRR is most severe in poorly drained and flooded soils. For example, hurricanes, tropical storms and, more recently, the artificial elevation of water ta bles adjacent to Everglades National Park, have resulted in se rious PRR-induced damage and mortality of avocado trees in southern Florida (Ploetz and Schaffer, 97; Ploetz and Schaffer, 99). PRR-resistant rootstocks have been selected at the Univer sity of California at Riverside for over years (Coffey, 9; Zentmyer, 9). In general, this program has relied on nat ural hybrids of the exican and uatemalan races of P. amer icana, vars. drymifolia and guatemalensis, respectively, and to a lesser extent, P. americana and P. schiediana hybrids. Although many of these selections have good resistance to PRR, they generally perform poorly in calcareous soils (Ben-Yaacov et al., 9; Ploetz et al., 99). oreover, they usually do not tolerate conditions in the lowland humid tropics as well as P. americana var. americana, the West Indian race. Work in Israel and the Canary Islands indicates that toler ance to PRR and to calcareous and saline soils exists in the West Indian race (Ben-Yaacov, etal, 9; Ben-Yaacov, 99; Table 2. Avocado accessions in the USDA-ARS National ermplasm Reposi tory in iami from which seed were obtained for screening against phytophthora root rot/ Pedigree/cultivar uatemalan Brookslate Collins seedling Ishim Nabal Nabal seedling Queen uatemalan x West Indian hybrids Collin Red seedling B Collinson irrardin ripina 2 Hiavvasee onroe Nesbitt Tonnage Suardia Winslowson seedling? exican exican hybrid Vero Beach RD exican x uatemalan hybrids Ein Vered Netaim Sharwil Tenerife Tensen exican x West Indian hybrid West Indian Arguello Aycock Red #3 Bernecker Biscayne Catalina Cellons Hawaii seedling Dade Dr. Dupuis 2 Family eneral Bureau Jose Antonio Kilo Ceiba axima Pollock Si mm on ds Waldin Wester Wilson Popenoe Complex pedigree accession number iami Year(s) tested,,,,,,,, ZA seedling progeny are from trees in the National ermplasm Repository for avocado at the USDA-ARS research station in iami, Fla. Proc. Fla. State Hort. Soc. :.

3 Table 2. (Continued) Avocado accessions in the USDA-ARS National ermplasm Repository in iami from which seed were obtained for screening against phytophthora root rot.z Table 3. ean phytophthora root rot ratings for open-pollinated seedling progeny.' 6 Pedigree/cultivar Irwing 7 Irwing 9 Irwing 3 arsheline Yama 3 iami accession number Year(s) tested 'All seedling progeny are from trees in the National ermplasm Repository for avocado at the USDA-ARS research station in iami, Fla. allo-llobet and Siverio, 99). In this paper we report pre liminary work to identify avocado rootstocks that could be grown in periodically flooded, P. dnnamomnnfested soils in southern Florida. aterials and ethods A large collection of avocado germplasm is established at the National Avocado ermplasm Repositiory (USDA-ARS) in iami. The collection was started in 96 and currently contains genetically diverse accessions. From 996 tol99, fruit were collected from a total of 2 accessions in this collection (Table 2). Seeds were removed from fruit, dipped in benomyl fungicide (.2 g L), and stored in an air-conditioned greenhouse until they could be planted (usually within 2 d) in a peat, bark, and sand potting mix. Seedlings were grown for 3- months in a glasshouse be fore they were replanted in potting mix that was infested with P. cinnamomi. Inoculum was of a virulent isolate of the patho gen that was grown on autoclaved millet seed. It was mixed uniformly, at a rate of 2.2 g (fresh weight) L of potting mix, in a large Hobart blendor. No more than 2 seeds were plant ed per.7 x.3 m metallic tray. After 6- weeks, seedlings were removed from trays, gently washed, and examined for root necrosis. A visual estimate of the percentage of roots on each seedling that were necrotic was made, and note was taken of the characteristics of seed lings that appeared healthy. In general, seedlings that had less than 3% root mortality and/or possessed a large and vig orous root system were replanted in infested soil in 3 L pots to ensure that they were not disease escapes. After 3-6 months, the selections were rated again. Results and Discussion A total of 2, open-pollinated seedlings were screened from 996 to. In general, very high levels of disease de veloped under the described conditions (Table 3, Figs. and 2). For example, an average of 97% of the roots of 'Ein Vered' seedlings were necrotic, and more than 9% of the seedlings of several other accessions were affected. However, there was considerable variation in susceptibili ty among seedlings from the different accessions (Table 3). In, seedlings from 'Simmonds' and 'Winslowson seedling' developed about one-half as much root rot as the most sus ceptible seedlings, and in seedlings from 'Catalina' de veloped about one-third the amount of damage suffered by the most susceptible group (Table 3 and data not shown). Al though seedlings from only eight accessions were tested in Cultivar Ein Vered Irwin 7 Sharwil Nabal Collinson Brookslate Netaim Tonnage Dade Wilson Popenoe Bernecker Nesbitt ripina 2 irrardin Queen Jose Antonio Dr. Dupuis 2 eneral Bureau Waldin Aycock Red #3 Hiawasee Pollack Cellons Hawaii seedling Collins seedling Family Arguello irrardin Winslowson seedling Simmonds Kilo Ceiba arsheline Nabal seedling Suardia Irwin 9 Tenerife Collin Red seedling B Irwin 3 Wester ripina 2 Biscayne Pedigree> n ean root necrosis (%) Tests in and occurred in potting mix artificially infested with Phytophthora cinnamomi Seed were from open-pollinated trees in the National ermplasm Repository for avocado at the USDA-ARS research sta tion in iami, Fla. > = uatemalen; = exican; = West Indian; = uatemalen x West Indian; = exican x uatemalen; = exican x West Indian. Proc. Fla. State Hort. Soc. :.

4 Table 3. (Continued) ean phytophthora root rot ratings for open-polli nated seedling progeny/ Cultivar Ishim onroe Tense n axima Catalina - totals Pedigree* n 33 2 ean root necrosis (%) 'Tests in and occurred in potting mix artificially infested with Phytophthora cinnamoml Seed were from open-pollinated trees in the National ermplasm Repository for avocado at the USDA-ARS research sta tion in iami, Fla. V = uatemalen; = exican; = West Indian; = uatemalen x West Indian; = exican x uatemalen; = exican x West Indian and, there was good agreement between results for these accessions in both years. There was also variation in the susceptibility of seedlings with different racial backgrounds (Table 3, Figs. and 2). For example, in mean ratings for seedlings from West Indi an cultivars were % lower than for exican x uatemalan hybrids and cultivars with a background. In, the same relative ranking was repeated despite the analysis of seedlings from a much different group of cultivars that year. Of relevance to the objectives of this work, far greater num bers of seedlings of West Indian cultivars and uatemalan x West Indian hybrids were tolerant of PRR than any of the oth er backgrounds that were tested. Using an arbitrary standard of less than % root necrosis, 23 of the 6 (3.%) uatema lan x West Indian seedlings and 3 of the 69 (.%) West Indian seedlings were tolerant, but only four of the 777 seed lings (.%) from all other pedigrees were so ranked. Four seedlings survived two rounds of screening: one each from 'Dade' and 'Pollock' and two from 'Catalina.' We plan to clonally propagate these individuals to test them fur ther in infested soil in the field. Work to identify additional tolerant individuals will focus on West Indian cultivars and uatemalan x West Indian progeny. exican/uatemalan uatemalan cultivars (n=) 7 hybrids (n=3) 6 O 3 3 I : IS 3 C uatemalan/west Indian hybrids (n-) West Indian cultivars (n=) 6.Q JSEL. 6 Percent Root Necrosis Figure. Frequency distributions of root necrosis classes for different racial backgrounds of avocado in. Data are for open-pollinated seedling prog eny from the National ermplasm Repositoiy in iami that are listed in Tables 2 and 3. n = the number of accessions that are represented for each racial backgrounds. Only accessions for which four or more seedlings were rated are included in these figures. Proc. Fla. State Hort. Soc. :.

5 c/> o l_ <D Q. 6 exican/uatemalan hybrids (n=2) %*fcs. uatemalan cultivars (n=3) K- P 2.> - D.E uatemalan/west Indian hybrids (n=) West Indian cultivars (n=7) a Percent Root Necrosis Figure 2. Frequency distributions of root necrosis classes for different racial backgrounds of avocado in. Data are for open-pollinated seedling prog eny from the National ermplasm Repository in iami that are listed in Tables 2 and 3. n = the number of accessions that are represented for each racial backgrounds. Only accessions for which four or more seedlings were rated are included in these figures. Acknowledgments We thank Zaragoza Alegria, Aime Vazquez and ike Winterstein for technical assistance. Literature Cited Ben-Yaacov, A.,. Bufler, A. Barrientos-Priego, E. De la Cruz Torre, and L. Lopez-Lopez. 9. A study of avocado germplasm resources, eneral description of the international project and its findings, p. 3-. In: Proc. World Avocado Cong. II. Ben-Yaacov, A.,. Zilberstaine, and I. Sela. 9. A study of avocado germ plasm resources, V. The evaluation of the collected avocado germplasm material for horticultural purposes, p In: Proc. World Avocado Cong. II. Ben-Yaacov, A. 99. The adaptation of the avocado tree to saline conditions in Israel by rootstock selection. Proc. World Avocado Cong. Ill (In press). Coffey,. D. 9. Phytophthora root rot of avocado, p In: J. Ku mar, et al. (Eds.). Plants Diseases of International Importance. Vol. III. Diseases of Fruit Crops. Prentice-Hall, Englewood Cliffs, N.J. allo-llobet, L. 9. Update of Canary Islands research on West Indian av ocado rootstock tolerance/resistance to Phytophthora root rot, p. -. In: Proc. World Avocado Cong. II. allo-llobet, L. and F. Siverio. 99. Evaluation of West Indian avocado seedlings's tolerance/resistance to Phytophthora cinnamomi Rands compared with clonal resistant root-stocks Duke 7, Thomas, and Toro Canyon, un der field conditions. Proc. World Avocado Cong. Ill (In press). Ploetz, R. C. and B. Schaffer. 97. Effects of flooding and phytophthora root rot on photosynthetic characteristics of avocado. Proc. Fla. State Hort. Soc. : Ploetz, R. C.J. L. Ramos,J. L. Parrado, B. Schaffer, and S. P. Lara. 99. Per formance of clonal avocado rootstocks in Dade County, Florida. Proc. Fla. State Hort. Soc. 2: Ploetz, R. C. and B. Schaffer. 99. Effects of flooding and Phytophthora root rot on net gas exchange and growth of avocado. Phytopathology 79:-. Zentmyer,. A. 9. Phytophthora cinnamomi and the Diseases it Causes. onogr. No.. Amer. Phytopathol. Soc. St. Paul. Proc. Fla. State Hort. Soc. :.