INSECTS Preliminary Root-Rating Results for Urbana Corn Rootworm Trial On July 13, a crew led by John Shaw, Illinois Natural History Survey, dug and washed roots from an experimental corn rootworm trial located just south of Champaign Urbana. We rated the roots for larval injury and present some of the preliminary results in Table 1. Some of the highlighted products (Table 1), such as Warrior T, are not labeled for corn rootworm larval control. Please consult the appropriate product label for rate and application instructions before making any treatment with these insecticides. FOR IMMEDIATE RELEASE No. 17 / July 21, 2000 Executive editor: Kevin Steffey, Extension Entomologist Available on the Web at http://www.ag.uiuc.edu/cespubs/pest/ For subscription information, phone 217.244.5166, or e-mail acesnews@uiuc.edu In This Issue Preliminary Root-Rating Results for Urbana Corn Rootworm Trial, 193 Scouting for Corn Rootworm Adults, 194 Sudden Death Syndrome on Soybean, 195 Behold White Mold? 196 Keep Your Eyes Open for Soybean Stem Canker, 197 Regional Reports, 197 The Urbana experimental corn rootworm insecticide efficacy trial was planted on April 28, late by today s standards. In general, most products performed very well and kept larval injury below a root rating of 3.0 (some light pruning of corn roots) on the Iowa State 1-to- 6 injury scale. The level of rootworm pressure in the experiment was quite good, with over two Table 1. Preliminary root-rating results for corn rootworm trial, July 13, 2000, Urbana, Illinois 1. Insecticide Rate Application Root rating 2 Aztec BD 2.1G 0.15 lb a.i./acre Band 2.00 Force 3G 0.13 lb a.i./acre Band 2.05 Fortress 5G 0.16 lb a.i./acre Furrow 2.10 Lorsban 15G 1.3 lb a.i./acre Band 2.13 Counter 20CR 1.3 lb a.i./acre Band 2.14 Lorsban 15G 1.3 lb a.i./acre Furrow 2.20 Force 3G 0.13 lb a.i./acre Furrow 2.30 Regent 4SC 0.13 lb a.i./acre Microtube 2.45 Aztec 2.1G 0.15 lb a.i./acre Band 2.60 Capture 2EC 0.09 lb a.i./acre Band 2.60 Warrior T 0.09 lb a.i./acre Band 2.71 Regent 4SC 0.13 lb a.i./acre Band 2.80 Capture 2EC 0.08 lb a.i./acre Band 2.85 Aztec BD 2.1G 0.11 lb a.i./acre Band 2.89 Warrior T 0.06 lb a.i./acre Band 3.06 NuFos 15G 1.3 lb a.i./acre Band 3.25 Warrior T 0.03 lb a.i./acre Band 3.45 ProShield Seed treatment 3.55 Gaucho 1.0 g.a./unit Seed treatment 3.60 Gaucho 1.3 g.a./unit Seed treatment 3.65 Furadan 4F 3 1.0 lb. a.i./acre Broadcast (at hatch) 3.80 Gaucho 0.6 g.a./unit Seed treatment 4.00 Fortress 5G 0.16 lb a.i./acre Band 4.09 Control 5.03 1 Plots were planted on April 28, 2000, in a field that had been planted to a trap crop of corn in 1999. 2 Iowa State University 1-to-6 root-rating scale used: 1 = no visible damage or only a few minor feeding scars, 2 = some roots with feeding scars but none eaten off to within 1.5 inches of the plant, 3 = several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed, 4 = one node of roots destroyed or the equivalent, 5 = two nodes of roots destroyed or the equivalent, and 6 = three or more nodes of roots destroyed. See issue no. 15 (July 7, 2000) of the Bulletin. 3 Furadan 4F was applied on May 26, 2000, to coincide with corn rootworm larval hatch. 193
nodes of roots destroyed in the untreated control. With the exception of the seed-treatment products, Furadan 4F, Fortress 5G (band application), NuFos 15G, and Warrior T (two reduced rates), the great majority of insecticide treatments kept root injury below the commonly accepted economic injury index of 3.0 on the Iowa State injury scale (several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed). Although the banded application of Fortress 5G failed to provide adequate root protection, the in-furrow treatment of this product offered a high level of root protection. The two lower application rates of Warrior T also failed to keep root injury below a root rating of 3.0; however, this product is not currently labeled for larval control. The higher rate of Warrior T (0.09 lb a.i./acre) did keep root injury below a rating of 3.0. NuFos 15G, which has the same active ingredient (chlorpyrifos) as Lorsban 15G, had root injury that exceeded the 3.0 root-rating level. This seems a bit curious, considering that Lorsban 15G performed very well in this experiment. On July 24 and 25, we will continue our corn rootworm insecticideefficacy plot evaluations in Monmouth and DeKalb, respectively. As soon as data from these experiments are available, we ll share them with you. We are very interested in how soil insecticides are performing in different areas of the state. Give us a call, or send us an e-mail message, if you experience corn rootworm damage that is greater than anticipated. Mike Gray and Kevin Steffey Scouting for Corn Rootworm Adults Emergence of corn rootworm adults is in full bloom throughout Illinois; peak population densities and peak egg laying will occur within the next few weeks. Therefore the time for scouting for rootworm adults is upon us. There are three different reasons for scouting for adult corn rootworms throughout most of state and a fourth reason in the counties in Illinois where western corn rootworms lay eggs in soybeans. The first reason to scout for rootworm adults is to watch for silkclipping injury that interferes with pollination. We discussed this in an article in issue no. 13 (June 23, 2000) of the Bulletin. The other three reasons for scouting for rootworm adults are to Determine the potential for rootworm larval injury next year if corn is planted after corn. Typically, results from this scouting effort suggest whether a preventive soil insecticide should be applied when corn is planted next spring. Determine whether you should prevent rootworm adults from laying eggs in corn this year to prevent rootworm larval injury next year. This practice has specific guidelines for corn after corn but not for corn planted after soybeans. As we have stated emphatically in the past, we strongly discourage attempts to prevent western corn rootworms from laying eggs in soybeans until we have more information about their movement and egg-laying behavior. Determine the potential for rootworm larval injury next year if corn is planted after soybeans in areas of Illinois where this behavior has been identified. Following are guidelines for scouting for rootworm adults in corn (two different objectives) and soybeans. Corn planted in 2000, corn to be planted in 2001 Is a preventive soil insecticide warranted? To answer this question, scout for rootworm adults at least once each week from mid-july through early September. During each scouting trip, count the number of beetles (both northern and western corn rootworms) on two plants selected at random in each of 25 areas of a field. The counts take about 45 minutes in a 40-acre 194 field. As you approach a plant, move quietly to avoid disturbing the beetles. Count the beetles on the entire plant, including the tassel, leaf surfaces, behind leaf axils, and on ear tips. Record the number of beetles you find on each plant, then determine the average number of beetles per plant for the field. If the average is greater than 0.75 beetle per plant in corn after corn (corn planted in both 1999 and 2000) or 0.5 beetle per plant in firstyear corn (something other than corn planted in 1999, corn planted in 2000) for any sampling date, a grower should plan to apply a preventive soil insecticide if corn is planted in 2001. Corn planted in 2000, corn to be planted in 2001 Preventing egg laying by rootworm females in 2000 This approach toward rootworm management is a viable approach if one adheres to the guidelines and is committed to a rigorous scouting program. Keep in mind, however, that this approach used year after year in some counties in Nebraska eventually resulted in a population of western corn rootworms resistant to both carbaryl and methyl parathion. Although we have not encountered rootworms resistant to insecticides in Illinois for many years, sole reliance on one control tactic is seldom a good idea. In general, the guidelines for sampling for rootworm adults to determine when to prevent egg laying are the same as discussed previously, although some consulting firms may suggest alternative combinations of numbers of plants and groupings of samples. However, virtually all recommendations for this approach for managing corn rootworms express the same thresholds. If the number of beetles reaches or exceeds 0.75 per plant, apply an insecticide when 10% of the females are gravid (with eggs). Consequently, you will need to be able to tell the difference between males and females and between gravid and nongravid females. Male and female western corn rootworms can be distinguished fairly easily. Female northern
corn rootworms are slightly larger than the males. Gravid females of both species have distended abdomens. Squeezing a gravid female s abdomen will reveal eggs within the liquid exuded. Companies that support this approach for management of corn rootworms usually provide more detailed explanations and vivid graphics. Fields that have been sprayed to prevent rootworm adults from laying eggs should be monitored weekly after treatment to accommodate immigration into the field or continued emergence within the field. A second application of an insecticide may be necessary if the number of beetles reaches or exceeds 0.5 per plant. Soybeans planted in 2000, corn to be planted in 2001 Is a preventive soil insecticide warranted? The saga of the variant of the western corn rootworm that lays eggs in soybeans dates back more than 5 years now. As you are well aware, the problem with western corn rootworms in corn planted after soybeans has spread from its origin in east-central Illinois and northwest Indiana. The problem has been verified from southern Michigan, western Ohio, throughout the northern half of Indiana, and in as many as 30 counties in Illinois. Consequently, monitoring for western corn rootworms in soybeans has been the best strategy for determining the potential for rootworm larval injury in corn planted after soybeans. Scouting for western corn rootworm adults in soybeans should be conducted from the last week in July through the third week in August by using yellow sticky traps (Pherocon AM traps). In the last week of July, 12 unbaited Pherocon AM traps should be distributed evenly throughout each soybean field, regardless of field size. The traps should be positioned just above the soybean canopy on metal fence posts. Each week, remove the old trap and replace it with a new trap. Count all western corn rootworms (male and female) found on each trap in each field, and determine the average number of beetles per trap per field. After the 1999 field season, we adjusted the thresholds to accommodate the new data we gathered. If you find an average of 10 beetles per trap per day, you can anticipate an average root-injury rating of 4.0 (one node of roots destroyed) the following season. If you find an average of 5 beetles per trap per day, you can anticipate an average root-injury rating of 3.0 (some root pruning, never an entire node destroyed) next year. The level of root injury that may result in economic loss varies according to the growing season and the hybrid selected. In general, root-injury ratings between 3.0 and 4.0 may cause yield loss. However, for certain corn hybrids during poor growing conditions, our research has shown that yield losses can occur with root ratings between 2.0 and 3.0 (minor root scarring to minor root pruning). For additional information on scouting soybean fields for western corn rootworm adults, please check out the following site: http://www.aces.uiuc. edu/ipm/field/corn/insect/wcr.html. As we begin to get a feel for the densities of corn rootworm adults in both corn and soybean fields this summer, we will report our findings in the Bulletin. In the meantime, happy scouting! Kevin Steffey and Mike Gray PLANT DISEASES Sudden Death Syndrome on Soybean 195 Sudden death syndrome (SDS) is a disease reported in most soybeangrowing areas of the United States and the world. In 1998, SDS was ranked second behind soybean cyst nematode as the most important disease of soybeans in Illinois. The soilborne fungus, Fusarium solani f.sp. glycines, which is the causal organism of SDS on soybeans, infects soybean roots. Under severe conditions, SDS can result in flower and pod abortion, premature defoliation, and yield losses. The use of fungicides to control this pathogen has not been effective, and crop rotation does not provide a viable control alternative. Although SDS potentially can be controlled by host-plant resistance and some progress has been made in developing resistant varieties, screening for resistance is difficult because disease expression is often environmentally controlled. Soil moisture appears to be one environmental parameter that influences the amount of SDS occurring in the field, as disease development appears to be favored by high soil moisture or rainfall and irrigation. Development of SDS in the field is more severe in compacted areas. Disease surveys conducted around the state have shown that sudden death syndrome occurs more frequently in fields under high production, in wet and compacted areas, and in fields with high populations of soybean cyst nematodes, Heterodera glycines. Research has also shown that the soybean cyst nematode (SCN) is not required for infection by the SDS pathogen. The presence of SCN appears to increase plant stress when both organisms are present, which in turn can increase the severity of foliar symptoms compared to those caused by the SDS pathogen alone. Symptoms produced by SDS begin as chlorosis and necrosis of the interveinal tissue of leaves, which then coalesces, forming large yellow and brown areas between a green midvein and green lateral veins. Other symptoms are rotting of roots, necrosis of the crown, discoloration of the vascular tissue in roots and stems, premature defoliation of the soybean plant, and abortion of the flowers and pods. When leaves drop off the soybean plant, they often drop from the top, leaving the petiole attached to the stem. Infected plants in the field prematurely turn yellow and then brown, whereas healthy plants remain green. The foliar symptoms of SDS seen in the field are similar to those of brown stem rot, but internal symptoms differ in that with SDS there is no pith discoloration. Some uniform reddish brown vascular discoloration can
occur with SDS, but without a streaking pattern. Leaf symptoms of the stem canker disease can be confused with SDS; however, stem canker has cankers on the lower stem, and plants with SDS do not have cankers. This disease is difficult to control for several reasons. First, there are still many unknown factors about this pathogen and how it is causing disease on soybeans. Second, there are no currently labeled resistant varieties. Currently, researchers at the University of Illinois are investigating mechanisms for resistance in several soybean varieties. Their research program is investigating whether the root system of the soybean plant may resist colonization or overcome infection using some unknown mechanism. In addition, researchers at the University of Illinois are investigating how tillage regimes and rainfall may influence disease development and root-system colonization. In the meantime, it is best to try to manage this disease by lessening the impact of SDS. 1. Learn to identify SDS in the field, as symptoms may appear similar to more common diseases such as brown stem rot or stem canker. 2. Select soybean varieties that mature at different times. Use either different maturities within a maturity group or use different maturity groups. Early maturing cultivars appear to be a better choice. 3. Delay planting or extend planting time so that all soybeans are not at the same growth stage at the same time. However, do not wait past the suggested time for planting in your area of the state. 4. Use cultural practices to improve drainage in low spots, reduce cyst nematode populations, and reduce soil compaction. 5. Crop rotation is of limited value because this organism can persist in the soil for many years. However, planting continuous soybeans is not recommended because this can increase other diseases. Loretta Ortiz-Ribbing Behold White Mold? According to the latest Illinois Weather & Crops report (7/17/00), 86% of the Illinois soybean crop is rated good to excellent and two-thirds of the crop is well into bloom. Now, about that price (more about that later). Because much of the crop is in the reproductive phase, it s time to watch for several midseason soybean diseases. For the time being, this advise is particularly true for northern Illinois and parts of southern Illinois, which have received a good amount of rainfall over the last week or so. Considering the fact that as little as 2 months ago many were talking about the 2000 drought, it seems quite odd even to think about white mold. Although I have not heard of any reports of white mold in Illinois at this point, should growers in the northern half of the state experience cooler weather along with continued rainfall, white mold may once again rear its ugly head in Illinois. White mold is favored by moderate temperatures (less than 85 F), normal or excessive rainfall, and high canopy humidity. Thus, you should monitor field areas that dry slowly due to a dense or lodged canopy or lack of wind, and areas where white mold has appeared in previous years. The first symptoms of white mold generally appear during growth stages R1 through R3 (beginning bloom through beginning pod) and are often aggregated (found in hot spots ) rather than uniform across the field. Infected plants initially have grayish green leaves (similar to the way frost-damaged plants would look) that eventually turn brown but remain attached. Eventually, all or part of the plant dies due to girdling stem lesions typically located several inches above the soil line. Side branches and pods may also be infected. Recognize that these symptoms can be confused with sev- Table 2. Comparison of three soybean stem diseases. White mold Stem canker Phytophthora Affected plant parts Stems, branches, Stem, usually near Roots and lower stem blooms, and pods node White fungal growth on infected area? Yes No No Root symptoms? None None Yes, most infections begin below ground and the blackto-brown discoloration extends upward along the stem Dead plant retains leaves? Yes Yes Yes Resistance available? Most varieties are susceptible, some have partial resistance Most varieties have good resistance Two types: race-specific resistance and general tolerance 196
eral other diseases including midseason Phytophthora, brown stem rot, and stem canker (see Table 2 for comparisons). However, plants that were killed or that are dying due to white mold typically have small amounts of cottony fungal growth on the surface of infected plant parts. Furthermore, at crop maturity, irregularly shaped sclerotia (hardened fungal survival structures) will be noticeable on and within white mold-infected stems and pods. White mold may go unnoticed until the sclerotia is found during or after harvest. If conditions are right for white mold in some fields or parts of your fields, what can be done at this point? Due to the depressed crop price, it is very unlikely that fungicide applications are economical. If applied properly, Topsin M can be used to manage white mold. Two applications are necessary, the first applied between 25% and 50% of full bloom and the second applied 10 to 14 days later. Good canopy penetration is crucial because the fungus is active in the top few inches of the soil and ejects spores from there into the lower canopy. Even if you don t plan to spray for white mold, accurate diagnosis is important for long-term disease management planning. Recently, the North Central Soybean Research Program introduced a White Mold Coalition Web site (http://www.plantpath.wisc. edu/ncsrpwhitemold/summ99.htm), which provides a great deal of userfriendly information about short- and long-term management strategies, including varietal resistance information. Bruce Paulsrud Keep Your Eyes Open for Soybean Stem Canker As discussed in the white mold article in this issue, moderate to excessive rainfall in some parts of Illinois may encourage a number of diseases, including stem canker. Stem canker symptoms usually appear in late July or early August, when the pods are starting to fill out, and persist until the crop matures. The fungus is favored by warm temperatures, normal or excessive rainfall, high canopy humidity, and crop damage (for example, due to hail). Stem canker is characterized by reddish brown to black and slightly sunken lesions usually located near lower leaf nodes. In addition, the fungus produces a toxin that causes leaflets above the canker to develop a bright yellow color between the veins. Enlarging lesions will eventually girdle the stem, causing the plant to wilt, wither, and die. Phytophthora root rot has plagued a number of growers this season, and in terms of future variety selection it is important that Phytophthora and stem canker not be confused with each another (see Tablle 2 for comparison). For example, let s say you have a field planted to a Phytoph-thora-resistant variety and you incorrectly identify a fair amount of midseason Phytophthora root rot in it this year. Based on this incorrect information, you will likely assume that the type of Phytophthora resistance you used is not appropriate for your field or area and may wrongfully select a different source of resistance for the 2002 crop year. If you are unsure about the diagnosis, seek assistance from an Extension educator, agronomist, or the University of Illinois Plant Clinic. Fifty years ago, stem canker was a major problem. However, most varieties available today have good resistance to this disease. If you are experiencing problems with stem canker in certain areas or with certain varieties, consider tillage and rotation to reduce the amount of infested residue, and consider using other varieties that have a good track record against stem canker. 197 If conditions are right for stem canker in your area, what can be done at this point? Due to the depressed crop price, it is very unlikely that fungicide applications are economical. There is a checklist on page 110 of the Illinois Agriculture Pest Management Handbook 2000, which will help you determine if a foliar fungicide application should be made to soybean fields. Provided you can achieve good canopy penetration, Benlate, Bravo, or Topsin M can be used to manage a number of foliar diseases (for example, anthracnose, stem canker, brown spot, and pod and stem blight). For maximum yield and seed quality, two applications are suggested, beginning at full bloom to early pod set (depending on the product) and once again 10 to 14 days later. Bruce Paulsrud REGIONAL REPORTS Extension Center educators, Unit educators, and Unit assistants in northern, west-central, east-central, and southern Illinois prepare regional reports to provide more localized insight into pest situations and crop conditions in Illinois. The reports will keep you up to date on situations in field and forage crops as they develop throughout the season. The regions have been defined broadly to include the agricultural statistics districts as designated by the Illinois Agricultural Statistics Service, with slight modifications: North (northwest and northeast districts, plus Stark and Marshall counties) West central (west and west southwest districts, and Peoria, Woodford, Tazewell, Mason, Menard, and Logan counties from the central district) East central (east and east southeast districts [except Marion, Clay, Richland, and Lawrence counties], McLean, DeWitt, and Macon counties from the central district) South (southwest and southeast districts, and Marion, Clay, Richland, and Lawrence counties from the east southeast district)
We hope these reports will provide additional benefits for staying current as the season progresses. Northern Illinois Second and third hay cuttings are being made as weather permits. Drier, cooler weather has slowed the development of fungal diseases in both corn and soybeans. Some fields have been sprayed, and hopefully the spray was justified. Limited cases of ergot have been observed in grasses and small grains. Insect activity continues at an extremely low ebb except for leafhoppers. In fields swept late last week in Stephenson County, leafhopper numbers had doubled from numbers observed in the same fields just 3 days before. Weather has now turned much cooler, so we should not see numbers build as rapidly. You need to continue to monitor for this pest, especially in the recently cut fields. Observations of Phytophthora in soybeans continue to increase, as well as observations of yellow stunted areas of corn and soybeans in wet areas of fields. Wildlife (groundhogs) have wiped out several rows of beans along bluffs and areas where dens are located. Wheat variety trials harvested in Ogle County last week had average yields in the 50- to 60-bushel range. Thistles are blooming or have bloomed; and, where they are not controlled or mowed, seeds are spreading by wind and other methods of movement. West-Central Illinois More rain fell in parts of the region, with reports of 4 inches or more in some areas. Crop prospects look excellent. Some Japanese beetles have been sighted, but no major problems have been reported. Septoria brown spot and bacterial pustule have been identified in some soybean fields. SDS has not yet been reported but is anticipated due to the recent cool, wet weather. SCN damage has not yet been identified in the region. Bird damage on double-crop soybeans following wheat has been reported. Yellow soybeans, other than those in poorly drained areas, have also been noted. Contributing Authors Mike Gray (m-gray4@uiuc.edu), Extension Entomology, (217)333-6652 Loretta Ortiz-Ribbing (l-ortiz@uiuc.edu), Champaign Extension Center, (217)333-4901 Bruce Paulsrud (paulsrud@uiuc.edu), Extension Specialist/Pesticide Applicator Training/ Plant Pathology, (217)244-9646 Kevin Steffey (ksteffey@uiuc.edu), Extension Entomology, (217)333-6652 198
U of I Extension Newsletter Service University of Illinois at Urbana-Champaign 528 Bevier Hall, MC-184 905 S. Goodwin Avenue Urbana, IL 61801 The Pest Management & Crop Development Bulletin is brought to you by University of Illinois Extension and Information Technology and Communication Services, College of Agricultural, Consumer and Environmental Sciences, University of Illinois at Urbana-Champaign. This newsletter is edited by Erin Cler and formatted by Oneda VanDyke, ACES/ITCS. Helping You Put Knowledge to Work University of Illinois U.S. Department of Agriculture Local Extension Councils Cooperating University of Illinois Extension provides equal opportunities in programs and employment. 199