report on PLANT DISEASE RPD No. 507 April 1998 DEPARTMENT OF CROP SCIENCES UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN ILLINOIS SOYBEAN DISEASE MANAGEMENT PROGRAM Soybean diseases reduce Illinois soybean yields by 5 to 15 percent annually, depending on the diseases involved, the varieties grown, the management practices followed, and various environmental factors. Approximately 15 different diseases are responsible for these yield losses. A comprehensive soybean disease management program can sharply reduce these losses in yield and grain quality. A successful disease-control program could involve just a single practice, but the long-term reduction of disease losses generally requires the application of several control measures. A comprehensive and integrated disease-management program should include the use of adapted, disease-resistant varieties; high-quality, disease-free seed; a well-drained, fertile seedbed; crop rotations; various tillage practices; balanced soil fertility and proper soil reaction (ph) based on a soil test; fungicides; insect and weed control; following other suggested cultural practices; and scouting and monitoring. The goal of an integrated disease-control program is to disrupt the combination of factors necessary for disease development: an environment favorable for disease-causing organisms (pathogens), susceptible plants, the presence of sufficient quantities of a virulent pathogen capable of reproducing and spreading, and adequate time for the disease to develop. Essential to a disease-control program, therefore, is an understanding of pathogens, disease cycles, which plant parts are attacked and when, and the factors involved in the spread and reproduction of pathogens. Table 1 (pp 6-7) lists the common diseases that are known to cause yield losses in Illinois. The maps following the text show the relative risk of losses from individual diseases on a regional basis. "Relative risk" does not mean that yield losses may not be higher or lower for individual fields; it simply means that the diseases commonly pose this potential risk in the areas shown. In general, disease losses rarely exceed 5 to 10 percent in any field regardless of how many diseases are present. Therefore, potential disease losses are not cumulative. Irrigated fields, fields in river bottoms or other areas where warm, wet conditions prevail, fields planted to narrow rows, fields where reduced- or no-tillage is used, fields planted with poor-quality seed, or fields where soybeans are grown continuously have a greater risk potential for disease loss. With the knowledge of the "risk potential" of the various diseases and of the life cycles of the pathogens that cause them, an integrated and comprehensive disease-control program can be planned. Various control practices that could be incorporated into a comprehensive program are discussed in this report. CULTIVAR MATURITY AND GROWTH HABIT The relative maturity of soybean cultivars can have a dramatic impact on the severity of soybean diseases. For example, early maturing varieties for a given area will generally be more severely affected by pod and For further information contact Dean K. Malvick, Extension Specialist and Field Crops Pathologist, Department of Crop Sciences, University of Illinois at Urbana-Champaign. University of Illinois Extension provides equal opportunities in programs and employment.
-2- stem blight, Septoria brown spot, anthracnose, Cercospora purple seed stain and leaf blight, and sudden death syndrome. Late maturing varieties are affected less because of the generally cooler, drier conditions prevalent later in the growing season. However, early maturing cultivars are generally less affected by brown stem rot than late maturing cultivars. To minimize the damage from charcoal rot, full-season cultivars should be planted as early as possible. Soybean growth habits can affect disease development. For example, yields of short determinant varieties are generally more affected by rain-splashed pathogens than the yields of indeterminant varieties. Diseases likely to be more severe on short-stature varieties include Septoria brown spot, pod and stem blight, and Cercospora purple seed stain and leaf blight. Differences in resistance may negate the effect of plant height on disease. SEED QUALITY Many important pathogens can infect soybean seeds. Seed infection often results in poor quality (i.e., low germination, vigor, yield, or a combination of these). The disease most commonly damaging to soybean seed quality is pod and stem blight. To reduce losses, soybean growers should plant only seed that has a germination rate greater than 70 percent in a cold germination test. The cold germination test is a better indicator of seed quality than the standard warm germination test. All seed lots should be tested before planting to assure good stands of vigorous seedlings with a potential for high yield. Plump seed, free from disease-causing organisms and cracks, is more likely to produce vigorous stands and sustain fewer losses from seed rot and seedling blight fungi. In general, seed-rotting and seedling blight fungi cause severe problems only where diseased, cracked, or low-vigor seed is used or where seedbed conditions do not favor rapid germination and emergence. The use of seed-treatment fungicides to increase germination of poor-quality seed is not recommended. The productivity of stands of poor-quality seed, even if properly treated, is not equal to that of stands of nontreated, high-quality seed. High-quality seed, produced in fields with a low incidence of disease, should be harvested as soon as it is mature, and handled carefully to prevent mechanical damage. Seed treatment fungicides have no effect on damaged seed. CROP ROTATION AND TILLAGE Crop rotation and clean tillage can be important practices in controlling diseases since many soybean varieties have little or no resistance to many common diseases. Practically all of the important fungal and bacterial diseases of soybeans survive between cropping seasons on and in soybean crop debris. Few of the fungi, bacteria, and nematodes that attack soybeans infect other crop plants. Therefore, when soybean crop residue is removed or thoroughly decayed and/or rotations with corn, sorghum, small grains, or forages are used, the disease-causing organisms lack a host on which to feed and reproduce, and eventually die. Crop rotation and tillage programs that promote soybean residue decomposition before the next crop of soybeans is planted will help reduce diseases such as pod and stem blight, anthracnose, stem canker, Alternaria leaf spot, Phyllosticta leaf spot, powdery and downy mildew, bacterial blight, bacterial pustule, Septoria brown spot, Cercospora leaf blight (purple seed stain), brown stem rot, Sclerotinia white mold and several other fungal and bacterial leaf diseases. Soybean cyst nematode populations can be reduced up to 50 percent for each year an alternate crop is planted.
-3- Producing soybeans with practices that do not allow crop residue decomposition may result in an increase in certain diseases. In these situations it is important to use all other available disease-control practices. These include planting resistant varieties, rotating to nonhost crops, scouting fields to determine the presence of diseases, using pesticides when needed, and planting high-quality, certified seed. Tillage practices that increase soil compaction may increase the severity of diseases such as Pythium and Phytophthora root rot and brown stem rot. ROW WIDTH Little experimental data exists on the effect of row width on disease incidence. However, as row width narrows, higher humidities and slower drying conditions are encountered. Therefore, as row width narrows, diseases such as powdery mildew, downy mildew, pod and stem blight, stem canker, and anthracnose, Sclerotinia white mold, and Cercospora purple seed stain and leaf blight may increase in severity. Research on Septoria brown spot has shown no difference in that disease's development as a result of row width. Charcoal root rot impact will be less severe where narrow rows are used with early planting because of the rapid canopy development and resultant lower soil temperatures and higher soil moisture levels. DATE OF PLANTING Date of planting can have a profound effect on development of certain soybean diseases. For example, late planting increases losses to charcoal root rot and soybean cyst nematode. Losses to charcoal root rot increase rapidly as soil temperatures increase. Early planted beans, however, appear more susceptible to sudden death syndrome. Planting into cold soils (less than 55 to 60 F) can delay emergence and increase seed decay and seedling blight. Soybean seeds may swell at lower temperatures but will not germinate, giving pathogens an extended opportunity to attack the seed. The period of high susceptibility to these diseases is from time of planting until the plant has 2 to 3 trifoliate leaves. The longer this period, the more severe these diseases may become. Planting high-quality seed in a warm (60 F or more), moist, well-drained, and fertile seedbed at the proper depth and spacing will ensure stands of vigorous, high-yielding seedlings. FERTILITY Adequate, balanced fertility can be important in reducing disease losses. Inadequate phosphorus or potassium can increase losses from soybean cyst nematode, charcoal rot, other root rots, and pod and stem blight. Vigorous plants are more tolerant of pathogens and are better able to produce an almost normal yield despite diseases. DISEASE SCOUTING AND MONITORING Scouting and monitoring soybeans is an important practice in long-term disease management. Scouting fields is especially helpful since diseases will probably be important only where they were present in recent years. Accurate surveys and diagnoses of problems through scouting thus permit more effective use of controls in succeeding crops, and early detection can improve the chances of reducing disease losses.
-4- Begin scouting your fields early in the season to detect areas that may cause problems as the crop matures. Low or flooded areas, and any part of the field where plants appear to be weak or to lack vigor should be checked more frequently. Many soybean diseases begin as circular areas in fields where the plants appear chlorotic or weakened. These areas often develop into disease centers as the plants mature. Periodic scouting of five areas of each field should begin in the seedling stage and continue throughout the season. Use the tables, maps, and other resources listed earlier to help you identify the important diseases, their relative risks, and their appearances. SPECIFIC SCOUTING PROCEDURES FOR SOYBEAN CYST NEMATODE RACE DETERMINATION Experienced personnel can determine the most prevalent race present in a field by planting one or several small areas (two to three rows, 5 to 10 feet long in areas of the field showing damage) to a Race 3- resistant variety, to a Race 3- and 4-resistant variety, and to a susceptible variety normally used by the grower. If no cysts appear on the roots of the Race 3- and Race 4-resistant varieties after six to eight weeks but cysts can be seen on the roots of the susceptible variety, the predominant race is 3. If cysts are obvious on the roots of the susceptible and the Race 3-resistant variety, Races 3 and 4 are both present. If cysts are obvious on all varieties, a race other than Race 3 or 4 is present. SOIL ANALYSIS Soil analysis for soybean cyst nematodes should always be done before a susceptible variety is planted. At present, a soil population below 5 mature cysts or 150 eggs and larvae per 100 cubic centimeters of soil is considered the threshold below which a susceptible variety can be planted without significant damage. Growers who want to plant a susceptible variety as soon as possible in the rotation should sample each nonhost crop to determine the remaining population. If populations increase after cropping to a resistant variety, the buildup of a new race should be suspected. Soil samples are collected to a depth of six inches from the field in question in the fall. Ideally, approximately 10 subsamples per five acres should be taken. Subsamples should be mixed and a composite sample should be sent to the University of Illinois Plant Clinic, 1401 W. St. Mary's Rd., Urbana, IL 61802, or to another designated soybean testing laboratory, for analysis. A nominal charge will be made for processing samples. FUNGICIDES SEED TREATMENTS Fungicide seed treatments will generally improve stands regardless of quality. However, the greatest benefits will be found (1) where low seeding rates are used; (2) where seed that is of poor quality because of low rates of fungal infection must be used; and (3) where seed is planted in a seedbed in which delays in germination or emergence are likely (reduced till or no-till fields). Fungicide seed treatments are not a substitute for high-quality seed and will not improve the performance of seed that is of low quality due to mechanical damage or physiological factors. Treated seed of low
-5- quality will not produce stands and/or yields equal to untreated high-quality seed. Therefore, only highquality seed should be considered for planting. A fungicide seed treatment should be used on seed planted to produce seed. Research has shown that fungicide-treated seeds produce seeds that have a lower incidence of pod and stem blight infection. Fungicide seed treatments will not improve germination of seed that is of low quality because of mechanical damage or physiological factors. There are many excellent seed treatment fungicides available. Table 2 (p. 8) is designed to assist in determining the need for seed treatments, to control Pythium and Phytophthora. Selection of the proper seed treatment is very important because of the specificity of certain fungicides for controlling only Phytophthora and Pythium. FOLIAR TREATMENTS Foliar fungicide treatments may reduce losses from Septoria brown spot, Cercospora leaf blight (purple seed stain), anthracnose, pod and stem blight, and stem canker. These diseases are most damaging when the weather is warm (70 to 80 F) and wet from early pod fill to maturity or when harvest is delayed. Foliar sprays of fungicides may increase yields 10 to 15 percent, increase seed quality, and reduce disease losses when such fields are planted to soybeans the following year. The use of fungicides should be based on expected disease severity. The six diseases just listed will not be as severe in cool, dry seasons and where adequate rotations have been used. The checklist in Table 3 (p. 9), can be used at early bloom to determine whether fungicide controls for the six diseases mentioned previously should be considered. A key factor in this checklist is the presence of black specks (pycnidia) on fallen petioles. Only brown, fallen petioles should be assayed, and more than two-thirds to three-fourths of these petioles should show pycnidia. If growers use the checklist and apply fungicides correctly, maximum benefits should be achieved. Less than optimal benefits will be achieved if fungicides are applied incorrectly or if disease severity does not warrant spraying. FUNGICIDE APPLICATION At present, aircraft are the best vehicles for applying fungicides to agronomic crops. Some aircraft may not be equipped or calibrated to do this job. It is therefore important to select an aerial applicator who is familiar with disease control and whose aircraft has been properly calibrated for uniform, thorough coverage of all aboveground plant parts. With the equipment now available, a reasonable job of applying fungicides requires a minimum of 5 gallons of water carrier per acre. Superior coverage may be obtained with more water, but the cost may be prohibitive. Conversely, a lower volume (under 3 to 4 gallons per acre) gives correspondingly poorer control. Five gallons of water can be applied uniformly using approximately 30 to 70 properly spaced nozzles, depending on the aircraft. The nozzles should be D-8 to D-12, hollow cone, with No. 45 or No. 46 cores. The final decision on nozzle number, size, swath width, and placement depends on the air speed, pressure, and volume desired. Droplet size is also important. Ideally, droplets should be 200 to 400 microns in size for thorough and uniform coverage. NEMATICIDES Nematicides may be useful in controlling soybean cyst nematodes and other plant-parasitic nematodes. Nematicides are suggested only where (1) crop rotations are not possible, and (2) resistant varieties cannot be used.
Table 1. Soybean Diseases that reduce yields in Illinois and the relative effectiveness of various control measures Resistant High or tolerant Crop Clean seed Disease varieties rotation plowdown quality Fungicides Other controls and comments Phytophthora root rot and seedling blight 1 2 3 Numerous races of the fungus are known. Avoid poorly drained areas and soil compaction. -6- Pythium, Rhizoctonia, and Fusarium seedling blights and root rots 2 3 Plant high-quality seed in a warm (55 to 60 F), well-drained seedbed. Deep planting may reduce uniform vigorous stands. Charcoal root rot 2 3 Early planting, deep and clean plowing, balanced fertility, narrow rows, and the avoidance of moisture stress will provide some control. Avoid high seeding rates. Soybean cyst nematode 1 1 3 3 (nematicides) Early planting and the elimination of susceptible weeds will aid in control. Avoid moving contaminated soil from field to field by equipment, water, or other means. Crop rotations of three years or more are necessary even when using resistant varieties. Maintain balanced fertility and use nematicides as needed. Soil analysis should be used in decision making. Pod and stem blight, anthracnose, and stem canker Cercospora leaf blight (purple seed stain), Septoria brown spot, Frogeye leaf spot 2 2 1 2 Fungicides are suggested to aid in producing high-quality seed. Grain producers may have higher yields in warm, wet seasons. Plant full-season varieties. 1 2 2 2 2 These diseases may be more damaging in narrow-row systems.
-7- Table 1. Soybean diseases that reduce yields in Illinois and the relative effectiveness of various control measures (cont) Resistant High or tolerant Crop Clean seed Disease varieties rotation plowdown quality Fungicides Other controls and comments Bacterial blight, bacterial pustule, wildfire 1 2 2 2 Seed should not be saved from fields that are heavily infected with these diseases Downy mildew 2 2 2 2 This disease may become more important in narrow-row culture systems. Sclerotinia white mold 2 3 2? The effectiveness of fungicide sprays is unknown at this time. Powdery mildew 1 Soybean mosaic, bean pod mottle, and bud blight viruses 2 (soybean mosaic) Plant seed produced in fields with a low incidence of soybean mosaic. Damage from bud blight may be reduced by bordering soybean fields with 4 to 8 rows or more of corn or sorghum. This may be especially helpful where soybean fields border alfalfa or clover fields. Before planting, apply herbicides to kill broadleaf weeds in fencerows, ditch banks, grass pastures, etc. Brown stem rot 1 1 2 Rotations of 2 or more years are necessary for control. Soybeans planted as end rows on corn fields aid in carrying over the disease. Early-maturing varieties are generally less affected than late maturing varieties. Sudden death syndrome See comments for soybean cyst nematode. Early planted or early maturing varieties appear to be more susceptible. 1 = highly effective control measures; 2 = moderately effective; 3 = slightly effective.
-8- Table 2 Soybean seed treatment checklist for reducing early season stand losses due to damping-off from pythium and phytophthora fungi Point value if Risk factors answer is yes Rainfall for the 7-day period before planting was: Below normal... 2 Normal... 1 Above normal... 4 Seedbed preparation was: Conventional tillage... 1 Rough surface (conservation tillage)... 2 No-till... 4 Germination at time of planting is less than 85 percent in a warm test or less than 70 percent in a cold test (such seed should be discarded if at all possible)... 3 Previous soybean stand in field was reduced by damping-off or Phytophthora root rot... 4 Level of resistance to Phytophthora root rot is: Susceptible... 2 Tolerant... 4 Resistant to one or more races... 1 Expected rainfall for 96 hours following planting is: Lower than normal... 1 Normal... 1 Above normal... 3 Low areas of field remain flooded for 48 hours following 1 inch of rainfall... 4 Seeding rate is less than 55 pounds per acre... 3 Field is planted to double-crop soybeans... 3 TOTAL POINTS AND SUGGESTIONS Less than 7 points: seed treatment will probably not be beneficial. 7-15 points: seed treatment may be beneficial if weather conditions do not favor rapid germination and growth. More than 15 points: seed treatment will be beneficial to stand development.
Table 3. Checklist to determine whether foliar fungicide application should be Made to soybeans -9- Point value if Risk factors answer is yes Rainfall, dew, and humidity up to early bloom and pod set are: Below normal... 0 Normal... 2 Above normal... 4 Soybeans were grown in the field last year... 2-3 Chisel-plow, disk, or no-till was used... 1 Pycnidia (black specks) are visible on fallen petioles, and Septoria brown spot is obvious on the lower leaves... 2 Early maturing variety (not full-season)... 1-2 Soybeans are to be used or sold for seed... 6 Yield potential is better than 35 bushels per acre... 2 Seed quality at planting time is less than 85 percent germination in a warm test... 1 Other conditions that favor disease development (weather forecast with a 30-day period of greater-than-normal rainfall and a field history of disease)... 1-3 NOTE: If the total point value is 15 or more, application will probably mean increased yields and higher seed quality.
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