Soybean Disease Atlas

2^nd Edition

Southern Soybean Disease Workers

CONTENTS

Soybeans continue to occupy large acreages of land in the southern United States. Soybean acreage for 1987 was 19,000,000 acres. However soybean production has its problems, the most serious being diseases. Recognition and control of these diseases is vital. In 1987, the average loss from all pathogens was 12.8 percent, or 75.3 million bushels. At the average price of $5.50 per bushel, this equals a 414.3 million dollar loss.

This publication has been prepared by the Southern Soybean Disease Workers to assist producers in proper soybean disease identification and to create awareness of the importance of soybean diseases. The intended audience of this publication is the southern soybean producer, so only diseases which are common in the southern region of the United States are presented. The information for the diseases presented is not exhaustive. More complete information is available in the American Phytopathological Society's Compendium of Soybean Diseases.

SEEDLING DISEASES

Fig. 1

Fig. 2

(Pythium, Rhizoctonia, Fusarium, and Phomopsis)

Seedling diseases are caused by a group of fungi, acting independently or together, that cause similar symptoms on young plants. These fungi occur wherever soybeans are grown, and are primarily seed-borne, although they also exist on debris or organic material in the soil. Damage may occur prior to germination, as the seedling emerges, or after the seedling is established, depending on which fungus is involved. Moderate to severe stand losses result in reduced yields and inefficient utilization of fertilizer or pesticides.

Early planting when soils are cool favors seedling disease. The cool temperatures slow seed germination and growth, and favors the growth of any seedling disease fungi. Any factor that delays germination and hypocotyl emergence results in a longer infection period and a higher incidence of seedling disease. Factors which retard seedling growth include poor seed quality, improper planting depths, poor seedbed preparation, and high rates of herbicides.

Characteristics of seedling disease include inadequate stands, lack of uniform stands, and death of young seedlings (Fig. 1). The presence of dark brown or reddish lesions on the stem or lower main root is evidence of seedling disease (Fig. 2). Seedling roots are often blackened and decayed. Other symptoms include lesions on the cotyledons, shriveled cotyledons, water soaked primary leaves, or soft young stems. Microscopic examination of the damaged seedling is necessary to identify the specific pathogen involved.

Seedling disease can be avoided by delaying planting until soil temperatures increase (above 20^oC), planting high quality seed, and utilizing an effective fungicide seed treatment. Seed treatments are relatively inexpensive and in most cases insures a good stand.

SAMPLE COLLECTING FOR DIAGNOSIS

Plant Sampling

Disease diagnosis is difficult because symptoms of some diseases are very similar, and symptoms vary depending on environmental conditions and plant growth stage. As a result a second opinion from someone experienced with soybean diseases may be beneficial. Unless this individual can visit the site, you may have to collect and submit high quality specimens. Following are some suggestions for collecting plant samples.

Disease samples should include both diseased and healthy plant tissue for comparison purposes. Suspect plants which are already dead are generally of little value. Whole plants are best because leaf symptoms may be caused by damage to the stem or roots. After the sample is collected it should be protected from deterioration and taken to the professional as soon as possible. Place the sample in a plastic bag and store in an ice chest or refrigerator to prevent drying. Do not place the specimen in direct sunlight or near extreme heat.

Finally submit pertinent information regarding cultural practices, pesticide applications, and cultivar. This information will be useful to the pathologist in disease diagnosis.

Soil Sampling

Soil samples are helpful in diagnosing a nematode problem or assessing potential problems. Before sampling, contact the local county agent or extension specialist to determine the procedure recommended for your area. The following is a general procedure which may be used.

When collecting soil samples for the purpose of determining the kinds and levels of nematodes present, it is best to divide the field into units of five acres. A total of ten soil cores from a depth of six to eight inches, containing soil and embedded root fragments, should be collected in a systematic manner from each of ten uniformly situated sampling blocks. The soil cores then are mixed thoroughly and a 1-2 pound subsample is bagged, labeled and submitted for analysis. When the objective of the sampling is to determine whether or not nematodes are responsible for damage in localized areas, separate samples are collected from adjacent symptomatic and non-symptomatic areas.

The time of year to sample soil is important. With field crops the best time to take samples is in the fall immediately after harvest. Nematode levels are generally highest at this time of year, and it is easiest to determine if a potential nematode problem exists for the following year. However, in some areas nematodes survive the winter, and may be sampled in the spring prior to planting. Consult with the local county agent or extension specialist for the best time to sample.

Nematodes are sensitive to heat and cold extremes, and to excessive drying. Keep the sample out of direct sunlight and store in a cool place until it can be processed for analysis. Samples should be processed as soon as possible.

DISEASE MANAGEMENT PRINCIPLES

Management of soybean diseases is based upon prevention. The following are some disease management suggestions that should be considered. Since management strategies vary from location to location, a local specialist should be consulted for specific strategies, and for cultivar and pesticide recommendations.

- Plant high quality, preferably certified seed. High quality, certified seed reduces the possibility of introducing pathogens into a field, and also produces vigorous seedlings that sustain less seed decay and seedling disease.

-Apply fungicide seed treatment. Fungicide seed treatments protect seed and seedlings from seed-borne and soil-borne pathogens. Seed treatments are inexpensive and effective.

-Use proper seed bed preparation, planting depth, and seeding rates. This will promote rapid seedling emergence and vigorous seedling growth, and help the seedling escape seed decay and seedling disease.

- Practice crop rotation with non-legume crops. Many pathogens survive between cropping seasons on crop debris. Continuous culture allows the pathogens to perpetuate and multiply. Rotation will reduce the survival and increase of pathogens within the field.

-Use deep plowing to bury plant debris. Pathogens survive between planting seasons on plant debris. Deep plowing will physically remove debris from the plant and also hastens decay. As the debris decays, the pathogens will die out.

-Plant disease resistant cultivars. Plant resistance is the most efficient and least expensive management practice. However, resistance to all known diseases is not available, and resistance may not last forever. Pathogens sometimes develop new races which overcome plant resistance.

-Apply nematicides. Although several effective nematicides are available for soybeans, their cost often makes them prohibitive. Apply at proper rates and follow label instructions.

-Use foliar fungicides. When disease pressure is high, fungicides are effective and profitable. Benefits include increased yield and improved seed quality. Apply at proper times and rates following label instructions.

-Practice good management. This includes good drainage, fertilization, irrigation, weed control, and insect management. This encourages healthy, vigorous growth that enables the plant to escape disease and be more tolerant to pathogens.

- Disease management is best accomplished using an integrated approach. This requires incorporating as many of the principles listed above that apply.

Special thanks to Kayla Mitchell for her assistance in typing the manuscript.