Diseases
Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
Diseases
Disease management of soybean is best achieved through proper rotation, balanced fertility, and the use of certified seed to reduce potential issues in the field. In Pennsylvania, many foliar diseases of soybean do not cause an economic impact. With that said, there have been severe epidemics of frog-eye leaf spot in fields where soybeans followed soybeans in no-till conditions. The more economically important diseases are those of the roots and stems, such as white mold, charcoal rot, and brown stem rot. These diseases cause early death of the plants, resulting in reduced pod fill and small seed size. Diseases that directly affect the seeds, such as downy mildew, purple seed stain, and Phomopsis seed decay, can reduce the size, quality, and appearance of the soybean seed. In general, the disease levels in Pennsylvania do not warrant a foliar fungicide application and an economic return from application is not likely.
Seed Treatments
The use of fungicide seed treatments is beneficial when early planting into cold, wet soils or no-till planting into heavy residue. In these conditions, the fungicide will help to protect the seed and seedling from seed decay and other early season diseases. In early planted soybeans, the successful use of fungicides may take precedence over thoroughly inoculating the seed with nitrogen-fixing bacteria. As the soil warms, proper inoculation should be considered more beneficial than fungicide seed treatments due to the reduced threat of seedling diseases and less time the seed is in the soil before emergence.
Similar to corn seed treatments, it is helpful to mix modes of actions to increase the protection on the seed. Dynasty (azoxystrobin) is labeled to control Rhizoctonia, Fusarium, and Phomopsis, but not Pythium or Phytophthora. Dynasty plus ApronMaxx RFC (fludioxonil and mefenoxam) then complement each other with ApronMaxx RFC providing control for Pythium and Phytophthora. In cold, wet soils, Pythium and Phytophthora will likely cause the most damage. Allegiance (metalaxyl) is another product similar to Apron, which is labeled to control Pythium and Phytophthora, but poor control of other seed rots. In order to maintain good protection, SoyGard (azoxystrobin and metalaxyl), a premix of Allegiance and Dynasty, is labeled to control all the major seed and seedling pathogens. Trilex 2000 (trifloxystrobin and metalaxyl), a mixture of Trilex and Allegiance, is labeled to control many of the seed and seedling diseases.
Regardless of fungicide treatment or if the soybeans purchased are pre-inoculated, it is always good insurance to add dry inoculants to the soybeans prior to planting. Always remember that the inoculant is a living organism and should be kept cool and in the dark (refrigerator) until ready to use. In some studies, fungicide treated seed has been found to reduce the number of live bacteria in the inoculant. In fungicide treated seed, it is best to apply dry inoculant, mix thoroughly, and plant the seeds within 4 hours to maximize the chance of nodulation.
Early Season Disease
Brown spot, also known as Septoria leaf spot, is a foliar disease identified as small, angular brown spots that begin on the lower leaves. This disease is dispersed mainly by rain splash, and moves up the plant with precipitation. Under severe conditions, brown spot can lead to rapid defoliation of the soybean plant. These outbreaks commonly occur after prolonged periods of rain and/or high relative humidity. The disease can be controlled easily with fungicides; however, under Pennsylvania growing conditions, disease levels warranting control are rarely observed. Soybean varieties resistant to brown spot are not available. Control recommendations include plowing under crop residues soon after harvest, planting disease-free seed, and rotation to other crops.
Brown stem rot has been increasing in occurrence Pennsylvania. Losses have not been determined, but those in other areas reportedly have been as high as 25 percent. Losses occur because of reduced pod fill, small seed size, and lodging of infected plants. External symptoms usually are not visible until late in the growing season. The lower portion of the diseased stems turns a dull brown color, followed by rapid dying of the leaves. The fungus responsible for this disease overwinters in the soil and in soybean debris. Severity appears to be greatest when cool weather occurs during pod fill followed by hot, dry weather. Rotation to a non-host crop is a recommended control method. Genetic resistance has been identified, but the level of resistance available in varieties recommended for Pennsylvania is unclear.
Phytophthora Root and Stem Rot is able to infect soybean at any growth stage. The use of fungicide treated soybean seed should help to reduce the early season losses. Typically symptoms start at the soil surface as black lesions that develop up the stem. The lesion can then girdle the stem, causing wilt-like symptoms and premature death. This disease is transported in water, and often found in areas of poor drainage such as highly compacted head rows or standing water in the field.
