Diseases
Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
Diseases
Disease management of soybean is best achieved through crop 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 fungicide seed treatments 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 (fludioxonil and mefenoxam) then complement each other with ApronMaxx 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. Cruiser Maxx is an insecticide and fungicide seed treatment using thiamethoxam to provide early season protection against insect pests and ApronMaxx to provide seed and early seedling protection against Pythium and Phytophthora. Acceleron is a fungicide seed treatment using pyraclostrobin and metalaxyl to control diseases and may include imidacloprid to provide early season insect control.
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. It is important to 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 potential fornodulation.
Early Season Disease
Seed rots and seedling diseases often occur when soybeans are planted into cold, wet soils. There are several common pathogens associated with these disorders, including Pythium, Rhizoctonia, and Phytophthora. Typical symptoms include ungerminated, rotted seed or germinated seeds with discolored roots and stems. Common causes are seeds imbibing water below 50°F (the lower end of germination), improper seed-to-soil contact (not imbibing enough water to germinate), and crusting (inability of the Sheppard’s crook to emerge). Fungicide seed treatments can help alleviate some of these issues, but best management practices should include planting into a proper seed bed to ensure a good stand.
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 spread mainly by rain splash, and moves up the plant with rain events. 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 tillage to incorporate crop residues soon after harvest, planting disease-free seed, and rotation to other crops.
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. These lesions 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 areas known to have standing water in the field.
