Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
Diseases
Fungicide seed treatment is especially important for beans planted early when soils are cold and wet. Under these conditions, seed decay and seedling diseases are major threats to plant populations as well as to ultimate plant vigor and yields. In early plantings, the fungicide treatment may have to take precedence over inoculation with nitrogen-fixing bacteria. In later plantings in warm moist soils, seedling diseases are less of a threat, so the successful use of nitrogen-fixing bacteria is the major concern.
If possible, do not add inoculant to seed freshly treated with fungicide. If a fungicide seed treatment is to be used, it should be applied to the seed several days before inoculant is added. Several specially prepared combinations of inoculant and fungicide are commercially available.
For growers who have problems with Pythium damping-off or early-season Phytophthora, a fungicide treatment of metalaxyl (Apron or Apron XL LS) may improve plant stands and yields, even with Phytophthora-resistant varieties. This treatment is labeled for use by commercial seed treaters only.
Seed diseases have become a major problem in Pennsylvania. When purple stain or pod and stem blight (moldy bean) become a problem, crop rotation, clean seed, and use of a recommended variety are essential.
Early Season Disease
Brown spot. Also known as Septoria leaf spot, brown spot is a foliar disease that under severe conditions 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 the disease 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. The occurrence of this disease appears to be increasing in Pennsylvania. Losses have not been determined, but those in other areas reportedly have been as high as 25 percent. Losses occur because of reduced numbers and size of the seed 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 nonsusceptible crop is a recommended control method. Genetic resistance has been identified, but the level of resistance available in varieties recommended for Pennsylvania is unclear.