Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
DISEASES
Mid-Season Disease
Charcoal Rot. Charcoal rot, also known as summer wilt, can occur during periods of hot, dry weather, which causes plant stress. Plants affected with charcoal rot often appear wilted or drought stressed but do not regain growth after rain. These plants often seem to have accelerated maturity and are easily found when the field is near R6-R7 since these patches have already lost their leaves. The disease causes a reddish brown discoloration of the taproot and progresses up the stem producing small, black microsclerotia (reproductive structures) that resemble charcoal dust on the stem. The microsclerotia can survive in the soil for 2 years or longer and the fungus can survive in the seed coat for up to 3 years. The symptoms are similar to white mold without the white, cottony growth and large sclerotia. The best control measures include using certified, disease-free seed and crop rotation. In a field with a history of charcoal rot, rotate to a cereal crop for 2 years. The disease can infect alfalfa and clover; therfore, if rotating to a forage crop choose a grass rather than a legume.
Frog-eye leaf spot. Early frog-eye leaf spot is characterized by dark brown spots surrounded by a red or brown margin. As the lesions age, the margins maintain their brown color, with a gray to white center, which may contain minute dark spots. This is a very common disease with usually little economic impact. In some situations, such as soybean no-till planted into soybean stubble, the disease can be severe and cause defoliation. The best method of control is crop rotation.
Downy mildew. Downy mildew usually starts as small, irregular-shaped yellow lesions on the leaf surface. On the underneath of the leaf a small gray or brown tuft is evident as the lesions age. In some situations, downy mildew will infect the pod and cause a wrinkled, discolored seed. This disease is relatively common in Pennsylvania with little known economic impact.
White mold. Localized outbreaks of white mold, also known as Sclerotinia stem rot, are common in Pennsylvania; disease is a serious threat to soybean production in the Commonwealth. Fields near river bottoms where the environmental conditions frequently favor disease development are most at risk for white mold. Average yield losses attributed to these epidemics are estimated to be between 10 and 20 percent. In severely diseased fields, the yield losses exceeded 50 percent.
Wilting of the plants is often the first indication that white mold is developing with a soybean canopy. Upon inspection, diseased plants are found to have stem lesions and signs of white, cottony mold growth. As the disease progresses, the leaves and stems of infected plants turn brown. Hard black structures known as sclerotia are often present on and within infected stem and pod tissue.
White Mold Management
White mold has proved to be a difficult disease to manage in Pennsylvania and many soybean production regions. Producers can reduce the risk of severe white mold by following the recommendations below.
Variety Selection. High levels of resistance to white mold are not currently available; however, soybean varieties do differ in response to white mold. Soybeans that are less susceptible to white mold are available for some maturity groups, and these varieties can effectively reduce the risk of serve white mold. When considering varieties with unknown white mold resistance, try to select a taller growing variety that with medium canopy width. Avoid planting fields with a history of white mold with highly susceptible varieties.
Crop Rotation. Each year a portion of the pathogen population will die because of adverse weather conditions or the activities of other organisms. Rotation to a nonhost crop such and corn, wheat, or other grass will provide time for the fungal population to drop and reduce the risk for severe disease. Fields with 5 to 25 percent disease should not be planted to soybean for at least 1 year. Fields with 26 to 50 percent disease should not have soybeans for 2 years, and fields with greater than 50 percent disease should be rotated away from soybeans for at 2 to 3 years.
Tillage. Tillage practices have been shown to influence the survival of the fungus. Deep plowing a field can be used to effectively bury sclerotia deep enough that they can no longer germinate and release spores. However, buried sclerotia are known to survive longer than those left on the soil surface, and subsequent plowing can bring them back to near the soil surface. In contrast, minimum tillage and no-till practices favor the natural decline of the fungal populations in the absence of a susceptible crop.
Canopy Management. Management practices that favor rapid canopy closure can result in an environment that is favorable for disease development. Increase the row spacing to 15 or 30 inches and keep plant populations less than 150,000 plants per acre when planting soybean into a fields that have had greater than 25 percent diseased plants.
Weed Control. Many broad leaf weeds can also be hosts of the fungus. Not controlling these weeds can allow the spread of the disease within a field and may reduce the effectiveness of the crop rotation.
Adjust Harvest Priorities. Fields with severe white mold and sections of fields that are heavily diseased should be harvested last to avoid the spread of the fungus within and between fields. Thoroughly clean all harvesting equipment before moving from diseased fields to locations where white mold is absent.
Chemical control. White mold typically infects through the flowers after fertilization. There has been no effective way of applying fungicides to the flowers. The use of the herbicide latofen has been shown to reduce white mold but causes severe burning to the soybean canopy. No consistently effective chemical control is currently available.