Mid-Season Disease
Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
DISEASES
Mid-Season Disease
Anthracnose can infect at any time during development, but symptoms typically appear during the early reproductive stages as irregularly shaped brown areas on the stems, petioles, and pods. Infected leaves often have reddish veins, leaf rolling, and premature defoliation. This disease may also girdle petioles, leading to premature defoliation. Pods that are affected may not produce seeds, produce smaller, brown, moldy, shriveled seeds, or show no symptoms at all. Infected seeds often do not germinate, or they die shortly after germinating with symptoms similar to damping-off. Typical signs of the disease are seen after maturation as dark specks covering stems, pods, and petioles. The causal agent of this disease survives in infected plant debris and seeds.
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 turgor 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. Charcoal rot can also cause corn stalk rot, so a corn-soy rotation will not be effective in reducing inoculum. The disease can also infect alfalfa and clover; therefore, if rotating to forage crop choose a grass rather than a legume.
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 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.
Soybean vein necrosis virus (SVNV) was confirmed in Pennsylvania in July 2012. Symptoms of the disease are mottled yellow discoloration radiating from main veins, often in younger leaves. These yellowed areas often turn reddish brown to brown or necrotic with time. This virus is transmitted by thrips, a tiny, slender-bodied insect about 1/32–1/5 inch long. Young thrips are often yellow and later have dark patches and fringed wings as adults. Thrips are very numerous in soybean fields and considered to have little economic impact, except transmitting this disease. This disease has been found to be widespread across Pennsylvania fields, and no yield loss data have been compiled at this time.
White mold also known as Sclerotinia stem rot, has caused localized outbreaks in Pennsylvania and poses a serious threat to soybean production in the Commonwealth. Fields near river bottoms where the environmental conditions (fog and extended periods of high humidity) 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 may exceed 50 percent.
Wilting of the plants is often the first indication that white mold is developing within 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. Further information and color photos can be found at http://pubs.cas.psu.edu/FreePubs/pdfs/xl0087.pdf
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 severe white mold. When considering varieties with unknown white mold resistance, select a taller growing variety with medium canopy width to help reduce the humidity within the canopy. 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 of 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.
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. Since the sclerotia (reproductive structure) is about the size of soybeans, fields with severe white mold and sections of fields that are heavily diseased should be harvested last to avoid spreading 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. Although many fungicides are labeled to control white mold, there has been no effective way of applying fungicides to the flowers. The use of the herbicide lactofen has been shown to reduce white mold but causes severe burning to the soybean canopy. No consistently effective chemical control is currently available.
