Late-Season Disease
Part 2, Section 4: Soybean Pest Management
Soybean Pest Management
DISEASES
Late-Season Disease
Brown stem rot (BSR) has been increasing in occurrence in Pennsylvania. Although infection often occurs early in the season, foliar symptoms of BSR often appear after pod set and include interveinal chlorosis and necrosis, leading to leaf rolling and leaf death. These symptoms are similar to sudden death syndrome and stem canker. The easiest way to diagnose this disease is by digging up the roots and splitting the stem from the crown up the stem to look for the characteristic browning often evident in the pith, or center of the stem. The fungus responsible for this disease overwinters in the soil and in soybean debris. Losses in Pennsylvania have not been determined, but those in other states reportedly have been as high as 25 percent. Losses occur because of reduced pod fill, small seed size, and lodging of infected plants. Severity appears to be greatest when cool weather occurs during pod fill followed by hot, dry weather. Control measures include using a BSR-resistant variety and proper crop rotation to corn, small grains, or forage legumes to reduce pathogen levels.
Sudden death syndrome (SDS) has not been a widespread issue in Pennsylvania, but there have been several isolated incidences. SDS, at onset, looks very similar to brown stem rot, beginning as interveinal chlorosis then necrosis of the upper leaves after flowering. Affected plants are often in wet or compacted areas of the field, and these areas may enlarge over time. To distinguish SDS from BSR, it is important to dig up the roots of affected plants. SDS is often associated with root rots, and light blue spore patches may be present near the soil line. Once dug up, split the stem from crown to a couple inches above the soil line. SDS will have a white pith, and BSR will have a brown pith and usually no root rot. Since both diseases cause premature death of the plants, the effect is often reduced seed size, reduced pod fill, and lodging of affected plants. The causal agent is a soilborne fungus and will remain in the soil. Best management practices include avoiding early plantings into cold soils, alleviating compacted and/or wet areas in the field, and using proper crop rotation. Seed treatments, in-furrow fungicide applications, and foliar applications of fungicides have had no significant effect in reducing the disease.
Diaporthe-phomopsis complex consists of stem blight, stem canker, and seed decay often found as one or more symptoms on affected plants. Stem blight is found on mature stems as linear rows of black specks on the stems and random black specks on the pods. Stem canker often begins during the early reproductive stages on the lower leaf nodes as small, reddish-brown lesions that expand often girdling the stems, causing wilting and plant death. The leaves remain attached after plant death. Phomopsis seed decay causes infected seeds to be shriveled, elongated, and cracked with a white, chalky appearance. Although infection can occur at any time after pod set, symptoms are not evident until nearing maturity. In fields with prior history, harvest as soon as possible because seeds throughout the plant may be affected under wet conditions. Control measures should include crop rotation and prompt harvest. Fungicide applications have not been consistent in controlling these diseases.
Seed Diseases
Phomopsis seed decay and Purple seed stain are two common seed diseases in Pennsylvania. These diseases can result in significant reduction in grain quality and yield losses when environmental conditions favor disease development. Losses are most significant when harvest is delayed by wet weather.
Cercospora Leaf Blight/Purple seed stain is one pathogen that exists as 2 very different symptoms within soybean. Cercospora leaf blight appears as leathery, bronzed leaves near the top of the soybean canopy and can be easily confused with ozone damage. Purple seed stain is easily distinguished by the pronounced purple to pink discoloration of infected seed. The discoloration may occur as specks or as large irregular blotches on the surface of the seed. In some cases, the entire seed coat may be discolored. Seed with purple seed stain may have reduced germination, and seedling diseases may also result from the seed infections.
Management of Seed Diseases
The fungi that cause Phomopsis seed decay and purple seed stain survive in crop residues left on the soil surface. Producers can reduce their risk of disease by using certified seed and crop rotation to limit exposure to the inoculum source. A one- to two-year rotation to corn or small grains should be sufficient to reduce the pathogen population in most fields. Tillage can also be used to hasten decomposition of crop residues, but these practices may have negative impacts on soil erosion.
