By Loren Giesler, Jenny Rees and Kyle Broderick
With soybeans at various reproductive stages, now is the time to watch for mid- to late-season diseases. As of June 29, the UNL Plant and Pest Diagnostic Lab in Lincoln has confirmed the following soybean diseases in the state thus far (see map for Districts):
• East District. Rhizoctonia and pythium damping off; phytophthora root and stem rot
• Southeast District. Phytophthora root and stem rot
• Southwest District. Rhizoctonia damping off; phytophthora root and stem rot
UNL Plant and Pest Diagnostic Clinic.
DISEASES OBSERVED: This map of Nebraska showing districts where the diseases are observed.
Phytophthora root and stem rot is a disease that we continue to see throughout the year. Symptoms right now include postemergence damping off where stems have a dark brown color on the exterior stem surface and lower branches. Discoloration of the stem extends from below the soil to 6 inches or more above the soil line. The taproot turns dark brown, and the entire root system may be rotted.
Leaves on older infected plants become chlorotic between the veins followed by general wilting and death, with leaves remaining attached.
While phytophthora is more common in low areas of a field, particularly in poorly drained and compacted soils, it may also occur on well-drained hillsides with heavy clay content that may be compacted. It has also been observed that dry conditions followed by heavy rain events, such as what we’ve experienced in portions of Nebraska this year, can result in higher levels of phytophthora.
For those experiencing phytophthora this year, future management includes:
• Using resistant varieties including a combination of good partial resistance and an Rps gene. Partial resistance alone will not be as effective during early growth stages or under high disease pressure.
• Cultural practices include anything that can improve soil drainage.
• Seed treatment fungicides containing mefenoxam or metalaxyl should be used.
Additional diseases to watch for as we approach pod and seed development include sudden death syndrome, brown stem rot, and sclerotinia stem rot.
Sudden death syndrome
Symptoms of sudden death syndrome include yellow or white spots on leaves in the upper canopy that eventually coalesce to form brown streaks between the veins. On those leaves, only the midvein and major lateral veins remain green. Advanced symptoms of premature defoliation rapidly progress (often less than 14 days), with petioles remaining on the plant. The plant will have a rotted tap root, and sometimes blue-gray fungal growth is present with high humidity.
Plants also tend to pull very easily due to the rotted tap root. Foliar symptoms can be confused with brown stem rot, so split open the stem to determine which is present. There will be no brown discoloration in the pith tissue with sudden death syndrome.
Management includes using more resistant varieties, use of fungicide seed treatment (ILevo), and testing your field for soybean cyst nematode as the combination of the two diseases increases potential yield loss.
Brown stem rot
Brown stem rot can be suspected in any pockets of stressed or dying plants. Foliar symptoms are not always present, but when present they will start with interveinal necrosis and spots that coalesce to form brown streaks between the leaf veins with yellow margins. These foliar symptoms look much like SDS. Therefore, it’s important to look at the stem and root symptoms.
Loren GieslerDISCOLORATION APPARENT: Brown stem rot is observed by splitting the soybean stem to see the discoloration at the stem nodes.
Plants affected by brown stem rot will have some root rot, but it won’t be as extensive as in plants with SDS. Upon splitting stems, plants with brown stem rot will have discoloration in the pith of the stem from the soil line upward. In some cases, discoloration will start at the nodes and may be visible when the stems are split. With SDS the root cortex will also be discolored and will be light-gray to brown, and may extend up the stem. Management includes use of more resistant varieties.
Sclerotinia stem rot
Symptoms of sclerotinia stem rot (white mold) are visible during pod development. Leaves will wilt and turn gray-green before turning brown, curling and dying. Infected plant parts generally have signs of the fungal pathogen as white, fluffy mycelium during humid conditions and sclerotia (dark fungal structures) on the surface of or embedded in the stem tissue. Infections will occur after flowering begins in the crop.
STEM LESIONS: With phytophthora root and stem rot of soybean, notice wilting of plants and dark stem lesions extending from soil surface up the stem.
Management for sclerotinia stem rot includes:
• using more resistant varieties (although soybeans vary in their response to Sclerotinia and this will not fully remedy the situation).
• moving to wider row spacing and avoiding irrigation during flowering (Narrow rows increase this disease as does irrigation during flowering.)
• maintaining longer crop rotations into corn and wheat (This has shown to reduce pathogen buildup and disease risk.)
• applying foliar fungicides at beginning bloom (R1) than at beginning pod (R3) to provide better control
Correct diagnosis is critical to managing any soybean disease, and samples can be sent to the UNL Plant and Pest Diagnostic Clinic.
Giesler is a professor of plant pathology, Rees is an Extension educator, and Broderick is coordinator of the Plant and Pest Diagnostic Clinic at the University of Nebraska-Lincoln.