With the extremely hot temperatures and relatively dry conditions that are forecast for much of the state over the next 7 to 14 days, there may be calls and questions about wilting soybean plants. With the extended period of wet conditions early in the season, many plants may have shallow root systems. These plants may be more prone to wilting under hot, dry conditions. The early wet conditions may also have been favorable for the development of root rot diseases such as Phytophthora root rot, Rhizoctonia root rot and Fusarium root rot. These root rot diseases may have weakened root systems, damaging roots and stem tissue. Again, these diseased plants would be more prone to wilting and even premature death if stressed by hot, dry conditions.
In the field it may be possible to distinguish between Phytophthora root rot, Fusarium root and tap rot and Rhizoctonia root rot if plants are carefully dug up and the soil gently removed from the root system. These diseases may occur alone but can also occur in combination which makes field diagnosis more difficult. At this point in the season there are no management options for these root rot diseases. Foliar fungicides will not control Phytophthora root rot, Fusarium root rot or Rhizoctonia root rot. Moderate and consistent weather conditions might help alleviate the stress on plants and keep some plants from dying but continued high temperatures or fluctuations in weather conditions may lead to more wilting and death of plants.
Phytophthora can cause seed rot, preemergence damping-off and early postemergence damping-off as well as late-season Phytophthora. Phytophthora seedling blight causes established seedlings to turn yellow, wilt and die. Generally the entire seedling is affected and roots may be poorly developed and rotted. As plants begin to flower and set pods, symptoms of late-season Phytophthora root and stem rot may develop. Infected older plants may show reduced vigor through the growing season, die gradually over the season or, as this year, die quite rapidly. Lower leaves may show a yellowing between the veins and along the margins. Upper leaves may yellow. Or the entire plant may have a yellow, off-color cast. The stems typically show a characteristic brown discoloration that extends from below the soil line upward. Eventually the brown discoloration may extend out several inches on the lower side branches of the plant. Entire plants may wilt and die. Withered leaves tend to remain attached even after the plant dies. Phytophthora root rot is more likely to occur in heavy, wet soils, low areas or compacted areas, but it may occur in light soils or better drained areas if heavy rains occur.
Rhizoctonia can cause seedling blight and root rot of soybean. Affected stands may have an uneven appearance and seedlings appear pale green in color and stunted in growth. The identifying feature of this disease is a small, reddish lesion on one side of the stem at or just below the soil line. This lesion develops into a sunken, cankered area at the point of infection. Sometimes the lesion will expand to completely girdle the stem. On severely infected seedlings, the entire hypocotyl may be discolored and shriveled into a dry, stringy or wiry stem. Rhizoctonia can also cause a root rot of older plants. On older plants the lower leaves may begin to yellow. The yellowing may be from the margin in resembling symptoms of potassium deficiency or may be a more general yellowing. Plants may be stunted and appear less vigorous than adjacent plants. When plants are removed from the soil, the root system may be poorly developed, lateral roots may be discolored or rotted and the stem may have a brick red discoloration beginning at the soil line and extending in either direction from the soil line. If plants are stressed by hot, dry conditions, severely infected plants may die. If cool, wet conditions occur after plants are infected with Rhizoctonia root rot, a flush of secondary roots may develop above the diseased portion of the stem. These plants may survive but are likely to remain stunted for the rest of the season.
Fusarium can also cause root rot of soybean. Infection is usually confined to roots and lower stems. The lower part of the taproot and the lateral root system may be discolored, deteriorated or completely destroyed. Older plants may be stunted, have an off-color or yellow cast and appear less vigorous than adjacent plants. When plants are removed from the soil, the taproot will show varying degrees of discoloration and deterioration. Discoloration may range from brown to purple-brown to almost black. The taproot and lower stem may show distinct lesions or may be rotted completely through. If the taproot is rotted through, it may break if the plant is pulled from the ground. If plants are stressed by hot, dry conditions, severely infected plants may die.
Once the crop has been planted, there is little that can be done to reduce incidence or severity of soybean root rot diseases. Additional stress from poor growing conditions, herbicide injury or other factors may compound problems with soybean root rot diseases. Prior to planting it is important to consider variety selection (especially in fields with a history of Phytophthora, fungicide seed treatment, crop rotation, seedbed preparation and conditions at planting.
REVISED: September 26, 2011