No. 22/August 29, 1997
Brown Stem Rot
Brown stem rot (BSR) and sudden death syndrome (SDS), two mid- to late-season soybean diseases, have prompted numerous calls from throughout the state. An earlier article in the Bulletin had noted that, although there were no reports of these two diseases yet, we expected to find them if weather conditions were to change to favor the fungal pathogens. Unfortunately, that has now happened. With the recent rainy spells and a return to cooler weather, both pathogens have become active. Both cause leaf discoloration and loss of yield, depending on time of infection and the stage of crop growth. BSR is more common in northern Illinois. SDS can be found statewide, although it is most commonly seen in the southern third of the state.
Brown stem rot, caused by the fungus Phialophora gregata (synonym, Cephalosporium gregatum), enters a plant through the roots and lower stem. Losses are greatest when cool weather occurs during the pod-filling stage (late July and the first half of August), followed by hot, dry weather. Losses of 17 to 25 percent may result from lodging, premature death, or the production of fewer and smaller seeds.
Brown stem rot is difficult to recognize before pod set because it has no external symptoms. When the stems of infected plants about midseason are split longitudinally, however, a characteristic, dark reddish brown discoloration of the vascular elements and pith is evident, extending upward from the roots or crown. Occasionally during hot, dry weather in late August or early September, wilting occurs, followed by a "scorching" (browning and dying) of the leaf tissue between the veins. The leaves blight and dry rapidly. Infected plants often look "frosted." The brown stem rot fungus reduces the efficiency of the water-conducting tissues in the stem. However, leaf symptoms may vary and should not be considered in diagnosis without splitting stems.
Disease development is optimum at air temperatures of 59° to 81°F (15° to 27°C). Little or no disease develops at temperatures above 90°F (32°C). Cool weather leads to more internal stem browning.
The brown stem rot fungus survives in soybean debris and in soil to a depth of about one foot. The fungus produces spores on all types of soybean residue except pods. Infection occurs through main and lateral roots, and the pathogen moves into the lower stem early in the growing season. The fungus spreads slowly upward in the water-conducting vessels. The pathogen may plug vessels partially or completely, interfering with the flow of water and nutrients. The fungus has been reported to be seedborne, surviving as mycelium within the seed coat.
- Grow soybeans in the same field only once in 3 or 4 years. Rotate with corn, sorghum, small grains, forage grasses, legumes, or other crops.
- Plant resistant cultivars in fields where brown rot is a severe problem. Cultivars that mature early tend to escape severe infection but generally yield less than later-maturing ones in the absence of the disease.
H. Walker Kirby,Department of Crop Sciences, (217)333-8414