Brown stem rot of soybean
Brown stem rot (BSR) is caused by the fungus Cadophora gregata. Characteristic foliar symptoms of BSR include chlorosis and necrosis between leaf veins, followed by leaf curling and leaf death. Leaf symptoms vary depending on soybean variety, fungal strain, and environmental conditions. In some instances, no foliar symptoms occur. Foliar symptoms can be similar to those of sudden death syndrome and stem canker and appear after early pod set. Stem symptoms usually occur prior to leaf symptoms and can occur even if foliar symptoms never appear. Externally, infected stems look healthy. However, when stems are split lengthwise, internal browning of vascular tissue and pith is evident, especially at nodes and in the lower stem. BSR is more severe when temperatures are cool and adequate soil moisture is present. Foliar symptoms are particularly sensitive to environmental conditions during reproductive growth stages of the crop and are suppressed when temperatures are high or soil is dry during these stages. Foliar and stem symptoms of brown stem rot. Image: A. Sisson
Split soybean stems to confirm pith browning, indicative of brown stem rot. Image: A. Sisson
Brown stem rot disease cycle.
The fungus does not produce long-term survival structures but can reproduce on soybean residue throughout the winter. The pathogen has a limited host range, so crop rotation to a non-host crop (corn, small grains, and forage legumes) will reduce pathogen levels. The most efficient way to manage BSR is using resistant varieties. Soybean cyst nematode (SCN) breaks resistance to BSR in most BSR-resistant varieties. However, many soybean varieties with PI 88788-derived SCN resistance also have BSR resistance. Varieties with Peking- or Hartwig-derived SCN resistance, however, may be susceptible to BSR.