Brown stem rot (BSR) is an economically important disease of soybean in Iowa and throughout the North Central soybean growing region of the United States. Leaf symptoms on infected plants vary and depend on the soybean variety, strain of the fungus and environmental conditions. In some instances, no foliar symptoms occur. Characteristic foliar symptoms include interveinal chlorosis and necrosis, followed by leaf curling and leaf death. 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. Externally, infected plant stems look healthy. However, when stems are split longitudinally, internal browning of vascular tissue and pith is evident, especially at nodes and in lower parts of the stem.
The fungus does not produce long-term survival structures but can reproduce on soybean residue throughout the winter. Severity of BSR and yield reduction is related to soybean variety, fungal population levels in soil and plant residue and environmental conditions. 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.
It is best to scout this disease R4 through R6, severity is greater in soils with low phosphorous and potassium and a soil pH below 6.5. Often occurs with soybean cyst nematode, which may increase severity of brown stem rot.
Variety selection: The most efficient way to manage BSR is with the use of resistant varieties. Many sources of resistance are available. It is also known that 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. Therefore, planting soybean with PI 88788-derived SCN resistance can reduce the adverse effect of SCN on BSR resistance. Varieties with Peking- or Hartwig-derived SCN resistance, however, may be susceptible to BSR.
Crop rotation and tillage: Phialophora gregata survives on crop residue. It has a limited host range, so crop rotation to a non-host crop (corn, small grains and forage legumes) will reduce pathogen levels. In fields where disease is severe, rotation to a non-host crop for three or more years is recommended. If tillage is being considered to decrease pathogen levels, great care should be taken to minimize soil erosion and maintain soil quality.
Photo by Tristan Mueller
Be on the Lookout for Brown Stem Rot in Soybean