Brown stem rot
(formerly Phialophora gregata)
Brown stem rot is a soilborne disease of economic concern that was first identified and confirmed in NY in 2013. Since the initial confirmation, it has been discovered to be fairly widespread throughout many soybean production areas in NY, though only at moderately low incidences. The disease is favored by cool, wet weather with temperatures below 80F during pod fill, followed by hot, dry weather. The fungus survives in the soil or on soy residue, and infects soybean plants through the roots and then colonizes the vascular system and is translocated throughout the plant. Symptoms are variable, and may not appear until early pod set.
Some varieties develop foliar symptoms, including interveinal chlorosis, followed by necrosis, which is indistinguishable from the foliar symptoms of other soilborne diseases, including sudden death syndrome and stem canker. Dead leaves remain attached to stems. Internal symptoms can be observed by splitting stems longitudinally, and include browning of the vascular system and pith extending upward from the soil line, which is often more prominent at the nodes. The disease reduces the number and size of seeds produced, and may result in yield losses of up to 25%.
The fungus is long-lived in the soil, and crop rotations may help in reducing the pathogen population. However, genetic resistance is widely available in many soybean varieties and is recommended as the best option for managing this disease in fields where it has been identified. Foliar fungicides are not effective, and are not recommended.