Brown spot on soybean. Like bacterial blight, brown spot occurs in Iowa every year in every field. The brown spot pathogen also survives in crop residues and can be seedborne too. In a wet season, it is also dispersed from the soil to soybean plants by splashing rain. Normally it causes no significant yield losses unless premature defoliation occurs.
Frequent rainfall is the primary condition for the occurrence of an epidemic.
In a wet summer, the disease progresses rapidly from lower leaves to upper leaves. Because the brown spot pathogen infects aged leaves, soybeans weakened by other diseases or improper farming practices become susceptible to this disease. It has been observed that relatively high levels of brown spot occur in fields with severe soybean cyst nematode damage. Soybeans with Fusarium root rot also have more brown spot. If you find brown spots in plants of abnormal growth, check whether there is other primary cause, such as nematodes.