By Loren J. Giesler, Extension Plant Pathologist
Brown spot is caused by the fungusSeptoria glycines and may also be called Septoria leaf spot. It was first reported in the U.S. in 1923 infecting soybeans in North Carolina and is now widely distributed through the north central states, the mid Atlantic states, and the southeastern U.S. Brown spot rarely affects soybean yield in Nebraska. In some years it may hasten maturity by causing premature defoliation.
Yield loss estimates due to brown spot range from 8% to 15% nationally and occur when 25% to 50% of the canopy prematurely defoliates. Severe brown spot infection usually results in smaller seed size. The disease is most severe when soybean is grown continuously in the same field, particularly in fields where tillage is reduced, since this is a residue-borne disease.
|Soybean residue||Lower leaves|
|Rusty brown or yellow||Dark spots|
Favorable Environmental Conditions
Brown Spot development is favored by warm, moist weather, which promote sporulation of the pathogen in the primary lesions. Severity increases due to increasing leaf wetness periods of 6 to 36 hours. Conidia are dispersed by wind or splashing rain. Its optimum temperature for development is 77°F, but disease symptoms develop at 60-85°F.
The onset of Brown Spot symptoms is influenced by the relative maturity of the soybean variety, and symptoms appear earlier in the season on an early-maturing variety. Varieties vary in their susceptibility to this disease, but resistance has not been identified.
Brown Spot is more severe in continuously cropped soybean fields. Reduced tillage systems will tend to have more brown spot as the pathogen overwinters in residue.
Application of fungicides to manage brown spot in Nebraska is typically not warranted with most varieties.
Yield loss as high as 10% has been observed in one fungicide trial conducted in south central Nebraska. In irrigated fields or in wet years with a variety which is known to develop brown spot significantly; a fungicide application may increase profitability. If applied, timing of the application should be in the flowering through pod fill stages depending on the time of disease development.