Brown Spot (Septoria Leaf Spot)

By Loren J. Giesler, Former Extension Plant Pathologist

Pathogen

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.

Video: Identifying Septoria Leaf Spot

Video: Managing Septoria Leaf Spot

Disease Symptoms

Infection occurs as early as the V2 growth stage on lower leaves. Under favorable weather conditions (warm, wet weather), the disease may continue to spread to the upper canopy. Late in the growing season, infected leaves may turn rusty brown or yellow and drop prematurely.

Symptoms start as dark brown, irregular spots on both upper and lower leaf surfaces. Lesions typically will have a yellow or chlorotic halo when held up to a back light. Adjacent lesions frequently merge to form irregularly shaped blotches. Leaves become yellow to rusty brown. Symptoms of Brown Spot can also develop on stems and pods of plants approaching maturity. Stem and pod lesions have indefinite margins, are dark in appearance and range in size from flecks to larger areas.

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.

Management

Genetic Resistance

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.

Cultural Practices

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.

Chemical/Biological Control

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.