Brown Spot (Septoria Leaf Spot)

White mold in soybean field

By Dylan Mangel | Extension Plant Pathologist

Pathogen

Brown spot is caused by the fungus Septoria 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. However, brown spot rarely affects soybean yield in Nebraska as it primarily impacts leaves near the soil surface.

Brown spot on soybean

Maximum potential yield loss estimates due to brown spot range from 8% to 15% 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. Cumulative state yield loss estimates are generally below 0.31% across the north-central region.

Video: Identifying Septoria Leaf Spot

Video: Managing Septoria Leaf Spot
 

Disease Symptoms

Infection occurs as early as the V2 growth stage on lower leaves. Symptoms start as dark brown, irregular spots on both upper and lower leaf surfaces. Lesions typically have a yellow or chlorotic halo when held up to a back light. Adjacent lesions frequently merge to form irregularly shaped blotches. 

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 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.

Soybean residue
Soybean residue
Lower leaves
Lower leaves
Rusty brown or yellow
Rusty brown or yellow
Dark spots
Dark spots

 

Favorable Environmental Conditions

Brown spot development is favored by warm, moist weather, which promotes sporulation of the pathogen in the primary lesions. Severity increases due to increasing leaf wetness periods of six to 36 hours. Conidia are dispersed by wind or splashing rain. The optimal temperature for pathogen 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. However, in irrigated fields or in wet years with a variety known to develop significant brown spot, a fungicide application may increase profitability. If applied, timing of the application should be in the flowering through pod fill stages depending on disease development.

 

Resources

Updated from a previous article by Loren J. Giesler, former extension plant pathologist