Several of you across the state have called in to report moderate Septoria brown spot infection on soybeans. The disease was present in many fields earlier this season as well, and continued favorable weather conditions have led to continued disease development. |
Brown spot is caused by the fungus Septoria glycines. Symptoms usually appear first on the unifoliate leaves very early in the season as brown spots that are somewhat angular to irregular in shape. Early-season infection is often misdiagnosed as normal leaf senescence. The spots develop both on the top and bottom surface of the leaf. Some leaves that become heavily infected turn yellow and drop off the plant. Under periods of warm, wet, or humid weather, the disease progresses up the plant.
Yield loss can occur if the plant is severely defoliated from the disease. Leaves that are infected later in the season often have a rusty brown appearance from numerous lesions. Leaf tissue around the lesions is usually yellow as well.
Brown spot lesions on soybean leaf.
The brown lesions can develop on the stems and petioles as well; it is hard to make a diagnosis from these stem lesions, though, because of their similarity to other diseases.
Where does the disease come from? The fungus survives and overwinters in infected soybean debris. The fungus produces tiny fruiting structures called pycnidia, which are black popcorn-seed-shaped fruiting structures. They are embedded in the leaf tissue, pointy side up, usually at the center of old lesions. You can easily see these with a 10X hand lens. Spores are produced in the pycnidia, and the spores are splashed by rain or sometimes wind up the plant.
Septoria brown spot foliar symptoms can be confused with bacterial blight. Look for the pycnidia in the lesions to diagnose Septoria in the field. Lesions on the stems and petioles can be confused with a number of fungal diseases such as pod and stem blight or stem canker. Again, look for the diagnostic pycnidia on leaf lesions.
Resistant varieties are not available. Fungicide treatments may be useful when symptoms are severe and other foliar fungal diseases are present. Use the risk-assessment checklist in Table 1 to determine if a fungicide application may be warranted. Recommended fungicides are Benlate, Bravo, and Topsin-M.--Suzanne Bissonnette