Septoria Brown Spot in Soybeans
Septoria Brown spot is caused by the fungal pathogen Septoria glyines and under normal conditions, the disease does not cause major yield loss in Canada. Were yield losses have occurred in the past is when the soybean plants are under prolonged stress conditions. This can result in premature defoliation which begins with the lower leaves and can work up the plant. A few of the most common stresses are dry conditions and soybean cyst nematode infection. In inoculated trials in the USA yield reductions of 20-30% have been observed. Cultivars differ in susceptibility but none is completely resistant.
What Does It Look Like? Symptoms of the disease appear first on the primary unifoliate leaves shortly after trifoliolate leaves have developed. In some cases depending on the amount of rain and splashing that has occurred the trifoliate leaves may have more lesions then the unifoliates. Disease symptoms begin as small, dark brown, irregular spots, 1-2 mm in diameter with or without a yellow halo which develop on upper and lower surfaces of lower leaves. Initial infections on primary leaves and cotyledons produce secondary inoculum that infects upper leaves as they develop. The fungus produces a toxin that contributes to yellowing.
Lesions may enlarge and coalesce, and frequently they are concentrated along the leaf veins or at the leaf margin (Fig. 1). Symptoms often disappear as the season progresses since as the weather turns warmer and drier. Humidity and moisture are important for brown spot development. Rapid yellowing and scenecence (death) of infected leaves occurs. Symptoms may be difficult to distinguish from those of bacterial blight, soybean rust and downy mildew. One way of distinguishing the disease from these others is that characteristic brown pycnidia (spots) are imbedded in the dead (necrotic) tissue of older lesions.
Figure 1: Septoria Brown Spot Back
Figure 2: Septoria Brown Spot
Figure 3: Septoria Brown Spot Soys
The fungus does overwinter on crop debris and can be spread by infected seed. In most cases seed infection is low in Ontario commercial seed but can be a problem in seed that has not been cleaned or has been kept for a number of years.
As I mentioned, the disease is more cosmetic then anything but development early in the season can lead to significant defoliation of the plants. There are differences in soybean varieties but we do not have good data for Ontario. Contact your seed supplier or consultant concerning more information about your specific varieties. As is always the case a good rotation with non-host crops such as wheat and corn will lower disease levels.