The first symptoms usually appear on older leaves in the lower canopy. Warm and rainy weather during the growing season may favor its progression up the plant. Small irregular dark-brown spots develop on both surfaces of the leaves, often only on one side. As the disease progresses, the spots enlarge and coalesce into irregular brown areas with or without a yellow halo, often starting along the leaf edge or the veins. Later, whole leaves turn rusty brown and yellow, and shed prematurely. However, damage is not comprehensive and rarely leads to yield losses.
Brown spot is a leaf disease caused by the fungus Septoria glycines, which overwinters on infected plant residues in the soil. It is most common during the mid- to late season as it is not seedborne. The development of the disease is favored by environmental conditions conducive to continuous leaf wetness. Extended periods of warm, moist and rainy weather and temperatures around 25°C are ideal for its growth. First infection occurs when splashing rain and wind scatters the spores onto the lower leaves. Secondary infections between plants will also occur in these conditions. However, the disease is primarily present on the lower leaves, and if weather conditions are warm and dry, it rarely moves into the upper canopy. Normally, it has a very low impact on yields.
In case of prolonged rainy weather, apply products containing Bacillus subtilis in early stages of the disease.
Always consider an integrated approach with preventive measures together with biological treatment if available. Damage caused by brown spot is usually minor. So, fungicide treatments are generally not recommended. Seed treatment with fungicides can be used preventively. In rainy years, apply fungicides of the group of azoxystrobin, chlorothalonil, mancozeb and pyraclostrobin on the above-ground plant parts (generally 2-2.5 g/l of water).