The symptoms vary slightly depending on the variety of plant concerned. In seedling growing from infected seeds, the pathogens usually cause damping-off of the newly emerged plants. In older plants, dark-brown, circular spots with a concentric growth and well-defined borders develop first on older leaves. Over time, the center of these target-like spots become thin and papery, eventually dropping out to give a "shot-hole" aspect to the leaf. As the lesions expand and coalesce, leaves can be engulfed by dead tissues, leading to premature defoliation. Spots can also appear on mature pods, which show shriveled, smaller and discolored seeds with signs of decay. However, because the disease usually occur during the maturity of the plant, it causes minimal yield loss and requires no management recommendations.
In soybean, Alternaria leaf spot is caused by several fungi belonging to the genus Alternaria spp. These pathogens can break in the pod walls, infecting seeds and making them the main transmission route of the disease between seasons. The fungus can also overwinter on susceptible weeds or non-decomposed crop debris. The secondary spread between plants is mainly airborne and is facilitated by warm, humid weather with predominant winds and rain splashes. Provided leaf wetness is optimal, the fungus germinates within hours and enter the tissues through natural pores or through insect wounds. Optimal temperatures for the development of the disease are around 20-27°C. Plants are most susceptible at the seedling stage and late in the season when leaves are mature. The incidence may be important on soybean crops irrigated during the post-rainy season, and favored by added physiological or nutritional stress for the plant.
No biological products are available against Alternaria leaf spot in soybean. Organic treatments include the use of fungicides based on copper (usually around 2.5 g/l).
Always consider an integrated disease management with preventive measures and biological treatments if available. This disease does not require specific management if it appears late in the season. Consider a fungicide application if infections occur early in the season and with favorable conditions for the growth of the fungus. In that case, products based on mancozeb, azoxystrobin or pyraclostrobin may be applied at the first sign of the symptoms. Do not delay treatment until a large outbreak is identified as it may be too late for successful control. Seeds treated with these fungicides are also effective to prevent the incidence of the disease.