The spotting may be worse in rainy seasons. Downy mildew causes a downy mold on the lower leaf surfaces but is not important on aster. No chemical control is listed.
Powdery mildew develops on the lower parts of crowded plants in late season.
Symptoms are a whitish coating on the leaves. Applications of benomyl or sulfur control powdery mildew. Bacterial crown gall causes the formation of rough, rounded galls. The galls form near the soil surface. Avoid infested soil and destroy infected plants.