Infections cause minutes, yellow to brown specks on leaves, stalk, sheath, and husks. As the disease progresses, the specks enlarge and become more numerous. The resulting patches or bands of diseased tissue may cover a good portion of the leaf. Their color usually varies from yellowish to brown and they are reminiscent of the symptoms caused by some sorts of rusts. However, contrarily to rust diseases, P. maydis lesions frequently develop in distinct bands across the leaf, particularly at its base. Another difference is that conspicuous dark brown to black spots appear on or immediately close to the main vein. In susceptible varieties, the middle vein may be covered with these lesions, and may turn from chocolate to reddish brown or purple in color.
Symptoms are caused by Physoderma maydis, a fungus that overwinters in infected crops debris or soil (up to 7 years in favorable conditions). The disease is more common in fields with continuous corn or with abundant crop residues, for example where reduced tillage practices are employed. Infection usually start in the whorl, where water tend to accumulate after rainfall or irrigation. From there, the secondary inoculum is spread via wind or splashing water to other plant whorls. This explains why symptoms are more conspicuous at the base of older leaves. Optimal conditions of light and temperature are also needed for that. Overall, this disease is not serious and has minor effect on yields.
There is no biological treatment against P. maydis available at the moment. Please notify us if you know of any. Cultural control is the most important practice to avoid its occurrence and possible outbreaks.
Always consider an integrated approach with preventive measures and biological treatments if available. There is no recommended chemical treatment against P. maydis as the occurrence is sporadic and the effect on yield should be minimal.