Northern Corn Leaf Spot
Symptoms vary slightly depending on the hardiness of the pathogen, the degree of susceptibility of the plant, and on environmental conditions. First symptoms usually appear in late stages of plant growth, either during the emergence of silk threads or at full maturity. Elongated to oval or circular light-brown lesions appear on the lower leaves, often surrounded by a dark margin. The length and width of the lesions depend on the strength of the pathogen and the type of plant used. In some cases, these lesions may also occur on leaf sheaths and husks covering the ear. Black mold is sometimes visible on the kernels.
Northern corn leaf spot is caused by the fungus Helminthosporium carbonum, which overwinters on corn residues in the soil. The spores on these debris serve as primary source of infection during wet weather. Secondary infection from plant to plant is wind- or rain-driven. The disease develops mainly on plants used in seed production and is therefore rarely a problem in the fields, where mostly resistant hybrids are grown. The progression of the disease is favored by moderate temperatures, humid weather, and minimal tillage of the field after harvest. If it occurs during the the grain filling phase, it can result in yield losses of 30 percent of more.
Most treatments named here have only been used in small scales. Essential oil from Indian bael (Aegle marmelos) is active against Helminthosporium carbonum, at least in laboratory tests. Different compounds isolated from leaf extracts of some maize varieties (resistant and susceptible alike) can be toxic to the fungus. Fungi isolated from the pith of stalk rot-affected maize plants seem also to parasitize on known plant pathogenic fungi, including C. carbonum.
Always consider an integrated approach with preventive measures combined with biological or organic treatments. On susceptible plants, a foliar fungicide application at the beginning of silking is probably necessary.