Numerous corn diseases, including those caused by fungi and bacteria, exhibit symptoms in the leaves. Fungal and bacterial inoculum can overwinter in crop residue on the soil surface and/or be spread by wind and water. Certain other pathogens must have a living host to overwinter, and spores must be carried on wind currents for new infections. Foliar diseases that establish prior to tasseling, or become severe within two to three weeks after tasseling and pollination, can result in significant yield loss. Major diseases affecting the leaf include:
Gray leaf spot
Gray leaf spot is the number one disease in all corn production. The fungus survives the winter on residue from the prior corn crop, providing a primary source of the inoculum, particularly when growers plant continuous corn or use a no-till soybean/corn rotation. Spores from the fungus are splashed onto the lower leaves early in the season. When these resulting infections produce spores, the inoculum can be carried by air currents higher into the canopy or into neighboring fields.
Eyespot, a fungal disease that overwinters on surface corn debris, can be a severe problem. Early symptoms include small, light green, circular lesions with yellow halos. Lesions develop brown/purple rings as they age and may merge, destroying larger areas of leaf tissue.
Northern corn leaf blight
Similar to gray leaf spot, the fungal inoculum causing northern corn leaf blight survives in the residue in the soil and moves up through the canopy. This disease differs from gray leaf spot, however, in that it prefers cooler conditions, tending to strike the corn crop later in the season.
Common rust is found everywhere in corn production. Both seed corn and sweet corn are very vulnerable to common rust, although the disease is not an economic problem in field corn. It tends to become less aggressive as the weather turns hot and dry.
Because of the changes in hybrid genetics over the last few years, southern rust has become more problematic. It has begun to spread all across the corn-growing region, starting in the South and being windblown to the North. Southern rust differs from common rust in that it thrives in hot conditions. As common rust starts backing off, southern rust explodes.
Southern corn leaf blight
Southern corn leaf blight lesions on leaves are seen at the mid-whorl stage through maturity. This fungal disease favors wet conditions and warm temperatures, as well as reduced tillage and continuous-corn production.
Goss’s leaf blight
As the bacteria that causes this blight overwinters primarily in crop residue, it favors continuous-corn production and reduced tillage. In addition to the leaf blight phase, this disease can also have a systemic wilt phase, which leads to a slimy stalk rot.
This fungal disease, which attacks the plant at various stages of growth, can appear early in the season as a leaf blight. Later it can attack the stalks above the ear leaf leading to “top die-back,” or lower on the stalk soon after tasseling leading to reduced plant productivity and lodging.
Stewart’s wilt, a bacterial disease, appears on the leaves as pale green, linear lesions that turn yellow and then brown. The margins of each lesion are wavy or irregular. The disease overwinters in its host and vector, the corn flea beetle.
Although primarily a disease of the stalks or ears, symptoms of common smut also appear on tassels and leaves. Tumor-like galls form on actively growing tissue and are white or greenish-white. The interior darkens and turns into a powdery, black clump of spores. Galls on leaves normally remain small, dry and hard.