Common rust. Typically, common rust on corn does not appear until later in the growing season, but this year it is already in many cornfields in Illinois. Common rust is caused by the rust fungus Puccinia sorghi. The rust pustule on the leaf is the main visible symptom. The pustules are cinnamon brown in color and circular to elongate in shape. They appear on both the upper and lower surfaces of the leaf and eventually turn brownish black with age. Rust spores are carried by wind from southern areas of the country, usually in mid-June to mid-July. If conditions are dry and spores fall onto growing corn, it is still possible for some infection to occur in the whorl, where moisture collects allowing spores to germinate. Yield loss can result when a significant percentage of leaf area is infected. However, the severity of this disease depends on hybrid susceptibility and the growth stage when infection occurs. Development of this disease is favored by humid, wet weather and by cool temperatures (60 to 77°F). Hot, dry conditions slow down or stop the development of the pathogen. |
Common rust on corn.
Southern rust. You may hear discussions about another rust called Southern rust. It is typically a problem in sweet corn but that is not to say that it can't occur in field corn. Southern rust is caused by a similar but different fungal organism called Puccinia polysora. The fungal spores of southern rust are also windblown and carried from tropical areas northward to the Midwest. The development of this pathogen is favored by warmer conditions than common rust. Symptoms of southern rust resemble common rust except for slight differences. First, pustules of southern rust are usually lighter in color (cinnamon brown to orange versus cinnamon brown), and they are more circular to oval in shape rather than more elongated. Second, southern rust pustules are mainly found on the upper surfaces of the corn leaves, whereas common rust pustules are found on upper and lower leaf surfaces. Third, the spores in the pustules of southern rust may erupt or break through the leaf epidermal tissue more slowly than those of common rust.
Other fungal leaf blights. There have been reports of Physoderma brown spot, anthracnose, and eyespot developing in Illinois cornfields. What is important to remember about all these leaf blights, including the rusts fungi, is not the individual identity of which blight you have but rather the percentage of leaf area blighted as a whole on your entire plant. Remember that control considerations may be justified when whole-plant infection reaches 15%. It is also important to remember that the type of symptoms or size and coloration of lesions for these corn leaf blights can vary with the genetic resistance of the hybrid that you plant.
Brown spot, anthracnose, and eyespot. Physoderma brown spot typically appears on cornstalks and husks later in the season, but lesions can also appear on the leaves and look similar to anthracnose. Brown spot lesions start out as yellow, round spots on the blade, sheath, stalk, or husk, which eventually turn dark brown to reddish or purplish brown. On the other hand, lesions caused by the anthracnose organism are brown, oval, or elongated spots surrounded by a dark-brown or purple border. Anthracnose lesions can also have a zone of yellow tissue around the spot, which usually contains black acervulus or fruiting bodies of the fungus. Symptoms of eyespot on corn are small, round, tan spots that have a dark border and are surrounded by a yellow area or halo.
Anthracnose on corn. (Photo courtesy of Gary Munkvold, Iowa State.)
Eyespot on corn. (Photo courtesy of Gary Munkvold, Iowa State.)
Control measures and control considerations for rust diseases and general leaf blights on corn were discussed in last week's Bulletin in an article by Suzanne Bissonnette. In addition, a Section 24c Special Local Need Label for Tilt does exist in Illinois through the end of the year. This 24c label is for control of rusts (Puccinia spp.) and eyespot as well as some leaf blights as discussed in that article. Read and follow all label directions and restrictions.--Loretta Ortiz-Ribbing