A lone symptomatic plant. Nearly every growing year in at least one corn field at the Northwestern Illinois Agricultural R & D Center, a lone plant can be observed exhibiting symptoms that look like those in the photo above (Figure). Round-ish, tan spots ranging in size from tiny specks to ½ inch in diameter cover many leaves of the plant. These spots begin to appear on the oldest leaves at the base of the plant, spreading to younger leaves over time.
What we know. Many bacterial and fungal pathogens are able to survive the Illinois winter in residue from previously infected crops. When environmental conditions (rainfall, relative humidity and temperature) are favorable, these pathogens can make their way onto growing plants through splashing rain or wind. Typically symptoms (leaf lesions) that result from these infections begin to appear in the leaves of the lower canopy.
Don't be fooled! Oftentimes the distribution of disease symptoms within and among fields can give us clues about disease. Patches of diseased plants within a field may indicate root disease, while widespread symptoms within a field points to diseases caused by a wind- or rain-disseminated pathogen. The foliar symptoms on the plant in the photo initially pointed to a disease caused by a residue-borne pathogen. However, the fact that none of the other plants in the field showed similar symptoms points to another cause.
Lesion mimic mutant. The symptoms on this plant's leaves are actually the result of a genetic mutation in the plant and not a pathogen-caused infection. A mutation in the plant's genes causes the plant to produce these leaf symptoms. The plant is called a 'lesion mimic mutant'. Genetic mutations often do not occur at high frequencies in hybrid populations, leading to individual lesion mimic plants in a field of thousands.
A plant exhibiting these symptoms may lead one to worry about the potential for a new pathogen in the area. However, one must view this plant in context; the fact that these symptoms can only be observed on individual plants, leads one to conclude that this plant was unlucky enough to have a lesion mimic mutation.
There have been sporadic reports of lesion mimic mutant corn plants from Indiana to Nebraska and reports indicate that some corn hybrids seem to have a higher frequency of these mutations than others. There is also some indication that symptom occurrence may coincide with periods of plant stress.
Diagnosis. If you are unsure about what you may be seeing in your corn field, there is always the option of sending a plant sample in to the University of Illinois Plant Clinic for diagnosis. Technicians at the clinic will be able to determine whether or not a microbe is responsible for causing the symptoms.
Tamra Jackson, Amy Timmerman and Jenny Rees. 2010. UNL CropWatch. Corn Diseases Update: Southern Rust Confirmed and Goss's Wilt
Johal, G.S. 2007. Disease Lesion Mimics Mutants of Maize. Online. APSnet Features. doi: 10.1094/APSnetFeatures-2007-0707
Bob Nielsen 2008. Purdue Plant & Pest Diagnostic Laboratory Picture of the Week: Genetic Leaf Discoloration Mutations in Corn