As was the case in 2009, we are seeing late, but rapid development of Northern Corn Leaf Blight (NCLB) in some Ohio corn fields. Why are we seeing such high levels of NCLB this year and how will this affect our yields? At the time of silk emergence (R1), foliar disease levels were very low in corn fields across the state, but shortly after R3 (the milk stage), lesions of NCLB began showing up on the middle and upper leaves of the plants. As the season progress, lesions continue to develop, blighting as much as 30% of the ear leaf in some fields as the crop approach the dent stage (R5). On most of the hybrids with the problem, the symptoms are very characteristic of a susceptible reaction to the disease, with one-to-six inch long cigar-shaped gray-green to tan-colored lesions on the leaves. 

Since the 2001 growing seasons, we have been seeing a fairly steady increase in the occurrence of northern corn leaf blight in the state. This may be due in part to an increase in the number of acres planted to NCLB susceptible hybrids. The relatively late occurrence of the disease this year was probably due to favorable weather conditions late in the season. NCLB develops best at temperatures between 66 and 80F, accompanied by extended periods of surface wetness (due to rainfall, dew, and high relative humidity).

For an epidemic of northern corn leaf blight (and any other plant disease for that matter) to occur, three basic conditions must be satisfied: 1) the fungus (Exserohilum turcicum) must be present; 2) the hybrid planted must be susceptible to the prevalent races of the fungus; 3) and the environmental conditions must be favorable. In Ohio, the fungus is present because it survives in crop residue in the field and continues to build-up from one year to the next. So when a susceptible hybrid is planted, then the only thing that is missing is favorable weather. We have had cool, wet conditions over the last few weeks. However, when the disease develops late, its impact on grain yield tends to be much less than when it develops early during grain fill.

Make a note of the susceptible hybrids and avoid planting them again next year. Planting resistant hybrids is the most effective method for control of NCLB. Partial resistance, which protect against all four of the known races of the fungus, is common among hybrids and should used to minimize problems in the future. On hybrids with partial resistance, lesions are smaller, of a lighter color, and produce fewer spores than lesions on susceptible hybrids.