By Paul Vincelli, Kentucky Pest News

Because of disease-favorable weather in 2004, gray leaf spot and northern leaf blight were potential threats to the 2005 crop. However, dry weather was so prevalent in the 2005 growing season that it helped suppress these and other leaf diseases.

The fungus that causes gray leaf spot survives in corn leaf residue that was previously infected. The same is true for the fungus that causes northern leaf blight. Since levels of these diseases were so low in 2005, this means that levels of inoculum-the spores that can start the disease process-are lower than normal this winter for both diseases. Thus, the risk from either disease for the 2006 season is probably slightly lower than normal. Producers can take these lower inoculum levels into account when selecting hybrids for next year, and may be safe planting a slightly more susceptible variety than in the past.

However, don't ignore these two diseases completely. The fungal pathogens are both still capable of causing significant damage, especially in fields of continuous corn, where surviving inoculum of these organisms tends to be highest. The spores of both fungi are spread in the wind, so even if your field has a low level of inoculum, neighboring fields may still provide the inoculum needed to lead to a damaging outbreak.

Furthermore, gray leaf spot and northern leaf blight are what are called polycyclic diseases. This means that the fungi that cause these diseases are capable of undergoing repeated infection cycles during the growing season. Thus, if weather conditions in 2006 turn out to be highly favorable for either disease, the disease potentially can build up on a susceptible variety from a small amount of overwintering inoculum into a fierce epidemic.

Since the weather is always an unknown this far in advance, it makes sense to choose hybrids with some resistance to both gray leaf spot and northern leaf blight, especially in fields of continuous corn. Even in rotated fields, having some resistance to these diseases is advisable if they are under conservation tillage, since the corn residue from the 2004 season is still capable of providing inoculum for the 2006 crop.