Spotty rains became the rule from late July through August in Illinois and Indiana. Areas that received thunderstorms got yield-saving moisture. But that moisture also may have promoted foliar diseases in corn. The same conditions that are good for grain fill are also good for some diseases.

The field from which the leaf pictured was pulled is an example. It was still relatively green and healthy in late August, but so was gray leaf spot. This fungal disease likes warm temperatures and humid weather. Drier areas received dews and even some fog in mid-August that the disease found to its liking.

Darcy Telenko, Purdue University Extension plant pathologist, notes that by mid-August, it wasn’t just gray leaf spot that was a concern in the eastern Corn Belt. Southern rust had been reported in about 20% of Indiana counties and roughly one-third or more of Illinois counties. This disease must blow in each year on storm fronts. It wasn’t limited to southern counties in the two states, either. Southern rust was found as far north as near Rockford in Illinois and near Fort Wayne in Indiana.

Meanwhile, tar spot was reported in about a quarter of all Indiana counties and a handful of counties in Illinois. Typically a more northern disease, it was detected in south-central Illinois and in several contiguous counties in the upper part of the southwest toe of Indiana, plus in all counties along the Indiana border with Michigan.

If you treated with fungicides, they should provide some control and protection, Telenko says. However, tar spot can outlast the effectiveness of a fungicide if conditions remain right. Once corn is past the dough stage, however, spraying would likely no longer pay.

Trouble brewing

Dave Nanda, director for genetics for Seed Genetics Direct, favors scouting intensely early and spraying if gray leaf spot lesions or other foliar diseases are detected. He found some gray leaf spot in the field he watches as part of the Corn Watch project in central Indiana. However, the farmer sprayed the field and effectively stopped the disease from developing further. It never reached the stage of severity shown in the leaf pictured here from an entirely different field.

The problem with letting foliar diseases go undetected or untreated earlier in the season is twofold, Nanda says. First, foliar diseases by themselves can cause enough tissue loss to reduce the size of the photosynthetic factory and cut yields. In some cases, like during a severe, late outbreak of southern rust in southern Indiana a few years ago, it can cost a grower 40, 50 or even more bushels per acre.

Second, stress from foliar diseases sets plants up for stalk and even ear rots later in the season, Nanda says. These can cause yield loss in their own right. They can also make fields more vulnerable to late-season windstorms that could cause lodging, leading to harvest loss.

Even now, Nanda believes in scouting fields. If stalks are beginning to deteriorate, mark those fields for early harvest, he suggests.