Up until the middle of the growing season, this year was largely a quiet one for fungal disease pressure in corn. However, in July and August, reports of southern rust and gray leaf spot blowing up in cornfields started to come into the University of Nebraska's Plant and Pest Diagnostic Clinic.

MANAGE FOR NEXT YEAR: Gray leaf spot (pictured) and southern rust were among the fungal diseases that showed up in Nebraska cornfields this year.

"Gray leaf spot and especially southern rust have blown up at the end of the season with higher humidity and temperatures," says Tamra Jackson-Ziems, University of Nebraska Extension plant pathologist. "Early on we had a few days that were really hot, but we didn't have the humidity that gray leaf spot likes. Gray leaf spot is kind of a picky fungus; it's pretty specific on the conditions it likes. But we do have a lot of it in the state now, and a lot of it overwintered from last year."

As November and December roll around and growers prepare to make hybrid selection decisions, it's important to remember the disease history of that field.

"In the last couple years, seed companies have reported better resistance to southern rust in their hybrids. That may be important in southern Nebraska where it's a more consistent problem. Otherwise, southern rust doesn't overwinter here; it has to blow in every year form the Southern states," says Jackson-Ziems. "We also have hybrids with some tolerance to gray leaf spot, which is residue-borne. So rotation, tillage, hybrid selection and, of course, a fungicide application are your best management options."

As most plant pathologists and agronomists will tell you, the most consistently beneficial time to apply fungicide is around tassel at VT, or shortly thereafter. However, in the last couple years, some growers applied fungicide early on — only to have gray leaf spot or southern rust develop later on, leaving many to wonder how late they can apply a fungicide and still see a return on investment.

"In many cases, some of these diseases have developed when it's been too late to talk about a fungicide application. Some of these diseases were in fields where the fungicides were applied weeks ago and wore off," says Jackson-Ziems. "In some of our older data, we showed applications made at dough were sometimes effective, at least those made in south-central Nebraska. It's still up in the air as to whether that would be an effective strategy or not. In some years it might be beneficial to wait a little bit longer."

What to watch for at harvest
As harvest approaches, Jackson-Ziems advises growers to be mindful of fields that have experienced foliar diseases or other kinds of stress. Those fields may see higher incidence of stalk rot or crown rot diseases.

"It's a good idea to identify plants that look like they died earlier than the rest of the field. You can push on them or squeeze the lower nodes to see if they are getting soft and at greater risk of lodging," says Jackson-Ziems. "If that's the case, it might be best to harvest those first or earlier than the rest."

There have also been reports of ear rot diseases like diplodia ear rot. For example, small raised, black fungal reproductive structures on infected kernels and stalks that are rough to the touch are symptoms of diplodia ear rot. Growers who have found symptoms of diplodia and other ear rot diseases need to be careful in how they manage their stored grain.

"If growers have a lot of ear rot diseases, it's best to avoid storing it," adds Jackson-Ziems. "But if they have to, having it dried and cooled as quickly as possible at harvest would prevent growth of those fungi. They'd have to get below 15% moisture if they have that fungi, and keep it cool so it doesn't create condensation inside the bin. However, that's easier said than done around the rush of harvest."