One phenomenon in parts of the Midwest this summer, especially in the eastern Corn Belt, is a large amount of common rust showing up on corn leaves. To call it an outbreak might be a bit premature, but it definitely came in early on many fields. Some believe its occurrence may be tied to the strong storms that moved up form the south, bringing heavy rains in May and especially in June.
"Common rust by itself is usually not a factor in reducing yields in commercial field corn," says Kevin Cavanaugh, director of research for Becks Hybrids, Atlanta, Ind. "On seed corn it's a different story. Common rust can cause economic losses in seed fields if it is not treated at the proper time."
So why were farmers on the fence about flying on fungicide when rust was the main thing they were seeing in their fields two to three weeks ago, when the decisions to spray or not spray were being made? Some figured that if rust was there, other diseases might be too. Others may have felt pressured by chemical dealers, who needed an answer. After all you can't bring in supplied of fungicides and line up applicators to spray thousands of acres overnight, if farmers decide to spray.
There actually were reasons to spray in some cases, maybe not others, Cavanaugh says. "The problem with rust is that while it doesn't affect corn yields by itself, it can set up plots for stalk rots later on," he notes. "The stalk rots could threaten yield loss if they cause lodging, through harvest losses in the field."
But it's not as simple as 'OK, I have rust, so I'll spray so I don't get stalk rot," the research director says. Next you needed to look at the overall disease package of the hybrid showing symptoms of rust on the leaves, particularly the upper leaves. Those are the ones that help pack in yield potential during grain fill.
If hybrids have a good diseases package against most or all stalk rots, then spraying may have still not been necessary,. He notes. But if a hybrid was susceptible to one or more stalk rots, spraying was likely a good idea. There's been enough stress in many locations this summer from too much water, or not enough at the right time, to allow stalk rot to get a foothold in plants predisposed by rust.
Common rust is a problem most people in the Corn Belt only deal with every few years, perhaps one year in eight, in commercial corn. This year happened to be favorable for the rust to develop. It's already present above the ear leaf in some fields.
Dave Nanda, long-time plant breeder and CI consultant, agrees wholeheartedly with Cavanaugh. "Absolutely, it can help set plants up for stalk rots," he notes. "Then it all depends upon the defensive package against diseases that the hybrid possesses."
Following fields with rust that were or weren't sprayed may be worthwhile this fall. Especially if you've got a hybrid subject tot one or more stalk rots, saw rust on the leaves, and didn't spray, start scouting that field relatively soon to determine if stalk rot may develop. It could be one of the fields you mark for early harvest.