Diseases Would Likely Come Too Late for Big Crop Impact

Both corn, soybeans, escape big disease pressure- so far.

This is a critical week for Indiana soybeans, in more ways than one. If a lasting break in the heat and more consistent rain doesn't come soon, yield impact may become real. Jim Newman, long-time ag climatologists, says the last half of August can be a critical time for soybeans, because pods are filling.

However, the soybean crop may not have to contend with Asian soybean rust for yet another year. Touted as a big threat in '05, it finally made it into Indiana, as far north as Lafayette, in '06, but only in October, when the crop was finished and some of the crop, at least, was already in the bin.

Greg Shaner, Purdue University plant pathologist, says that if rust doesn't appear within
the next few days, it likely won't be a factor if it does appear. Even if it would appear now, spraying may not be justified. He made these comments just a few days ago to an early August open session of the Purdue Crop Diagnostic Training Center, held at the Purdue Agronomy Rese4arch Center outside West Lafayette.

There is rust present in southern states, Shaner notes. But even there, he says, pathologists and Extension educators say not everyone would be justified in spraying, due to the progress of the crop and economic threshold for damage. The only way spores would likely get this far north now is if they ride storm currents northward. Computer modeling shows that's what apparently happened in September, '06.

"What we learned in '06 was that rust spores can make it this far north, even though in that case, they arrived too late to be of a concern," Shaner says. Soybean rust fungus is not believed to overwinter this far north, so new spores must blow northward into the region each season if the disease is to ever get a foothold.

Most of the confirmations last year in Indiana, which were few, were in counties in southwest Indiana. However, one important event occurred, Shaner says. "The first time you find rust in a state, you must send it to the USDA lab in Maryland for positive identification before you can officially confirm it and announce it to farmers. We did that last year. So the next time we discover rust anywhere in the state, we can announce it immediately and take steps to control it."

Leaf disease won't likely be a threat for corn yields in most cases this year either, Shaner confirms. Although many fields were sprayed with Headline fungicide at tasseling, fields were clean anyway, he notes. Some gray leaf spot has crept onto susceptible hybrids within the past couple weeks, especially on early hybrids. But in most cases, those are drying up quicker than usual anyway, often confounded by drought stress.

Since many areas have stayed dry, how did gray leaf spot finally at least show up, anyway? "Humidity levels begin rising in late July, especially at night," Shaner notes. "That's what the organism needs. This fungus can start to germinate, then wait up to three days if conditions change, for favorable conditions to return."

The bottom line, though, seems clear, although not yet 100% certain. This is likely to go down as a low disease year for corn and soybeans in Indiana. Other factors may have severely impacted yields in some fields, primarily a lack of water all-season long and heat late, but disease likely won't be the limiting factor.

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