The hotter than usual weather that's gripped Iowa and much of the Midwest this summer has had an adverse impact on crops, particularly corn. Several Iowa farmers interviewed this week say rumors of poor pollination and problems with grain fill are indeed true.

Farmers, particularly those south of Interstate 80 across Iowa, where little rain has fallen for nearly a month, say the hot nights this summer have been particularly hard on corn. Daytime temperatures in the 90 to 100 degree range aren't so bad if weather can cool off into the 60s at night. But temperatures have been in the upper 70s and into the 80s at night, which doesn't relieve the stress.

A southern Iowa farmer, Ron Gordon of Creston, observed on August 3 "The corn plants are holding up and look good, with nice leaves. But we're not getting good fill of the kernels on the ears. If we don't get some rain soon we won't have a good yield. Test weights may be low."

The hot weather of July 2011 represents a sharp change in weather patterns from recent years when Iowa saw relatively few summer days when the temperature reached above 90 degrees. The stretch of 95 degree-plus temperatures that hit in July of this year coincided with the pollination period for corn. Corn pollinates and fills out ears best at temperatures below 90 degrees.

The news on this year's Iowa and U.S. corn crops isn't reassuring

As the calendar moves through the first week of August, agronomists are now getting a better handle on this year's corn crop prospects, and the news isn't reassuring for many areas in Iowa and the Corn Belt.

On August 1, Iowa State University Extension climatologist Elwynn Taylor estimated that this year's national corn yield average will likely be about the same as the 153 bushels per acre that was harvested in 2010. That yield was down from 164 bushels per acre harvested in 2009. The reason last year's yield was down was down from 2009 was because of record rainfall in July 2010.

"This summer, people were concerned about pollination in corn when we had the very hot weather during July," says Jim Fawcett, an Iowa State University Extension field agronomist in eastern Iowa. "I don't think that hot weather had a big impact on pollination this year. But it's just not good for corn yields to have such hot weather, particularly the warm nights when the weather doesn't cool down after being so hot during the day."

The Iowa corn crop is moving through its development stages pretty fast this summer. "We are already seeing some denting of corn in some fields," says Fawcett, who saw denting of corn in fields during the last several days of July and the first couple days of August. "That's not good when you see denting this early. It's bound to have an impact on yield, shaving some bushels off of the yield potential this year."

Japanese beetles and spider mites are raising questions from farmers

Farmers are reporting Japanese beetles in fields and people are seeing them in yards and gardens too, this summer. "This insect has been a bad problem in certain areas of Iowa this year and farmers are wondering about spraying an insecticide now to kill this pest in corn and soybeans," says Fawcett. "Japanese beetles were clipping silks in corn, as of the last several days in July and the first few days of August. But most of the corn is done pollinating by then, which means these pests can clip all the silks they want after pollination."

That's typical behavior of Japanese beetles. They tend to wait until the corn is done pollinating before they move into fields. "It really doesn't make sense to be spraying Japanese beetles in corn at this point," says Fawcett. What about beans? "With soybeans, you need to have 20% defoliation before it pays to spray," he answers. "That much damage looks horrible, so normally Japanese beetles don't do that much defoliation of soybeans. Usually it doesn't pay to spray soybeans unless you are getting a huge amount of defoliation."

Spraying for Japanese beetles can increase spider mite problems

If you do spray for Japanese beetles, there could be unintended consequences with spider mites. "I didn't think we'd be talking about spider mite infestations this year," says Fawcett. "But it's been a feast or famine situation in Iowa with the weather. It seems as though some areas of Iowa this summer have either gotten a lot of rain or haven't received hardly any. The very dry areas of the state are starting to see spider mite infestations."

One way to attract spider mites to your field is to spray an insecticide you didn't need to spray. Most insecticides don't work on spider mites. What happens is the insecticide kills the other insects but leaves the spider mites there. Thus, the spider mite population then increases tremendously. Particularly, the use of pyrethroid insecticides can cause spider mite problems. "This is a good reason to not spray an insecticide on any field unless you really need it," says Fawcett.

Goss's Wilt disease of corn is starting to show up in Iowa this summer

Goss's Wilt is a corn disease that struck a number of fields in central Iowa a year ago. "We are starting to see some of this disease confirmed in eastern Iowa," says Fawcett. "It showed up on some corn in western Iowa earlier this summer. This disease isn't a widespread problem, but it can be pretty devastating in the fields that it hits."

Goss's Wilt is a bacterial disease, not a fungus. So the fungicides don't help control Goss's Wilt, he notes. This disease tends to cause the top part of the corn plant to die. The plant will also have very large lesions or cuts on the corn leaves, which are produced by Goss's Wilt infection. This disease kind of gives the plants a greasy or shiny appearance because of the bacteria. It is a disease you should be looking out for this summer.

"If your corn gets Goss's Wilt, keep in mind there isn't anything you can do to control it this year," says Fawcett. "But the next time that particular field is to be planted to corn, try to select a corn hybrid to plant there that has more tolerance to Gross's Wilt."