Insects, Disease Take Varied Toll

Charcoal Rot

It’s not surprising last season saw a mixed bag of insect and disease pressure. An exceptionally wet spring followed by some hot, dry summer months set the stage for a few different players to threaten crops. And with so much corn-on-corn acreage being planted in recent years, it’s becoming clear that the practice is having some fallout.

The mild winter felt throughout the Midwest this year may well play a role in increased infestations in 2012. For instance, densities of some pests such as soybean aphids, corn flea beetles and bean leaf beetles — which overwinter in the region — may be more significant, says Michael Gray, professor and assistant dean at the University of Illinois.

And because there have been greatly reduced moisture levels and no deep frost in some areas, growers in states like Iowa are concerned about soil conditions producing both insect and disease problems.

Bt Corn Failures

A good deal of buzz coming off of last season centers around the suspected or confirmed resistance of western corn rootworm to the CryBb1 protein in Bacillus thuringiensis (Bt) hybrids in Iowa, Illinois, Minnesota and Nebraska. Illinois’ Gray was called to a number of fields to investigate severe corn rootworm pruning of these hybrids, planted in northwestern Illinois fields, in continuous corn production for many years. He saw lodged stalks, numerous rootworm adults, and plants with two to three nodes of roots completely destroyed. Yield losses were significant. Gray is concerned that most growers are not scouting large-scale corn and soybean fields. “Fueled by high commodity prices, the new form of IPM is insurance pest management, rather than integrated pest management,” he says. Some growers are using multiple tools against corn pests in one season, even though the insects are not detected. And they’ve been doing it for several years.

“We are applying enormous selection pressure to this insect species,” he emphasizes.

Jason Kohorst, agronomist with West Central Cooperative (WCC), Ralston, IA, says his clients had more corn rootworm feeding than normal. “That doesn’t necessarily mean it was resistance,” he says. “But ultimately when growers see a trait that they’re buying is not bulletproof, they immediately think there’s resistance.” In response, he’s seeing a large shift, with growers putting either liquid or dry insecticide on with their planters to prevent some of that resistance.

Northeastern Iowa is the site of the first confirmed Bt resistance; other locations in the Midwest will require further detailed laboratory work to prove the problem, says Gray.

Efforts to find out if resistance has developed in Nebraska are underway, says Jeff Bradshaw, extension specialist at University of Nebraska-Lincoln. “Resistance to Bt corn is as real as glyphosate-resistant Palmer amaranth and kochia — and for many of the same reasons,” he says. “We will continue to lose the effectiveness of any individual control tactic for which we rely on as our sole source of pest, pathogen, or weed control.”

Aphids feeding in corn was another big — and relatively new — issue in Iowa, says Erin Hodgson, Extension entomologist with Iowa State University. The northern half of Iowa was heavily infested from mid- to late August, she says. Many growers were not aware of this emerging pest complex and were caught off guard as fields entered the dent stage.

“Universities do not yet have a treatment threshold established for this pest, so that makes management decisions more difficult,” she admits.

Last season was a big black cutworm year in Indiana, reports Christian Krupke, Extension entomologist with Purdue University. Pheromone traps in the state alerted growers to incoming pests. Western bean cutworm moths (WBC) also made their way into the state.

Ron Hammond, Extension and research entomologist with The Ohio State University, was most interested in WBC this year, with pheromone traps in that state catching almost 4,000 adults. “We are still not seeing larval infestations, and we’re not yet seeing significant infestations like our neighbors in Michigan and Ontario,” he says.

More Corn Suffering

Goss’s wilt made a startling attack on corn last season, with concerns for a replay this year. “It was the number one conversation topic,” says Daren Mueller, Extension program specialist at Iowa State University. “We are still trying to pinpoint why we saw so much disease, but the inoculum levels have increased over the past few years. And there we some very susceptible hybrids planted in 2011.”

“Goss’s wilt was our biggest issue last year,” agrees WCC’s Kohorst. “It’s substantially affected the way we place our seed varieties for 2012, and it’s changed some growers’ tillage practices.” In fact, he says the problem has “created a kind of frenzy among the growers as they’re scrambling to figure out what they can do to prevent the disease.”

In Illinois as well, the disease was observed in more counties than ever before, reports Carl Bradley, Exten­sion specialist with the University of Illinois. The most severe cases appeared in fields that had been cropped to corn for several consecutive years, allowing build-up of the bacterial pathogen that causes Goss’s wilt, he says.

The disease has been confined to northwest Indiana so far but may be confirmed in other parts of the state this year, says Kiersten Wise, plant disease specialist, Purdue University. “Its symptoms are easy to confuse with scorch and other environmental disorders or deficiencies,” she explains. “It must be diagnosed with at least two methods — microscopic observation of bacterial streaming; and confirmation of the causal bacteria using organism-specific test strips.” These strips are marketed for field use and can be used to rule out Goss’s wilt, but if a test is positive, the sample should be sent to a diagnostic lab for confirmation, Wise says.
Both Mueller and Bradley recommend growers plant resistant hybrids the next time they put corn in infected fields. In fact, Bradley suspects releasing hybrids with better levels of Goss’s wilt resistance will now become a priority for seed companies, since they’ve seen this weakness in 2011.

In addition, Wise says tillage and rotation will help reduce the bacteria population present in the field for the following corn crop.

Overall, a major factor for disease development in 2011 was the unusual weather. In Illinois, weather patterns seemed to change drastically beginning in July for much of the state, turning very hot and dry, says Bradley. Foliar diseases did not pose a problem, but in areas in the south where susceptible varieties were grown, frogeye leaf spot caused some yield reduction.

Unfortunately, little can be done for in-season control of many diseases. “Foliar fungicides can be helpful for foliar diseases of corn and soybean, but they do not control many of the diseases that are prevalent under hot, dry conditions such as charcoal rot and Fusarium ear rot,” says Bradley.

Continued lack of moisture will be a concern for reappearances of dry-weather disease problems in 2012, including charcoal rot, Fusarium ear rot and Aspergillus ear rot (and the aflatoxin it produces), say specialists. Current commercial hybrids do not have strong genetic resistance to Aspergillus, but some biological products are available to help reduce the fungus, says Wise. Another strategy is to reduce plant stress by maintaining good plant nutrition and irrigating when possible.

Indiana did see some Diplodia and Gib­berella ear rot in April planted corn, Wise reports. A caution: She points to new research that shows the presence of western bean cutworm increases the severity of Gibberella. WBC-resistant hybrids and insecticides may be combined with fungicide applications to help control this problem, but not all fungicide products are effective (or are labeled) against Gibberella ear rot.

Soybeans Avoid Disaster

Soybean aphids were Iowa’s primary pest in 2011, but treatable infestations were generally located in the top third of the state.

“Soybean aphids should be watched for carefully in the Midwest in 2012, advises Gray. “Densities could reach economic levels, especially if we have a mild growing season.”

In Iowa, Illinois, Indiana and Ohio, soybeans did not experience any major diseases. But they won’t likely be getting a pass for 2012. This year, growers will be going back to fields planted to soybeans in 2010, a season that saw very high levels of sudden death syndrome.

Then too, if dry weather persists, soybean cyst nematode (SCN) will be “a big factor,” says Mueller. Wise recommends growers sample fields this season for SCN if they haven’t done so within the past five years. A genetic test would also be helpful, as the genetic type of SCN within the field may influence variety resistance expression, she says. Growers may need to rotate their SCN resistance source.

Although in-season weather has the largest impact on plant diseases, the mild winter seen in most of the Midwest may have allowed some pathogens to survive a little better, says Bradley. Growers may want to be on the lookout for Stewart’s wilt, a bacterial disease vectored by corn flea beetles — insects that might have survived the warmer winter.

“Because most corn these days is treated with a seed insecticide, most corn flea beetles should be kept under control,” says Ohio’s Hammond.

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