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Drought Impact Part Two: Check Fields for Insect and Disease Damage

Damage by Japanese beetles. Photo by J. Obermeyer, courtesy of Purdue University.
Damage by Japanese beetles. Photo by J. Obermeyer, courtesy of Purdue University.

Earlier this week, Bob Streit, owner of Central Iowa Agronomics in Boone, Iowa, reported on drought conditions in central Iowa. In this post, he discusses weed, insect and disease observations. It’s part of Case IH’s efforts to help you Be Ready to meet challenges as they occur.

Weeds have been extremely tough to kill this year. Farmers need to recognize that to prevent early-season weeds, they have to apply herbicide before weeds germinate, which is around the first week of April. This year, they germinated about March 23.

Insect Activity
Corn borer moths have been nearly nonexistent, but we’ve seen heavy infestations of hard-shell insects, like rootworm and Japanese beetles. They went after the silks and threatened corn pollination. In our area, there have been almost no aphids but we’ve seen lots of spider mites. If you find three silver-dollar-size colonies on a corn-plant leaf at the ear leaf or above, you may want to consider pulling the trigger on spraying for spider mites.

In soybeans, treatment is recommended if there is a moderate population of spider mites in the middle of the plant and there are mites moving into the top third. Normally, a fungus called Neozygites wipes out spider mites, but the dry weather has minimized its presence. The important fact about spider mites relates to their ability to multiply 40-fold every four days. As a result, a field’s mite population can explode in nine days. In 1988, the difference between spraying and not spraying an infested soybean fields was 12 to 18 bushels per acre.

We have also seen thrips (Order Thysanoptera), which are tiny, slender, cigar-shaped insects with fringed wings that live on the bottom of soybean leaves. We’re counting 100 to 200 per leaflet, which can translate into more than 4,000 per plant. Universities haven’t yet determined a threshold for thrips in the Midwest, but in Argentina the insects that did the most damage to beans during their 2011/2012 drought were those sap-sucking insects. If farmers spray for spider mites, they should obtain control of the thrips as well.

Photo of a spider mite, courtesy of University of Minnesota Extension.

Additional insect pressure is widespread. Japanese beetles are still a problem. They like to congregate in areas receiving illumination, such as fields near town or well-lit farmsteads. Their leaf-feeding habits can be damaging to soybeans, while their silk-feeding tendencies can hurt corn pollination.

Rootworm challenges continue for many growers raising second- and even first-year corn. In many cases, the traited corn did not stop their feeding. As a result, the reduced root mass left the plants more susceptible to drought stress, lodging and rotted roots. Silk-feeding during the pollination phase also caused damaged to corn that was not being monitored closely enough to observe beetles feeding on their favorite food source. Long term, the jury is still out on what management program is best for controlling the insect. Regardless, it appears that beetle management will need to supplement any current program.

Disease Challenges
Trained crop scouts still see evidence of Goss’s Wilt, but there’s disagreement of what it is and how it travels. Both the leaf and stalk symptoms are less prevalent than in 2011. If we receive rains, the added moisture could cause symptoms to increase and the disease would be more apparent. Having the disease reduce the effectiveness of moisture transport during a moisture-stressed year was something we feared. Several products are being applied this summer in documented, replicated university and grower trials to validate their efficacy against Goss’s Wilt.

Root diseases have been a problem this season, too. Even from emergence time, we have been digging and seeing browned and blackened roots. So far, common diseases like Fusarium and Pythium have been diagnosed. In other cases, the symptomology doesn’t seem to fit any particular disease description.

Good Planning is Key
It’s important for farmers to monitor their fields year-on-year for long-term planning. Knowing what challenges your crops have withstood over the past three to five years will help you plan ahead for future challenges in terms of the seed traits and crop protection products you need. Keep in mind that certain varieties of seed corn were in short supply this spring. With the weather challenges this summer, expect more of the same from your suppliers for 2013.

It appears farmers with richer soil (i.e., more carbon and better water-retention qualities) and who have developed soil for the deepest, healthiest root systems, are faring better in the drought conditions. We are also seeing that the application of the newer biologicals and micronutrient mixes are helping with plant health and are mitigating a portion of the dry weather problems. We have generally remained patient optimists and think many fields still have the potential for 160- to 200-bushel yields. However, those fields will still need at least four to six inches of rain to realize that potential. At this point, one inch equates to an increase of about 12 bushels of corn or four bushels of soybeans.

For more information, email Bob or visit his Facebook page. How would you rate your corn and bean crop? What insect challenges do you see and how are you dealing with them? Your comments could help other producers.

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