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What's up for corn and soybean diseases

The telltale small yellow lesions are early symptoms of frogeye leaf spot. This disease, once mainly limited to Southern soybeans, is increasing in the Midwest.

Diseases are stealthy yield robbers that can plague your corn and soybeans. Fortunately, steps like scouting and fungicide use can curb their impact. Here are some answers to questions about diseases and ways to control them.

1. Disease thresholds: do they exist?

Brown spot, frogeye leaf spot, and Cercospora leaf spot are all soybean diseases that can be curbed by fungicides.

But there are no thresholds developed for these diseases, says Mark Carlton, Iowa State University (ISU) Extension field agronomist. Start scouting at R1 (beginning flowering) and watch for disease movement and rainy conditions with high humidity. These factors can increase the odds of a positive payback from fungicides of up to 8 to 10 bushels per acre. Fields with continuous no-till beans, particularly with frogeye leafspot, also favor a fungicide payoff.

In corn, the threshold for gray leaf spot (GLS), developed in the mid-1990s, consists of lesions on the third leaf below the ear leaf or higher occurring on 50% of the plants at tasseling. This threshold is being reevaluated because of the new hybrids and fungicide products that have been developed since then. For now, however, the threshold stands.

It's also important to consider high-risk cases, such as fields in continuous corn with high residue in river bottom areas, says Carlton. Hybrid ratings for GLS resistance should also be considered.

“Timing is important,” says Carlton. He advises applying fungicides following tasseling and continuing into the R2 (blister) stage.

2. Will fungicides aid hail-damaged corn?

BASF officials say Headline fungicide products can do it. BASF obtained a supplemental label for Headline from federal regulators in 2009 that includes benefits other than fungal disease control, including better hail tolerance.

Yield responses on hail-damaged corn from Headline can coincide with 12- to 16-bushel-per-acre responses that BASF says it consistently sees from Headline applications on corn, says Nick Fassler, BASF technical market manager for fungicides.

Responses hinge upon yield potential and the severity of plant damage. Headline won't help corn with heavily stripped leaves. It is beneficial, though, to less damaged plants. It can protect those new leaves forming below the tassel that contribute to yield during grain fill, says Fassler.

University of Illinois (U of I) tests in 2007 and 2008 showed no interaction between simulated hail damage and foliar fungicide for yield or foliar diseases severity. Scientists simulated hail-damaged corn with a grass trimmer. Researchers applied two strobilurin fungicides – Headline and Quadris – at tasseling and compared it against simulated damaged corn with no strobilurin fungicide application.

“No interaction between simulated hail damage and foliar fungicide was observed for yield or foliar disease severity,” says Carl Bradley, U of I Extension plant pathologist.

He recommends hail tolerance be taken out of the equation for considering fungicide applications Instead, focus on factors like disease risk factors and disease observation.

“Hail will not influence fungal disease pressure,” he says. “Fungi already can penetrate the leaves without any help from hail. One exception is common smut, but common smut is not listed on the (Headline) label.”

3. Are any new action modes set for 2012?

One new fungicide on tap for 2012 from BASF is Xemium fungicide, pending regulatory approval. It belongs to a fungicide class called carboxamides. Xemium has a mode of action called a succinate dehydrogenase inhibitor (SDHI). Carboxamides, new to the corn and soybean market, give farmers another option to the strobilurin and triazole fungicides they now use.

Xemium will be marketed under several different brand names. Priaxor will be the foliar fungicide brand you'll see in corn, soybeans, canola, and sunflowers. It will feature a 2:1 ratio of F500 and Xemium. F500 is the active ingredient in BASF's Headline strobilurin fungicide.

Strobilurin fungicide-resistant Cercospora sojina, the causal agent of frogeye leaf spot in soybeans, surfaced in 2010 and 2011 in fields in Tennessee, Kentucky, Missouri, and Illinois.

One way to forestall resistance is to use multiple modes of action. BASF officials say the dual modes of action in Priaxor are a step in this direction.

Systiva will be the seed treatment formulation of Xemium. Proposed uses are for corn, soybeans, wheat, and other crops.

4. Can the fungicide application window be widened?

Fungicides perform best when applied at recommended times. In the case of BASF's Headline, company officials say the optimal time to apply Headline in corn is the VT through R2 stages (full tassel through blister).

Aerial applicators face a tight time schedule trying to cover all fields during this time. BASF is researching formulations designed for chemigation and encapsulation that could help widen this application window.

Large-scale application trials are again slated for 2012 for slow-release technologies containing Headline that would help widen the current application window.

“It is too early to declare victory,” says Markus Heldt, president of BASF's crop protection division. However, the technology is promising and may appear on the market in several years, he says.

5. What's this winter' s disease impact?

One downside to this mild winter is enhanced survival for insects that normally don't survive overwintering. The outlook for this year's insect pests is hard to say, notes Bradley. However, it's possible more Stewart's wilt may be observed in corn.

Stewart's wilt is a bacterial disease transmitted to corn plants by the corn flea beetle. There is a close relationship to Stewart's wilt observations and the number of flea beetles that successfully survive the winter, Bradley says. In the mild winter that we've observed so far, it's likely that flea beetle survival will be high, which could relate to a higher incidence of Stewart's wilt in the 2012 season.

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