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Watch where you put less-resistant hybrids

You’re scouting an irrigated cornfield the first week of July. Gray leaf spot is off to a healthy start. Then you check the disease rating chart for that hybrid and see that it rates 6 of 9 for gray leaf spot, with 1 being most tolerant and 9 being susceptible.

One farmer ran into exactly that situation in ’09. Fortunately, the coolest July ever stymied gray leaf spot and let his hybrid escape yield loss.

How to interpret disease ratings from seed companies is the subject of this month’s Crops Corner. Panelists include members of the Indiana Certified Crop Adviser’s group: Greg Kneubuhler, G & K Concepts Inc., Harlan; Jeff Nagel with Ceres Solutions, Lafayette; and Darrell Shemwell, Posey County Co-op branch manager, Poseyville.

I’m sorting through seed literature, making final decisions on which hybrids go where. My seedsman recommends two hybrids for irrigated ground. One rates a 4 for gray leaf spot on a 1-9 scale, with 1 being most tolerant. The other rates 6 of 9. Are these good choices for that field?

KNEUBUHLER: The fungus, cercospora, survives on corn residue. Spores surviving in the spring infect the new crop. Favorable conditions include warm, humid weather with long dew periods. Conditions typically present themselves around late July and August. If the previous crop was corn and it exhibited gray leaf spot, plant a variety with a better rating.

If it’s first year corn, the potential for gray leaf spot is not as great, although weather conditions may increase its effects. It’s very important to get with your seedsman to place varieties. They change so frequently that selecting them can be very challenging.

Also, be cautious reading (seed) literature. Ratings systems vary from company to company. A rating of 4 with one company may not be the same as a rating of 4 with another company.

NAGEL: Selection and placement of hybrids is important to help optimize performance. Disease ratings are one aspect of hybrid selection. Your seed specialist is likely also looking at other factors to guide his recommendation.

The hybrid with the lower rating (a 6 here) is more likely to be impacted by gray leaf spot if favorable environmental conditions develop during the growing season. Monitor the field for disease. Make an informed decision to apply a foliar fungicide if return on investment looks favorable.

Monitor for all diseases, not just gray leaf spot. The cool, wet summer of 2009 was favorable for Northern corn leaf blight. Some hybrids were quite susceptible.

SHEMWELL: I’m assuming the previous crop on this irrigated ground was corn. If this is the case, the hybrid with a 4 rating would be the best choice for the field, if the two hybrids have equal yield capabilities. Usually gray leaf spot has the potential to be more severe on corn following corn.

However, this doesn’t mean that a hybrid with a lesser (not as tolerant) rating can’t be planted on second-year cornfields. If you’re planting either one of these hybrids under irrigation, you should strongly consider spraying them at the early tasseling stage with a fungicide.


TOUGH COMPETITOR: Panelists agree you need a plan in place if you plant hybrids that are moderately tolerant to moderately susceptible to gray leaf spot.

This article published in the January, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.