With expanding herbicide and corn-rootworm trait resistance, why let fungicide resistance get a seat at the table? Fungicide resistance in corn has yet to be identified. Checking resistance at the gate makes good agronomic and economic sense.
"You only need two ingredients for fungicide resistance to develop: .mutations and selection pressure," says Paul Vincelli, Extension professor, Plant Pathology, University of Kentucky. "Mutations are out of the grower's control. He won't know they are there until he starts using a fungicide. It is the fungicide application that selects for resistance."
Strains of the Cercospora fungus that causes frogeye leaf spot resistant to strobilurins have been found in soybeans in eight states, including southern Illinois, Kentucky, Tennessee, Indiana and North Carolina. Researchers know what to expect with other diseases and in other crops based on that experience.
"We now have some good tools for looking at genetic diversity," explains Carl Bradley, associate professor, Plant Pathology, University of Illinois. "We found that resistance appeared as individual selections, not the spread of a resistant isolate. It was the same mutation, but the fungicide-resistant Cercospora isolates were genetically diverse."
As corn growers increasingly use fungicides to improve standability, as opposed to disease protection, the likelihood of similar resistance developing in corn diseases increases, Vincelli says. Data suggests fungicides can enhance stalk strength in higher plant populations, he acknowledges. But, at what cost to the bottom line and enhanced resistance risk?
Bradley advises reviewing data carefully when considering suggested benefits. He points to research conducted since 2008 at 33 different Illinois with multiple sites at each location. While results indicate a 5.8-bushel per acre average return, that number alone can be misleading. He reminds growers it reflects a range of returns across all locations and sites.
"We looked at the amount of disease present and the response at each site," he says. "Compared to the untreated check, we saw low disease pressure (10% leaf area with lesions) and a 3-bushel response in 20 out of 33 locations. In those with greater than 10% disease severity, there was a 10.2-bushel response. The primary yield response comes from controlling disease," Bradley says.
Corn breeders have provided a soft landing when resistance does develop, Vincelli says. "Breeders have done a good job selecting for resistance to common pathogen outbreaks of gray leaf spot and northern leaf spot," he says. "I don't expect large-scale damage when resistance does develop, as long as breeders continue to select for disease resistance. If we get fungicide resistance and we don't have high levels of disease resistance in the crop, that could be a problem."
Bradley points out most hybrids recommended in central Illinois have some level of disease resistance. However, companies vary in how they report resistance levels.
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“You may see a hybrid with a moderate resistance rating, but if you read comments that it is not recommended for corn on corn, it could be because of susceptibility," he says.
The disease susceptibility level can influence yield response and implied benefits from fungicides. Bradley describes the potential for a higher yield response from a racehorse hybrid without a good disease-resistance package, but high yield potential, versus applying a fungicide to a workhorse hybrid with higher disease resistance.
Another facet to current fungicide use that may reduce or at least delay the risk of resistance development is the trend toward multiple modes of action in products. "We haven't seen resistance building with multiple modes yet," says Bradley. "We’ve been monitoring sensitivity to the strobilurins, which have the highest risk of developing resistance, and so far we've seen no resistance in gray leaf spot or northern corn leaf blight."
Evaluate application decision
Tight profit margins in 2014 may play an important role in setting back fungicide resistance potential in corn. While mutations may still occur, selection pressure may take a breather as growers reconsider "insurance" applications. Bradley points out even the 6-bushel return, while profitable in recent years, might have been considerably less profitable in 2013, depending on when corn was sold. He recommends carefully evaluating risk in 2014.
"Make your application decision during the growing season based on risk of weather pattern, hybrid susceptibility and whether the field is in rotation or continuous corn, which has a higher risk of foliar disease," Bradley says. "It will help with reducing potential fungicide resistance as well as the bottom line."
In 2011, Vincelli reported a 20-bushel increase with a single fungicide application in two of three field-size trials in the absence of disease pressure. In 2013, he worked with three large grain producers on commercial-scale, replicated trials that used a single fungicide, but included other products. In the face of low to moderate disease pressures, increases varied from six to 12 bushels. At $4 corn, he estimates the six-bushel increase would not likely have covered costs and even the 12-bushel increase would not have exceeded costs by a great deal.
"Fungicides are most likely to provide a benefit under moderate to severe pressure, but substantial increases are sometimes possible even in the absence of significant disease pressure," says Vincelli. "If undecided, leave several untreated strips in a treated field and see for yourself if yield increases under your farming conditions."