Because of the dry weather, there have been few foliar disease problems on corn and soybeans during the 2012 growing season in Iowa—so far anyway. Across the state many farmers and agronomists are asking if they should apply a fungicide on corn or soybeans. Also, some farmers are asking when to apply it. Like the past few years, there have been very little early season foliar problems on both crops.
In soybeans, frogeye leaf spot, Cercospora leaf blight, and brown spot have been reported on seedlings. Brown spot is really the only foliar disease still being reported, although soybeans usually grow out of any early season infection. Likewise in corn, very few foliar diseases have been found prior to tasseling this summer.
Iowa State University Extension plant pathologists Alison Robertson and Daren Mueller have written several previous articles, including 2011 summaries for corn and soybean, fungicide application timing for soybean and a summary of responses on corn. This year they are receiving questions about spraying fungicides on drought stressed crops and also about applying half rates of fungicides. The two ISU crop disease specialists address both of these topics in the following article.
Should you apply fungicide to crops when there are no disease symptoms?
"The greatest responses to foliar fungicides we have seen over the past five years in Iowa have been when disease could impact yield of corn or soybeans," says Mueller. Similar data have been reported from the north central region and even nationally.
"We have found however, that when applying foliar fungicides in absence of significant disease, selecting the right product may make a small difference," he adds. "In a four-year study funded through the Iowa Soybean Association and soybean checkoff involving hundreds of treatments on soybean across more than 20 total locations, we found that products containing the strobilurin fungicide had a greater response in the absence of disease (see accompanying chart). When disease was present, there were no differences between products containing a strobilurin, a triazole, premixes of a strobilurin and a triazole, or a carboximide."
ISU experts hesitate to recommend application on drought-stressed crops
For drought-stressed corn and soybean crops, there is limited scientifically valid data indicating if foliar fungicides will improve water use efficiency and increase yields, say Robertson and Mueller. One published report on grapevine shows that strobilurins have no effect on water use efficiency (Diaz-Espejo, 2012). Considering the biology of pathogens and the plants, drought stress will reduce the chances of foliar fungal pathogens becoming established. Drought-stressed plants will also have lower yield potential. "Both of these facts give us pause in recommending fungicides for drought-stressed crops," says Mueller.
Another question: Are diseases developing resistance to foliar fungicides?
A caution when spraying fungicides – you should scout fields after applying the fungicides and look for applications that do not appear to affect disease severity. The potential for diseases to develop resistance to fungicides is real, say Mueller and Robertson.
Resistance to the strobilurin fungicides (Headline, Quadris, one of two active ingredients in Stratego YLD and Quilt) in the fungus that causes frogeye leaf spot was reported in Tennessee, Kentucky and Illinois in 2010. Resistant strains of the fungus were again found in 2011 in the same states and also Missouri. Mueller and Robertson say reduced sensitivity to strobilurin fungicides has been reported for more than 30 pathogens and is often detected within a few years of the product being used on a crop.
What about applying half-rates of fungicides instead of the full labeled rate?
There is a reason why chemical companies recommend the application rates that are on labels. Research indicates that the recommended rate is most effective against targeted pathogens, but risk management also is strongly considered.
"That's the second topic we are currently getting questions from farmers about--Is it a good idea to apply a half rate of fungicides?" says Mueller. "When half rates of fungicides are used, this increases the chances of fungi being able to survive these applications, and consequently increases the chances of fungicide-resistant strains of the fungus developing and becoming established. We encourage all farmers and agronomists to follow label rates for application, to ensure that these highly effective disease management tools will be around for a long time."