All Eyes on Frogeye

Frogeye leaf spot (Cercospora sojina), a once-obscure foliar disease, has become recognized as a serious threat to soybean yield in the Midwest and South. Adding to the disease’s notoriety, the University of Illinois recently discovered that some isolates of frogeye leaf spot are resistant to the popular strobilurin class of fungicides.

Carl Bradley, plant pathologist at the University of Illinois, confirmed strobilurin resistance in samples of the frogeye pathogen collected last season from two fields in Tennessee, one field in western Kentucky and two fields in southern Illinois. Compared to susceptible populations, the resistant isolates required from 200 to 7,000 times greater concentrations of strobilurin fungicide to control spore germination. Strobilurins, also called QoI inhibitors, include azoxystrobin (Quadris), pyraclostrobin (Headline), fluoxastrobin (Evito), trifloxystrobin (one of the compounds in Stratego, which also contains propiconazole, a triazole fungicide). “For a long time, frogeye was not considered to be a major yield robber,” says Jason Bond, associate professor of plant pathology at Southern Illinois University. “But this was one of the diseases that, even with a low level of infestation, caused yield loss. And in some of the fungicide treatment plots, when disease was moderate to severe, we saw 10-to-16-bushel yield losses. It can rob a lot of yield.”  

Regional Study

Bond led a five-year, 12-state study of frogeye leaf spot and charcoal rot that began in 2006. Through the project, funded by the North Central Soybean Research Program (NCSRP), Bond and his colleagues screened 600 lines of commercially available soybean germplasm for resistance to the two diseases, and conducted extensive fungicide field trials in an effort to improve frogeye control. Bond says roughly 100 of the soybean lines showed at least some resistance to the frogeye pathogen. A resistance gene called Rcs3, found in 13 of the tested lines, is the most effective, fighting off all of the known races of the pathogen. Lines containing Rcs1 and Rcs2 are each susceptible to some races of C. sojina, of which dozens have been identified worldwide. Public breeders in Missouri and Illinois are working to breed Rcs3 into elite soybean lines. In the meantime, probably two-thirds of the time, other sources of resistance would be OK,” Bond says. “However, we don’t have a complete picture of the races in production fields.”  

A Perfect Storm

A management plan that starts with a frogeye-resistant soybean variety will be increasingly important as strobilurin resistance becomes more widespread. Strobilurins act at one crucial site in fungi’s cells, wrecking the power plant that helps the pathogen generate energy. But it takes a substitution of just one amino acid for another in the fungus’ DNA to render the fungicide ineffective. So far, scientists have identified three of those swaps in dozens of pathogens, from Alternaria in potatoes, tomatoes, pistachios and apples to Septoria in wheat, and classified the strobilurin class of chemistry a high risk for resistance. To make matters worse, Cercospora sojina is renowned among plant pathologists for its genetic variability. Attacking a mutation-prone fungus with a single-site fungicide creates a sort of “perfect storm,” – the conditions for big trouble. “It’s seemingly happened overnight, but that’s probably not accurate,” notes Melvin A. Newman, Extension plant pathologist at the University of Tennessee’s West Tennessee Research and Education Center in Jackson, Tenn., who found the first resistant frogeye isolates in a grower’s field. “I think the genetics were already there and we’re just bringing it out by spraying. It’s not just a Tennessee thing, and it’s not that the farmer has done something wrong spraying the strobilurin.” Bradley says plant pathologists believe the frogeye pathogen does not tend to move long distances from season to season. If that holds true, the bad news is that spraying the same population again and again with a favorite fungicide creates high selection pressure. But the good news is that a resistant population emerging in one area is less likely to blow into distant fields. Bradley notes that more research is needed to fully understand if the resistant isolates identified from the different locations came from a single source, or if they were selected independently from local sources.  

2011 Recommendations

If strobilurin resistance is indeed emerging in isolated populations, growers should adhere to established frogeye leaf spot management strategies and pay close attention to the results, say the experts. Here’s a frogeye leaf spot checklist:

• Consider whether frogeye is likely to be a problem at all.
• On river-bottom fields and other damp areas where frogeye tends to thrive, start by planting a resistant variety
• Pay special attention to frogeye leaf spot in early-planted fields
• Response to fungicides is generally greater in those fields
• Scout
• If frogeye symptoms appear or warm, wet conditions favor the development of the disease, apply a fungicide.
• An application at the R3 stage of growth – the beginning pod stage – tends to be most effective

“I don’t think farmers will make their problem worse by putting out a strobilurin in 2011,” Bond says. “I think there’s probably a very high percentage that will still get the same level of control as they have been getting in previous years.” Newman agrees. “If you still want to spray your strobilurins, I have no problem with that, especially if producers use a frogeye-resistant variety,” he says. “I would be careful to watch the crop to make sure you’re getting good control of frogeye. And if you’ve experienced problems with control, then switch to another class of chemistry, like the triazoles.” If you switch to a triazole or a strobilurin-triazole mix, warns Bradley, just be careful to pick a triazole that controls frogeye. Researchers will be scrutinizing frogeye fields this year in search of insight on the genetic variability and the virulence of the pathogen, and they’ll be tracking the development of fungicide resistance. For growers, says Newman, the big change for 2011 will be staying on guard for signs of resistance, especially as high soybean prices increase the likelihood that growers will be spraying to protect yield potential. “It’s not the end of the world,” Newman says. “The sky’s not falling in. But it behooves us to pay close attention to frogeye in the field to make sure we know what’s happening.”