In 2010, the first reports of strobilurin-resistant frogeye leaf spot emerged out of Tennessee and Illinois. A year later, the resistant fungal disease was found in Missouri, Kentucky and Louisiana.
Alert plant pathologists were on the lookout for the strobilurin-resistant disease and, in 2012, it was discovered in Arkansas fields.
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“The difference between Arkansas and some of the other cases was we saw field-wide failures,” says Travis Faske, University of Arkansas plant pathologist. “Strobilurins – like Quadris or Headline – were applied and the frogeye kept increasing in severity. The fungicides just wouldn’t shut the frogeye down. We collected isolates from those hard-hit fields and confirmed strobilurin resistance in the lab.”
Faske and colleagues initially reported that the strobilurin-resistant frogeye had been identified in seven counties. Recently, their lab confirmed it in another two counties.
“It was identified all the way from Clay County in the north all the way down to Chicot County in the south. Thus, it is widely distributed.
“The most severe yield loss you could experience from this would come along with premature defoliation. That would happen at about 30 percent disease severity.”
Most of the affected fields Faske went into had about 15 to 18 percent disease severity.
“If you compound that with some of the other fungal diseases in the field, there was probably some early defoliation in isolated cases. Strobilurin-resistant frogeye leaf spot certainly contributed to increased disease severity although it’s hard to say exactly how much yield loss that caused.”
Most of the producers that recognized strobilurins didn’t work early on, did apply a second fungicide, a triazole. “With that second shot, they found the frogeye didn’t progress.”
The resistance is a result of classic selection pressure over time, says Faske. “It’s so unfortunate because strobilurins are good fungicides.”
Among Faske’s other comments:
On severity and management…
“I’ve done some sensitivity assays with strobilurin fungicides and our isolates have a similar EC50 (effective concentration to kill 50 percent of the resistant isolates) to what’s being seen in Tennessee. Also, based on molecular assays, the mutation in (Arkansas) isolates is similar to that in Tennessee and Illinois.
“There are ways to tackle this. You can use host-plant resistance and there’s quite a few varieties out there that are resistant to the disease. The University of Arkansas – backed by the soybean check-off program – screen about 300 varieties of soybeans every year at the nursery in Newport, Ark. We put the results on www.arkansasvarietytesting.com . Host plant resistance is the most effective and economical tactic to manage this problem.
“As far as cultural practices to combat resistant frogeye, producers should know the fungus overwinters in crop residue. Tilling in the crop residue reduces the innoculum amount. Crop rotation does the same thing.
“Row-spacing can also make a difference. Planting beans in rows rather than drilling allows for air flow, which tends to reduce frogeye infection.”
“Thankfully, some of the fungicides do still work. The triazoles are effective – some a bit more than others.
“Since this problem has only been around since 2010, limited trials have been done. We’ll know more in coming years what fungicides have the best efficacy for isolates found in Arkansas.
“Growers will continue to plant susceptible varieties. They’ll likely use triazole fungicides or, at least, a mixture – something with a strobilurin along with a triazole for control.”
On 2013 expectations…
“Of course, we don’t know if 2013 will be a frogeye year, or not.
“Most of the time frogeye seems to relatively consistent in the Arkansas River Valley in comparison to the the Delta region. In 2012, conditions were more favorable for frogeye in the Delta than in the past. Resistant isolates were in the area at a low population but increased in 2012 with more reproduction due to favorable disease conditions.
“Awareness of resistant frogeye is on the upswing. Those who experienced the lack of response to strobilurins were alarmed and letting others know. Word-of-mouth has moved pretty fast.”
Plant pathologists “are doing our best to let folks know, as well. It’s a topic at every grower meeting I attend this year. I think we’ve done fairly well in getting the word out on this.
A warning not to misuse triazoles…
“Folks should understand there is triazole resistance in the fungal genera Cercospora. That’s nothing new. It’s in peanut and sugar beets and is a bit harder to detect because the resistance is quantitative. The problem develops slowly, over time as the population of resistant isolates builds up.
“If we abuse the triazoles and resistance develops we could be in a situation with almost no effective fungicides.
“One important thing: some of the triazoles do cause some plant injury. That injury can look very similar to Sudden Death Syndrome (SDS). If someone applies a triazole and the entire field looks as if it has SDS that may be due to phytotoxicity. The severity of that reaction differs between varieties and triazole fungicides.”