MISSISSIPPI STATE – A statewide survey that Mississippi State University began this summer will continue next year as researchers look for a particular fungal disease that is developing resistance to chemical control.

In 2012, soybean fields in two Mississippi counties were found to have frogeye leaf spot fungi resistant to strobilurins, the class of fungicides commonly used for late-season disease management in soybeans.

Jeff Standish, an MSU graduate student in plant pathology, is involved in the ongoing project to track the distribution of frogeye leaf spot and Cercospora sojina, the fungus that causes it. The Mississippi Soybean Promotion Board funds this study through the Mississippi Agricultural and Forestry Experiment Station.

“I am testing samples for strobilurin resistance, and if I find resistant samples, I will sequence the gene to positively confirm the presence of fungicide resistance,” Standish said.

This year, Standish collected 128 samples of frogeye leaf spot from 49 Mississippi counties where producers observed the disease. He collected the greatest number of samples from Bolivar County, the state’s biggest soybean producer. Frogeye was not observed in many central and southeastern counties.

“Samples were returned to Starkville and cleaned and dried to remove dirt and insects,” Standish said. “I put half of the leaves from each sample in a moist chamber to promote fungal production of spores. The other half was preserved for future analysis if needed.”

Standish took steps in the lab to isolate the pathogen so fungicide-sensitivity analyses can be made.

Trent Irby, soybean specialist with the MSU Extension Service, said frogeye leaf spot can be a significant disease of soybeans.

“Frogeye leaf spot attacks the leaves, and percent yield reductions can be directly correlated to the percentage of the leaves covered with frogeye leaf spot,” Irby said. “This disease can have a major impact on soybeans, but generally, it is more of a problem in fields planted to a susceptible variety.”
Tom Allen, Extension plant pathologist at MSU’s Delta Research and Extension Center in Stoneville, said the fungicide screening program was initiated in Starkville and Stoneville during the 2013 growing season.

“In the past, strobilurin fungicides have been suggested as a normal management practice and were applied at R3, an important reproductive growth stage,” Allen said. “In trials conducted during the 2013 soybean season, fungicides with more of an ability to treat the disease – notably the triazole class of fungicides – were more effective at reducing yield loss when applied in the presence of frogeye.”

Allen said continued monitoring for the presence of strobilurin resistance in frogeye leaf spot will provide soybean producers with valuable information to help them manage the disease and reduce the likelihood of yield loss or crop failure.

Maria Tomaso-Peterson, a MAFES researcher and Standish’s major advisor, said the purpose of this research is to get ahead of fungicide resistance in this disease before it develops.

“We’re being proactive in identifying whether or not we have strains of this fungus that are resistant to the strobilurin fungicides, and if so, to what extent,” Tomaso-Peterson said. “The fungal population progressively develops resistance, allowing the resistant spores to increase in numbers throughout a soybean field. This is called practical resistance.”

The next step will be to develop a fungicide program that helps slow down the development of practical resistance to the fungicide.

Fungi have a higher likelihood of becoming resistant to strobilurin fungicides because this class of chemicals has what is known as a “single-site mode of action,” which means it has only one way of attacking disease-causing organisms.

“Strobilurin fungicides target fungal respiration, so we will use a rotation of fungicides and tank mixes that include different classes of fungicides,” Tomaso-Peterson said. “We also will suggest planting resistant varieties, and where practical, using crop rotation, so soybeans are not planted on the same land each year.”

Tomaso-Peterson said strobilurin resistance in turf grass pathogens took only a few years to develop, but specialty crops such as turf grass require many more fungicide applications each year than do row crops. So far, it has taken more than 15 years for fungicide resistance to manifest in soybean diseases.
“We suspect there is probably practical resistance to this fungicide, and Jeff is doing this research to give us a better idea of the scope of the problem,” she said.