Popular fungicides failing, may cause hard choices for apple growers
Janna Beckerman said apple scab is now resistant to popular fungicides in Indiana and Michigan. Orchard growers will have to change management techniques. (Purdue Agricultural Communication photo/Tom Campbell)
WEST LAFAYETTE, Ind. - Orchard growers have started finding that some of the most commonly used fungicides are no longer effective at controlling apple scab, according to a Purdue University study.
Janna Beckerman, an associate professor of botany and plant pathology, said that extensive, long-term use of four popular fungicides has led to resistances in apples in Indiana and Michigan, the focus of her study.
"The fungicides that are regularly used to control scab have started to fail," said Beckerman, whose findings were published in the early online version of the journal Plant Disease. "But the most disturbing thing we found is that many of the samples we tested were resistant to all four fungicides. It's kind of like multidrug resistance in antibiotics. This is full-blown resistance."
Apple scab, caused by the fungus Venturia inaequalis, is highly destructive to apples, causing brown lesions on leaves and fruit. A single lesion can reduce an apple's value by 85 percent. Over time, the scabby lesion will crack and allow insects, other fungi and bacteria inside, causing a loss of the crop.
"It can cause orchard failures," Beckerman said. "An orchard grower that has this could lose blocks of an orchard, or depending on the amount of diversity in the orchard, they could lose the entire crop."
It's thought that when organisms adapt to form resistance, that change will weaken the organism in some other way. Beckerman said the study, done with Purdue graduate student Kim Chapman and Michigan State University professor George Sundin, showed apple scab, on the contrary, is becoming resistant to fungicides with no apparent fitness penalty to itself.
"Having these multiple resistances to fungicides doesn't debilitate them in any way," Beckerman said.
Apple scab samples were treated with dodine, kresoxim-methyl, myclobutanil or thiophanate-methyl. About 12 percent of the apple scab samples tested was resistant to all four fungicides.
The only options apple growers have, Beckerman said, is to use older fungicides that are tightly regulated, require more frequent application and are more expensive.
"It's going to change how growers manage their orchards," Beckerman said. "The more susceptible apple cultivars, like McIntosh, will become more chemically intensive to manage. Growers have few options as it is, and this will limit their options even further."
Beckerman said she and her collaborators would work to develop faster tests to detect fungicide resistance in apple scab to help growers change management plans in a timely manner. The U.S. Department of Agriculture, Purdue University and the Michigan Agricultural Experiment Station funded the research.
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Identification of Resistance to Multiple Fungicides in Field Populations of Venturia Inaequalis
Chapman, Kimberly S., Sundin, George W., and Beckerman, Janna L.
Venturia inaequalis, the causal agent of apple scab, is controlled primarily by fungicides. Long-term, extensive fungicide use has led to the development of resistance to multiple fungicides. To assess fungicide resistance, isolates of V. inaequalis were collected from Indiana and Michigan orchards. Single-spore derived isolates were evaluated by mycelium growth assays with previously determined fungicide baseline concentrations and corresponding thresholds for growth on media containing dodine, kresoxim-methyl, myclobutanil or thiophanate-methyl. Of 195 isolates tested, 5.2%, 0.7%, 57.0% and 92.6% of isolates were found to be resistant to dodine, kresoxim-methyl, myclobutanil and thiophanate-methyl, respectively. This is the first report of kresoxim-methyl field resistance in these states. Isolates resistant or shifted to a single fungicide were often found to have multiple fungicide resistance. Of all isolates tested, 38% were identified as resistant or shifted to two fungicides and 12% were resistant or shifted to all four fungicides tested. No fitness penalty was found for isolates resistant to multiple fungicides based on a statistical analysis of mycelial growth and conidial production. Fungicide resistance results in fewer effective fungicide modes of action for apple scab management; however, one option may be difenoconazole, which was found to inhibit growth of myclobutanil-resistant or shifted isolates.