July 14, 2011 – 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.
July 14, 2011 – 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.
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| Janna Beckerman said apple scab is now resistant to popular fungicides in Indiana and Michigan. Orchard growers will have to change management techniques. (Credit: Purdue Agricultural Communication photo/Tom Campbell) |
“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 multi-drug 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
per cent. 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 per cent 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.
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