KANANASKIS, Alta. – A new way to manage disease in potatoes might be needed as more cases of fungicide resistance are reported, says a Manitoba potato specialist.
“Resistance can develop by the intensive use or misuse of fungicides,” Tracy Shinners-Carnelly told the Potato Growers of Alberta annual meeting in Kananaskis Nov. 14.
Fungicide resistance is similar to that found in weeds and insects, developing over time with repeated use of the same chemistry.
“It is not possible to prevent resistance but our goal has to be to delay it as long as we can so we can keep these fungicides effective as long as possible,” she said.
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Some fungicide groups are categorized as high risk products that may only attack a problem with a single mode of action. Others are multi-mode and attack a number of different biological systems within the fungus.
Shinners-Carnelly said each label has a fungicide group code so it is important to read the instructions and learn the group’s resistance rating.
The National Potato Council in the United States has published a full list of products, their chemistries and common names of products on its website.
She said farmers need to integrate cultural practices with fungicide use, including lengthened crop rotations, disease forecasting tools, regular field scouting, volunteer plant management and elimination of disease sources by changing how culled potato piles are handled.
“When you have a healthier crop, it is more able to withstand some of the pressures we put on it, like disease,” she said.
Fungicides work best when they are applied preventively before the disease is a real problem.
Resistance occurs because a naturally resistant spore may exist within any population of spores.
That spore survives the fungicide application, reproduces and spreads disease.
After a couple of generations the resistant population is much larger and may outnumber the susceptible ones so the fungicide does not appear to work anymore.
“The re-application of the same chemistry is perhaps pushing that population to be the resistant one,” Shinners-Carnelly said.
Nothing is 100 percent effective but using alternating chemistries, crop rotations or tank mixes combining multi-site products could worsen or improve the problem of resistance.
“Here in Canada, the way our products are labelled and the way tank mixes are registered don’t always support good resistance management practices,” she said.
However, tank mixes using two different chemistries with different modes of action could get the job done.
One product may kill 85 percent of the spores and the other may get the remainder.
Because it is hard to know what rate will work, farmers sometimes try to save money by using fungicides at below the recommended dose, which can result in inadequate coverage and missed disease.
An example of how resistance grows is found in the United States, where a highly effective chemical against early blight was repeatedly used in one season in the late 1990s.
It had a single mode of action and resistance appeared in one year’s time.
Researchers collected more than 4,500 samples from 11 potato growing states and found the resistant strain in 96.5 percent of the samples.
Idaho, Washington, Oregon and Wyoming did not use the product as extensively as some other areas yet the resistant varieties were found in 12 to 60 percent of the samples.
