Silver scurf relieves prairie potato producers of their gold.
“Like many places, we can have some pretty gnarly potatoes in Saskatchewan if we don’t look after our disease issues,” said plant pathologist Jill Thompson of the University of Saskatchewan.
Thompson spoke about the problem at the Saskatchewan Seed Potato Growers Association annual conference in Saskatoon Nov. 29.
The fungus thrives in warm, moist conditions, just like those typically found in the first three months of storage after harvesting potatoes or after bagging fresh packed tubers.
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The fungus first appears as a tan or silver-grey patchiness of the skin. It turns into a sooty coating and in extreme situations causes the skin to be sloughed off.
“Consumers don’t want to buy mouldy potatoes,” she said.
As a result, those potatoes aren’t sent to stores and become a loss for producers. In some cases, seed potato growers are being asked to test contamination levels of their product.
It is a challenging problem because the fungus is widely distributed in the soil.
“It’s a growing problem. I mean it’s alive and the problem is on the rise,” Thompson said.
But she said there are several points in the production chain where farmers can shut down the disease process.
It starts with seed treatments.
Idaho studies showed that multiple fungicide applications are the most effective and they reduce the development of pathogen resistance.
She recommends mixing or alternating fungicides such as Mertect SC, 500 grams per litre, thiabendazole, and Senator PSPT, 10 percent thiophanate methyl.
Producers can also alternate between these and Maxim PSP, 0.5 percent fludioxonil.
“Using combinations and alternating (fungicides) is critical to preventing resistance. And from what we know about dry rot, we will see resistance,” said Thompson.
Soils with higher than average levels of iron, copper or nitrate seem to show limited natural resistance to the fungus.
Studies in Quebec indicate soil chemistry can limit spore viability and help organisms that are potentially harmful to the fungus.
“Unfortunately we are still a long way from being able to offer a solution based on this idea. But it does give us options when it comes to research paths to follow,” she said.
After seed treatment, harvest management can be one of the lower cost methods of controlling fungus.
Potatoes that are stored dry and in cool conditions tend not to develop the disease.
“It is counter to the way we manage potatoes today, but it might be worth a try when it’s possible,” she said.
Storage facilities should be well cleaned and treated with quaternary ammonium compounds, with the doors closed, for several days.
The doors should then be opened and the space thoroughly dried before more potatoes are stored.
Thompson recommends producers separate early and late crops, including isolating air flows that could carry spores between inventories.
Biosafe StorOx, a bactericidal, fungicidal potato storage treatment based on hydrogen peroxide and acetic acid, can be sprayed onto potatoes entering storage or added to the ventilation system.
This treatment will be licensed early next year.
In high dosages and with continuous application, the low-cost system has had some success in controlling fusarium dry rot, bacterial soft rot and silver scurf, said Thompson.
“Companies and researchers are working on the problem. We may even see (resistant potato varieties) some day, but it won’t be anytime soon. So producers will need to learn to manage the problem for the foreseeable future,” she said.
