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Jeff Miller will be presenting his latest study on pink rot management at PAA's annual meeting this summer. Photo: Jeff Miller/Miller Research

Fine-Tuning Pink Rot Fungicide Timing

Jeff Miller, a researcher with Miller Research in Idaho, will be presenting his latest study on management of pink rot (Phytophthora erythroseptica) at the 2019 Potato Association of America (PAA) annual meeting.

This persistent soil-borne disease causes tuber breakdown and yield loss. Miller’s field trial is looking at the effect of the timing interval between spraying a phosphite fungicide for pink rot suppression and applying irrigation water.

Miller provides some background information to put his study in context. “For many growers in Idaho and in other places, pink rot of potato is a pretty serious disease problem. It is controlled by a number of methods including irrigation management and crop rotation. But with some potato varieties that are really susceptible, the use of pesticides is required,” he says.

“One fungicide that has been very effective is metalaxyl or mefenoxam [Ridomil]. It has been a pretty easy way to manage the disease; growers could put it on once in-furrow or perhaps twice on the foliage to control the disease.” But, over time, mefenoxam-resistant pink rot has become more and more common in potato-producing regions.

“One of the next best options is phosphite-based fungicide products. However, phosphites are not as good as the old mefenoxam-based products. They are very weak as fungicides, so you have to use pretty high rates and apply them frequently.”

One question around phosphite performance is whether these fungicides would work better if the potato plants were given more time to absorb the fungicide before the treated crop is irrigated or receives precipitation. For example, research by some scientists in Canada found it takes about 48 hours to get almost the full uptake of a phosphite application.

“Let’s say a farmer sprays the phosphite fungicide and then turns their irrigation system on a day later. Is that irrigation going to wash off some of the fungicide before the plant can absorb it?” asks Miller.

So, with funding from the Northwest Potato Research Consortium, Miller is leading a two-year trial to answer the question: Do growers need to wait 48 hours from the time they apply a phosphite fungicide to when they irrigate?

“Our experiment was really quite simple,” he says. “When it came time to make applications of the fungicide based on the growth stage of the plant, we knew when the irrigation was going to go over the trial, so we spaced the fungicide applications at 48, 24, 12 and 6 hours before that irrigation. At the end of the season we measured how much pink rot developed.”

At the PAA meeting, Miller will be presenting the findings from 2018, the trial’s first year. These findings are good news for irrigation farmers — and for dryland farmers trying to time their phosphite applications with the weather forecast in mind.

“We had significant control of the disease if our fungicides were applied 12 hours or more before the irrigation. At six hours the results weren’t as good; in fact, it looked like there was a reduction in performance. But we didn’t see a difference between the 12-, the 24- and the 48-hour time intervals,” he says.

“Especially here in Idaho, we have a very arid environment and growers’ centre pivots are going almost non-stop at times. If growers had to wait 48 hours after spraying the fungicide before they water, that would have created quite a hardship for them,” Miller explains.

“But we’re saying you don’t have to wait that long. If you can give it a day, please do, but you don’t have to. You just need to wait 12 hours, half a day, before you turn the irrigation back on, and most growers can do that.”

 

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