Phosphite resistance might not be to blame if potato protection is starting to lack.

One question Jeff Miller has been getting a lot recently is if phosphite resistance is a thing? He has been hearing this more often as people are using phosphites more and are starting to see more disease on their potato crops.

“The questions have been asked, are we developing resistance in the phosphites? Or perhaps is irrigation interfering with the use of our product?” Miller, president and CEO of Miller Research, says during a potato storage management workshop in Portage la Prairie, Man. on Dec. 8, 2021. “Or is the timing of application optimal? Is there a way to improve how when we make those sprays?”

To find out if this is in in fact the case, Miller and his team have completed a number of tests.

First, they experimented to see if application rates affect protection levels. Miller and his team tested in a lab to see how fungus reacts to application concentrations. At a lower rate the fungus grows but as the fungicide concentration increases the relative growth rate drops. It was found it takes almost 10 times the concentration of phosphorus acid as it does Ridomil to get the same level of growth reduction.

“The take home message here is that the phosphorus acids are weak fungicides when it comes to managing pink rot. And that’s why we have to use rates that are so high,” Miller explains. He adds current recommended usage rates are high and are necessary for adequate disease control. Grower shouldn’t use reduced rates as they won’t see as good results.

“It does suggest that or indicate that we aren’t seeing resistance to the phosphites. This also does point out that the phosphites just aren’t as strong of a fungicide as the Mefenoxam is.”

The second tests they did looked at application timing. A previous study done by Gefu Wang-Pruski with Dalhousie University found the longer the amount of time a phosphite has to absorb into a plant the more uptake there is. Miller says this shows that if you want to irrigate you need to choose an interval which will allow for phosphite uptake in the plant.

Miller has heard recommendations of waiting 48 hours after phosphite application to irrigate. However, in southern Idaho where Miller Research is based, they’re unable to wait 48 hours due to how hot it gets during the summer.

“Usually in the heat of the summer, we may be able to have the pivots off for half a day, maybe a day, we might be able to go 24 hours, but there’s no way we could go 48. So that could be a deal killer out the gate,” he says, adding this could be why growers are finding phosphites aren’t working as well.

Miller’s team then tested irrigation timing. They used a 48-hour revolution on a centre pivot. Plots throughout the field were sprayed in order for the pivot to water them at various time frames.

“What we took from this is that maybe we don’t need to wait 48 hours. In fact, it doesn’t appear that we’re gaining anything by waiting longer than 12,” he explains, adding they tried to replicate these results in future but didn’t get adequate pink rot infection in field.

Miller doesn’t doubt though there is more phosphite uptake the longer you wait to irrigate, but he found plant protection levels off at around 12 hours.

Photo — Jeff Miller, president and CEO of Miller Research, presented during a potato storage management workshop in Portage la Prairie, Man.

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