Researchers looking to stem powdery mildew on cherries

Powdery mildew can be seen on these cherries, causing them to be unsuitable for export markets.

PROSSER — Powdery mildew on agricultural products can cause economic hardship for growers looking to export their products.

Some countries will reject products like cherries if they are found to have powdery mildew, Dr. Claudia Probst of the Washington State University Extension Office said.

That is why she and researchers at the extension service are looking at the effects of different fungicides at different stages of crop growth.

“We looked at timing,” Probst said, adding three fungicides have been used on different cherry blocks. Another block has been used as a control.

The fungicides used for the research, which is in its second year, include Fontalis, Quintec and Pristine. Researchers have also tested the effects of stylet oil post-harvest and Luna Sensation in the nursery.

Researchers discovered there is no ideal timing for the application of fungicides. But, combining or rotating applications are more effective than using one specific fungicide.

“Growers like to apply fungicides on a rotation,” Probst said.

She said applying stylet oil three times post-harvest was effective in reducing chasmothecia, when spores prepare to over-winter.

The applications took place once every two weeks, Probst said.

“We saw some success, but not much, unfortunately,” she said.

The rotational block on which Quintec and Fontalis was applied showed the greatest reduction of powdery mildew infection, she said.

“But there is a difference in how fungicides treat either fruit or leaf infections,” Probst said.

Pristine, for example, was only consistent in reducing fruit infection. “It’s a combination fungicide,” she said.

That means it uses different components, spurring additional study in the nursery. In the third year of research, Probst said researchers will be studying the effects of the components in Pristine separately, as well as a combination fungicide.

Nursery studies have already resulted in findings that Luna Sensation reduces chasmothecia. That is important because it means fewer spores are released at the beginning of the season, when the irrigation is turned on, Probst said.

She said Luna Sensation will be studied in the orchard in the coming year to determine whether results in the nursery translate to mature trees.

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