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Ask Your PCA: How should strawberry growers manage powdery mildew?

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Issue Date: May 12, 2021
Gary Omori PCA, AG RX, Oxnard
Gary Omori PCA, AG RX, Oxnard

Powdery mildew can be a problem throughout the season in strawberries. Ideal conditions for infection are dry leaf surfaces, high relative humidity and cool to warm air temperatures. These conditions will increase problems with powdery mildew.

Initially, the leaves will become infected with powdery mildew and have small, white, powdery colonies on the underside. Untreated, they will enlarge to cover the entire lower leaf surface and cause the edges of the leaves to roll up. Purple-reddish blotches will appear on the upper and lower surfaces of leaves.

Infected flowers produce deformed fruit or no fruit at all. Infected immature fruits will become hardened and shriveled. If severely infected, the plant can die.

Powdery mildew overwinters as mycelium—the vegetative part of a fungus—on the leaves. The pathogen also survives as mycelium and chasmothecia—closed, spore-bearing structures—from nursery plants. In California, the coastal growing regions and northern nurseries are more susceptible to the disease, and it is particularly severe in greenhouse and plastic tunnel systems.

It is advised to monitor for powdery mildew. The first signs of the disease are leaf distortion and discoloration.

Apply fungicides as soon as the disease is detected. This is especially important for sulfur applications. Fall control of powdery mildew is also advised, to reduce disease development the following spring.

Good cultural practices will help prevent disease buildup, too: Avoid overhead irrigation and excess use of nitrogen, and use less-susceptible varieties when possible.

Currently, there are enough chemicals available to allow for rotation and reduce the risk of resistance building. It's important to manage available chemicals and use cultural control methods—all the tools in the toolbox—to prevent resistance from building.

Calibrate equipment and change nozzles to ensure the best possible application. Good coverage is important, especially if there are limited opportunities to make a spray application.

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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