Signs of powdery mildew are a characteristic curling of leaves which is followed by appearance of a white powdery coating on the underside. Purple-reddish blotches appear on the upper and lower surface of leaves.
Infected flowers produce deformed fruit or no fruit at all. Severely infected flowers may be completely covered by mycelium and killed. Infected immature fruits become hardened and desiccated. Infected mature fruits become seedy in appearance with white powdery mycelium evident.
Powdery mildew can overwinter on leaves of remnant plants or may be introduced on planting material.
Spores are spread by wind. Ideal conditions for infection are dry leaf surfaces, high relative humidity, and cool to warm air temperatures. Overhead irrigation, rain, and dew inhibit the development and spread of this disease, so this pathogen tends to cause more problems in greenhouses and high tunnels.
Resistance to fungicides is an issue with powdery mildew. Sulphur may be used as a preventative.
To control powdery mildew, apply fungicides at the first sign of disease, that is leaf distortion and discoloration. Under high tunnels, targetted, intermittent brief applications of overhead irrigation may help prevent the spread of this disease but need to be weighed up against the risk of increasing Botrytis.
In general, avoid overhead irrigation and excessive use of nitrogen fertiliser.
For the latest information on chemicals registered for control of powdery mildew, consult the APVMA website.