Powdery Mildew

Leaves infected with powdery mildew initially have small, white powdery colonies on the undersides of leaves. These colonies enlarge to cover the entire lower leaf surface, causing the edges of the leaves to roll up. 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 and support spore-producing colonies that look powdery and white.

The disease overwinters as mycelium on leaves in California, so it is most likely introduced into the field through planting material or spores from neighboring fields. Spores are wind disseminated and short-lived. The pathogen also survives as mycelium and chasmothecia (closed spore-bearing structures) on plants coming from nurseries. Ideal conditions for infection are dry leaf surfaces, high relative humidity, and cool to warm air temperatures. Accordingly, the disease is mostly limited to the coastal growing regions and northern nurseries and causes very little damage in inland growing regions. Powdery mildew is particularly severe in greenhouse and plastic tunnel production systems.

During routine field surveys, watch for the leaf distortion and discoloration that are the first signs of powdery mildew, especially in fall and spring. To control powdery mildew, apply fungicides when disease is first detected. This is especially important for protectants such as sulfur. Controlling powdery mildew in the fall reduces the amount of disease that develops the following spring, and controlling foliar disease helps prevent fruit infections. The standard nursery practice of removing leaves from transplants during harvest and packing helps minimize introduction of the disease, although inoculum may still be present on crowns. Cultural practices are important in helping to prevent disease buildup.

Avoid overhead irrigation and excess use of nitrogen. Use less-susceptible cultivars, such as Albion, San Andreas, and Fronteras, where practical.

Apply mined sulfur or insecticidal soap on organically certified strawberries. Use less-susceptible cultivars where practical. Select field sites where environmental conditions are not conducive to disease development.

Apply fungicides about 1 month after planting and again 3 to 4 weeks later. Make additional fungicide applications when plants begin to bloom.