Powdery Mildew

Spring is the time of year when it is common to see white powdery patches appear on leaves. This bane of gardeners is called powdery mildew, and it can occur in autumn, as well.

Powdery mildew on butternut squash (Kate Russell)

Cucumber, melon and other cucurbits are susceptible to powdery mildew. You may also see it on tomatoes, roses, snapdragons, chrysanthemums, peas, artichoke, beets, grapes and wheat. 


What starts as a small white spot, powdery mildew expands to engulf an entire leaf as the nutrient-sucking fungi bleed the life from your garden. It can be found on either side of a leaf and sometimes on stems.


Powdery mildew identification

Powdery mildew is a fungal disease. It is caused by different types of fungi (e.g., Erysiphe spp., Sphaerotheca spp.), depending on which plant is affected. Contrary to common belief, moisture and humidity are not needed for these fungal beasties to appear.


The white powder seen on leaves is actually thin layers of fungal tissue (mycelium). Other symptoms of powdery mildew include:

• yellowing leaves
• leaf die off
• distorted leaves
• yellow patches on leaves (especially on tomatoes, peppers, onions and artichoke)

How does powdery mildew grow?

Powdery mildew fungi simply need living plant tissue to survive and thrive. To make matters worse, their spores are carried on the wind, so the battle never ends. The reason powdery mildew seems to disappear in the heat of summer is that these microorganisms prefer shade and temperatures between 60° to 80°F. Our California summers are simply too hot for the spores to reproduce. Instead, they remain dormant until conditions improve.


Not only does powdery mildew cause leaf loss, it can also weaken a plant. This lowers production and increases susceptibility to other pests and diseases, such as citrus blast. Leaf drop can also lead to sunburn damage.


Powdery mildew management
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Prevention and vigilance are the best ways to counteract powdery mildew. These tips can help, but nothing will eliminate powdery mildew in the garden:

• Plant resistant varieties
• Change the species of annual plantings to break the fungal life cycle
• Monitor plants for new spots of white mycelium and remove infested leaves
• Plant in sunny areas
• Provide good air circulation (use spacing and pruning)
• Avoid applying excess fertilizer
• Apply horticultural oil (not within 4 weeks of applying sulfur, or your plants will BURN)
• Apply sulphur products before symptoms appear (only when temperatures are below 90°F)
• Apply biological fungicide (not as effective as oil or sulphur)
• Apply fungicides, as a last resort

Some people recommend spraying plants with a baking soda solution, but this can add too much salt to your soil. 


More recent research shows that spraying the soil with milk before planting and then spraying the leaves of susceptible plants when the disease is most likely to start appearing can significantly reduce the incidence of powdery mildew in your garden.