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 at other times of the year.

Powdery mildew on butternut squash

​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 grows on both sides of the leaves and sometimes on the stems.

What is powdery mildew?

The white powder on leaves is thin layers of fungal tissue (mycelium). Other symptoms of powdery mildew include yellowing leaves, leaf distortion and death, and yellow patches on leaves (especially on tomatoes, peppers, onions, and artichokes). Cucumber, melon and other cucurbits are also susceptible to powdery mildew. You may also see it on peas, artichoke, beets, grapes and wheat.

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 do not trigger these fungal beasties

How does powdery mildew grow?

The only thing powdery mildew fungi need to survive and thrive is living plant tissue. Spores can travel in the wind, so the battle never ends. These microorganisms prefer shade and temperatures between 60°F to 80°F, so the disease may seem to disappear in summer, but don't be misled. Spores remain dormant until conditions improve.

Powdery mildew weakens a plant, lowering production and increasing the susceptibility to other pests and diseases such as citrus blast. Leaf drop can also lead to sunburn damage.

Powdery mildew management

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

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

Biological fungicides can also be applied, though they are less effective than oil or sulfur. Some people recommend spraying plants with a baking soda solution, but this can add too much salt to your soil.

Recent research shows that spraying the soil with milk before planting can reduce powdery mildew, followed by spraying the leaves of susceptible plants.