What is Powdery Mildew? 

If you want to know how to get rid of powdery mildew, it’s helpful to know what it is. Powdery mildew is a fungal disease that forms on the foliage of plants. In the vegetable garden, powdery mildew is a common disease on cucumbers, squash, tomatoes, and greens. Although these crops are its favorite prey, powdery mildew can be a disease for any plant, given the right conditions. 

Powdery Mildew in Zones 9 and 10

Powdery mildew is widespread in Zones 9 and 10 and any area that has coastal influence and fog. Powdery mildew thrives in moderate temperatures of 60-80 degrees. In Zones 9 and 10, you can find powdery mildew year-round. It can be particularly damaging to your summer crop of squash, cucumbers, and tomatoes. Because it is so common in the vegetable garden, if you want to know how to get rid of powdery mildew, you must understand how to identify it and manage or prevent this damaging disease. 

Powdery Mildew on Tomato Leaves

Powdery mildew causes irregular yellow blotches on tomato leaves and white “powdery” looking growth on its stems and older growth. If you are Growing Tomatoes in the Fog Belt, you will find this a common issue. Luckily, if you identify the disease early on and manage it quickly, you can still get a bountiful harvest from your garden. 

White powdery growth on leaves and shoots can be a sign of powdery mildew. This disease affects many plants, and one of several fungi can cause it. Manage powdery mildew by growing resistant plant varieties and altering the growing environment. In some situations, fungicide treatments might be required for susceptible plant species. 

The UC IPM Powdery Mildew Quick Tips makes these suggestions for the management of the disease. 

Powdery Mildew on Squash Leaves

Most new gardeners have their first run-in with powdery mildew on their squash leaves. The small white blotches start on the leaves’ undersides or older growth and soon cover the whole plant. If the disease is not managed early in its growth stage, it can quickly overwhelm a gardener and take over the entire squash plant. It’s important to note that a garden with powdery mildew can still be productive and happy. It’s up to the gardener to identify the disease early and manage it for the best outcome. 

Alter the Growing Environment to Make Plants Less Susceptible

Powdery mildew, like any plant disease, requires a specific growing environment to thrive. By changing the growing environment to an environment less favorable to powdery mildew, gardeners can decrease the disease’s occurrence. Here are some quick tips on how to get rid of powdery mildew. 

• Grow plants in sunny locations. 
• Provide good air circulation by pruning excess foliage. 
• Fertilize appropriately; too much nitrogen causes lush foliage and shade, providing conditions for fungal growth. 

How to Treat Powdery Mildew

The first thing for a gardener to do after identifying they have powdery mildew is to alter the growing environment. Not sure you have powdery mildew? See this helpful UC IPM link: Powdery Mildew on Vegetables. 

In any organic gardening or growing practice, you want to use Integrated Pest Management as a tool to manage disease and pests. This method starts with the most environmentally friendly option to control pests and diseases. Learn more about UC IPM here. 

When you identify that you have powdery mildew, start with non-chemical actions. First, alter the growing environment to ensure that the disease can not thrive. Next, use these cultural methods. 

• Wash spores off infected plants with overhead sprinkling. To prevent other disease problems, do this midmorning, so moisture dries rapidly. 
• Prune out small infestations and remove infected buds during the dormant season. Quickly remove infected materials so you don’t spread spores. Do not compost diseased plants—trash and clean pruners after. 

For suggestions to control severe powdery mildew using commercial oils or plant oils such as neem oil, sulfur, or biological fungicides, see the UC IPM link’s recommendations: Powdery Mildew on Vegetables.  

Another resource is How to Treat Powdery Mildew on Plants, which includes some additional explanations about conditions that promote powdery mildew growth.