Garden Help Desk: Next season is the time to address powdery mildew on squash plants
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- Leaves infected with powdery mildew may turn yellow before they turn brown and crispy. When the infection is severe, like it is on these leaves, fungicidal sprays won’t be helpful for the infected leaves, but it may be useful in protecting the newer leaves if most of the plant is healthy and uninfected.
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- When a squash plant has mostly healthy leaves and just some powdery mildew spotting on some leaves, applying a fungicidal spray can stop the spread of the fungus and protect younger new leaves.
Courtesy Anna Seaver
Leaves infected with powdery mildew may turn yellow before they turn brown and crispy. When the infection is severe, like it is on these leaves, fungicidal sprays won't be helpful for the infected leaves, but it may be useful in protecting the newer leaves if most of the plant is healthy and uninfected.
My zucchini looks like it has powdery mildew. Most of the leaves have white spots and a lot of them are completely covered in white. What is the best spray to use to save the plant?
It’s powdery mildew season in local gardens with squash plants. It’s a common fungal disease that can thrive in warm growing conditions, so it typically shows up by late summer.
The early fall weather with warm days and cooler overnight temperatures in our valley make it easy for powdery mildew to bring the squash harvest to an end. Leaves that are infected by powdery mildew may not be able to produce enough carbohydrates to feed the plants and the fruits on the plants. The plants will be less vigorous, and the quality of the fruit will be reduced. Severely infected leaves will turn yellow to brown and can become crispy. They may break apart in windy weather.
At this point in the season, with leaves that are covered by the white mildew and other leaves with multiple spots and splotches like the photos I’ve included of my own zucchini, it’s unlikely that any spray will help your plant stay productive. I’ve chosen to remove my plant and clean up this part of my garden rather than try to stay ahead of the mildew.
It may be too late to rescue your own squash plant this fall, but there are things you can do next season to help your zucchini harvest last longer into the season.
Courtesy Anna Seaver
When a squash plant has mostly healthy leaves and just some powdery mildew spotting on some leaves, applying a fungicidal spray can stop the spread of the fungus and protect younger new leaves.
The first line of defense against powdery mildew is to choose varieties of squash, pumpkins and melons that are resistant to powdery mildew fungi. Seed descriptions and variety names for powdery mildew-resistant squash may include the designation PM. Resistant varieties aren’t immune to powdery mildew, but they usually become infected much more slowly than susceptible varieties.
You’ll need to search through a few seed catalogs or check online for resistant varieties. If you absolutely must have your favorite varieties, and they aren’t powdery mildew resistant, there are a few things you can do.
Space your squash and melons plants more widely in the future to slow down the spread of infection and to improve air circulation around your plants.
You’ll have fewer problems with powdery mildew if you get in the habit of going out at least every other day looking for the first signs of infection. Check both the upper and undersides of the leaves. The sooner the signs of powdery mildew are seen, the more successful you can be with control treatments.
At the first signs of infection, begin treating with fungicides right away. Remember, once the infection is severe, fungicides won’t be effective. Most of the effective fungicidal sprays work well to stop the spread of the disease if the damage is minimal, but there are only a few options that are “curative.” None of them work well once the leaves are severely affected, so the time to treat is when you see the first signs of the disease earlier in the season.
The goal with spraying is to at least protect the new growth on your plant while there is still plenty of good growing weather and possibly preserve some of the infected leaves.
There are many different brand names and active ingredients available for both organic and conventional gardeners. Look for a product that is labeled for use in vegetable gardens. The label should also include powdery mildew in the list of controlled diseases. Many fungicidal treatments need to be repeated every seven to 10 days to protect the leaves.
Read the label carefully, apply as directed and repeat as recommended on the label for the frequency and number of treatments allowed. Don’t apply any oil-based spray when your plants are drought-stressed or when you know temperatures will rise above 90 degrees before the oil can be completely dry. Oils also shouldn’t be applied within two weeks of a sulfur spray.
If you happen to find only a few infected leaves and you don’t want to use any spray, you can remove and dispose of just those leaves to slow the spread of the disease.
Powdery mildew overwinters on infected plant debris, so this fall, and every fall, clean out your garden completely. Remove or turn under any plant debris to reduce the number of spores that will be produced next year. Don’t leave any infected plant material in your garden.
