How to protect vining vegetables from powdery mildew

powdery mildew on zucchini
Zucchini plant with powdery mildew. Photo: Marissa Schuh, University of Minnesota Extension.

Powdery mildew is one of the most common diseases in the vegetable garden and, contrary to what you may think, it’s not dependent on wet conditions to take hold and spread.

The leaves of afflicted plants will appear to have been dusted with flour. Summer and winter squash varieties, melons, cucumbers and pumpkins are susceptible to powdery mildew.

The impact of powdery mildew can range from slightly reduced yields from a single plant to spreading throughout your garden across multiple crops, depending on spacing and regular maintenance.

Powdery Mildew

Powdery mildew is a fungal plant disease that thrives in hot, dry weather common in mid to late summer. The fungi grow on the surface of plant leaves, clogging pores and blocking the sunlight plants need to thrive.

It can be introduced to your garden from purchased plants or from spores in the soil. Once established, it spreads via wind, insects and garden tools and hands or gloves that have come into contact with infected plants and then touched healthy plants.

Powdery mildew first appears on leaves as whitish to grayish blotches that are dry to the touch. It will then spread quickly, covering most of the affected leaves and then the stems, as well. Infected leaves will take on a dull white cast and become brittle before curling up and falling off. If powdery mildew is allowed to spread to the entire plant, it will die.

In squash varieties with natural white markings on the leaves, you can determine whether or not plants are infected with powdery mildew by rubbing the leaves. If a white powder comes off on your fingers when you rub them, your plants have been infected.

Plants with powdery mildew will produce fewer and smaller fruit. Fruit may also become scorched as leaves fall off and they are exposed to too much sun.

Preventing powdery mildew

It may be cliche, but it’s no less true — an ounce of prevention is worth a pound of cure. Preventing powdery mildew from infecting your plants is the best way to limit the damage it can cause.

Rotate your crops. Powdery mildew spores can overwinter in garden soil. You can prevent last year’s spores from affecting this year’s crop by planting vining vegetables in different locations of your garden on a three- to four-year rotation schedule.

Space your plants out. Following seed spacing recommendations, considering the space plants need at their mature size and ensuring good air circulation will reduce the spread of powdery mildew and increase the amount of light your plants’ leaves get.

Choose a sunny spot. Powdery mildew spore germination increases in the shade. You can reduce disease establishment and spread by planting squash and other susceptible vegetable varieties in full sun.

Spray plants with water. Powdery mildew spread rapidly in hot dry weather. You can reduce disease development by spraying plants with a hose or overhead watering them. Doing this in the morning is ideal so the leaves have time to dry off before night, which can encourage other plant diseases to take hold.

Avoid overfertilizing. Overfertilizing encourages foliage growth, making plants bushy and susceptible to infection.

Plant powdery-mildew-resistant varieties. Some vining vegetable varieties are resistant to powdery mildew. Those seed packets will be marked with “PMR” to indicate they are powdery mildew resistant.

Getting rid of powdery mildew

Powdery mildew is common and spreads quickly. Gardeners should monitor their vining plants regularly in order to act on the first signs of disease.

Infected leaves. Remove infected leaves at the first sign of powdery white to grayish spots.

Severely infected plants. Remove severely infected plants and plants that don’t seem to be recovering well to avoid the spread of powdery mildew to surrounding plants. Make sure to put severely infected plants in the trash and not the compost pile. Burning infected plants and debris is another option.

Don’t touch healthy plants after touching diseased plants. Removing leaves and plants infected with powdery mildew can help get rid of the disease. However, coming into contact with healthy plants with gloves, hands or tools used to tend to unhealthy plants can further spread the disease. You should wash your hands and disinfect your tools after touching infected plants.

Minimize the disease with organic sprays. Organic sprays are most useful when they’re used early to address a powdery mildew infection. They won’t cure severe infections. Neem oil is one of the most effective sprays for mild to moderate infections. Infected leaves should be sprayed on both the upper and lower surfaces every week until there are no more symptoms and every other week after that to prevent reestablishment. Only spray when temperatures are below 90F and when bees aren’t active. Sulfer sprays and stylet-oil fungicides can also be effective in treating powdery mildew.



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