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Raise, Grow, Harvest, Eat, Repeat

A blog for growers, consumers, and backyard gardeners to grow, eat, and connect in the local food system.

The Mildews of Summer

Last summer, I focused on profiling tomato blights which you can still find here. As we've dealt with a rainy, colder June and now July so far, it may be that we start to see mildews particularly downy and powdery. Both of these mildews can cause problems for many plants but many times we only consider it targeting the cucurbits like pumpkin, squash, cucumber, watermelon, and other ones. Let's discuss mildews further.

Like other plant diseases, the mildews are caused by a pathogen. Downy mildew is caused by a water mold while powdery mildew is caused by a fungus. Both mildews do similar damage. They invade healthy plant tissue of the plant causing it to start breaking down. You may see a lot of this on the large cucurbit leaves in the beginning as yellowing. The yellowing will be scattered on the leaves. If it is downy mildew, these yellowing symptoms will be on the underside of the leaves.

Pictured below is downy mildew. While the yellowing starts on the underside of the leaf, as it worsens it can be noticeable on the top of the leaf.


Powdery mildew like the name entails produces powdery like symptoms on the top of the leaves and other parts of the plant. Pictured below is the powdery mildew.


Both of these mildews require humid conditions. Air temperature plays a large role in the spread and development of the mildews. Downy mildew thrives in cooler temperatures near 65 F compared to powdery mildew needing 80 F. Since we've had cooler, wetter weather in June, it's possible that we may be seeing downy mildew right now. It can be common to have both of these mildews in a growing season.

Preventing downy and powdery mildew begins at planting. If you have proper spacing between plants, this allows for good air flow that can keep the mildews from spreading. This allows plants to dry out after rains and keep them from having wet foliage for prolonged periods. There are some varieties that are disease resistant to these mildews. If you spot powdery mildew in your field, sometimes you can remove the foliage and still have a productive plant. If the plant has succumbed to the disease, you want to remove it from the field as it is a breeding ground for the disease. With these cucurbits, it is important to be mindful of working in the fields when it is too wet as you could be spreading disease from plant to plant. Chemical control is always an option.


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