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Gardening: Summer cooldown means mildew on plants

Powdery mildew, shown here, will soon start appearing on squash, pumpkin and cucumber leaves as our weather begins to cool.  (Pat Munts/For The Spokesman-Review)
Powdery mildew, shown here, will soon start appearing on squash, pumpkin and cucumber leaves as our weather begins to cool. (Pat Munts/For The Spokesman-Review)
By Pat Munts For The Spokesman-Review

Could it be possible? The temperature is going to be in the mid-80s through next Tuesday. Traditionally we start cooling off about mid-August and it looks like we may be doing just that. Now it’s time to get out, pull all the weeds and get ready for the transition into September.

Once the nights start cooling down and we get higher humidity or some rain, powdery mildew will start taking over the leaves of squash, pumpkin, cucumbers and maybe beans, beets, carrot, eggplant, melons, parsnips, peppers, tomatillo, tomatoes and turnips. Powdery mildew is a fungus that develops when the weather cools in the late summer and early fall. It appears as a powdery gray or whitish haze on plant leaves as the temperatures swing from cool nights followed by warm days. Over time the fungus can create a thick cover of powder. The powder is made up of spores that are shedding and spreading to other plants. Once it is established, it can’t be removed and can reduce the leaf’s ability to photosynthesize. If you want to control it, you need to start treating it now before it appears.

Overfertilization can stimulate new growth which is more susceptible to the fungus. Powdery mildew is more likely if your plants are crowded or the air circulation among them is poor. There are many cultural ways to prevent the mildew from establishing itself. Watering with overhead sprinklers might help. The water on the leaves helps wash off the spores before they can settle in. Do this is the morning so the plants have a chance to dry off during the day.

You can treat the susceptible plants with a commercial organic fungicide that contains sulfur as the active ingredient. If the box stores don’t have it, check with the independent garden centers or order online. If home remedies are more your style, there are some simple sprays that can be made from household products. They can be used as a preventative or on existing infections.

Sodium bicarbonate (baking soda) and potassium bicarbonate can be mixed and applied as a preventative. Potassium bicarbonate is the same as low-sodium baking powder. Mix one tablespoon of the baking soda and ½ teaspoon liquid soap such as Castile or Dawn dishwashing soap in a gallon of water. Spray the plants liberally, soaking the top and bottom leaf surfaces and any affected stems or fruit. Repeat applications once a week.

A milk-based spray works especially well on zucchini, melons and cucumbers. Mix one part milk to two to three parts water and spray liberally as above. In all these sprays, it is thought that they change the chemistry of the leaf surface making it inhospitable to powdery mildew. Neem oil by itself won’t treat the mildew but when a couple of tablespoons are added to these sprays, it can give them an extra boost.

Use a handheld sprayer that hasn’t been used for other chemicals and soak the leaves thoroughly through the last of the harvest.

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