Powdery mildew is a common disease problem on squash, cucumbers, melons and many plants. It's really a common problem late in the summer when you get toward the end of August. With dewy nights and temperatures between 60 and 80 degrees, it is the perfect situation for this fungus to take over.
And it's ubiquitous in the environment, meaning it's just out there and there's not a lot you can do to prevent it! Don't worry if you start getting it in late August and September. The plants are winding down anyway. But if it's happening earlier in the season, especially if it's on roses, phlox, lilac, birch trees, other kinds of plants that also get powdery mildew, then you need to control it.
There's two different things you can do. One, you can be using baking soda. One tablespoon of baking soda in one gallon of water, with a little dash of liquid soap is the recipe. Just spray it on the leaves early in the season as a preventative measure.
If you have phlox or bee balm, for example, you can also use resistant varieties like 'David' and 'Orange Perfection' phlox, and 'Marshall's Delight' bee balm.
The other option is to spray Serenade. It's Bacillus subtilis. This bacteria fights the fungus. Spray this, again, just as the disease is starting and it will slow it down. And all you really want to do is slow it down enough that you can get those melons and squash and cucumbers throughout the season.
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