Squash Problems Gone Wild, Or yes, Scouting in the Garden Matters

By Joan Allen

This week’s blog photo is from my vegetable garden. I give lectures on the fundamentals of integrated pest management (IPM) and one of the first practices on the list is to scout or monitor your garden or field frequently to catch problems early, when you have a much better chance of keeping damage to a minimum.

Well, I am well aware of this recommendation, but failed to do it in my own backyard. So what happened? My zucchini has succumbed to hordes of squash bugs and both that and the cucumbers have been hit hard by powdery mildew.

Let’s start with how to check for these two major cucurbit problems, even though it’s too late this year in many gardens. In the case of squash bugs, look for the reddish groups of eggs on the undersides of the leaves. If found, squash them. You’d think this would be easy, although a bit disgusting, but they’re pretty tough. Squeeze hard. This can be done with nymphs and adults (shown in photo) too, or they can be drowned in soapy water. In the photo, there’s an adult in the upper left area of the group with a nymph on its back. Adults will have mature wings.

There are also some insecticidal products that are labeled for squash bug but they vary in effectiveness. Get the nymphs with insecticidal soaps. Avoid using any insecticidal products when bees/pollinators are active. The little yellow flecks on the leaves in the photo are caused by squash bug feeding. They pierce the plant cells and suck out the contents through straw-like mouth parts. You can see how all of these steps would be easier if you took the time to check early on, before the problem became all too obvious. Favored host plants are squash and pumpkins but other cucurbits may also be attacked. There is one generation per year and the adults overwinter in sheltered places.

Powdery mildew is a very common and damaging disease of cucurbit crops. As powdery mildew progresses, covering both upper and lower leaf surfaces with a white powdery growth, photosynthesis is reduced, impairing growth and reducing yield. Plants typically become infected around the time fruit begins to form and mostly on the older leaves.  The powdery mildew that affects cucurbits is not the same fungus that causes powdery mildew on other, unrelated, plants. Disease is favored by warm temperatures and high humidity. Promote good airflow around plants to reduce humidity using ample plant spacing or vertical supports for vines. Spores are wind-borne and can travel long distances so crop rotation is not effective in this case.

Good scouting for those first small white spots will alert you that it’s time to apply preventive fungicides. There are a variety of active ingredient options here including organic options such as biological fungicides (Bacillus subtilus QST 713), potassium bicarbonate, copper, or wettable sulfur. Conventional fungicides include chlorothalonil and others. These are preventive and must be used to protect plants, not as a curative solution. An alternative and effective solution is to look for powdery mildew resistant varieties for your garden.