Harvest Potatoes / Powdery Mildew on Squash

Question: When are potatoes ready for harvest? D.P.

Answer: New or early potatoes can be dug seven to eight weeks after planting. The potatoes will be one to two inches in size. Potatoes are mature and full-sized when vines die, usually in late August or September. Dig and lift the potatoes out of the soil with a spading fork, taking care not to pierce the tubers.


Question: The leaves on my pattypan squash are covered with powdery mildew. Should I treat with something? B.C.

Answer: Powdery mildew is easily identified by white to light grey powdery growth on the upper, and sometimes lower, surface of leaves and the stems.

The disease is caused by several closely related fungi that live in plant debris or on infected plants. Summer squash, winter squash, cucumber, lilac, ninebark, beebalm, rose, phlox and zinnia are highly susceptible to powdery mildew. Each powdery mildew fungi is fairly host specific; meaning the powdery mildew species that affects squash does not infect roses.

Powdery mildew fungi need high humidity, but not rainfall, to infect and spread. Foggy or hazy mornings create a perfect environment for powdery mildew. Shady locations, crowded plants, or areas with poor air movement are good for disease development.

For most plants powdery mildew is primarily an aesthetic problem. In squash, pumpkins and cucumbers the disease can cause severe leaf loss resulting in reduced crop yields. Treatments are preventative but you can treat now to decrease spread of the disease to uninfected leaves. A mixture of 1 ½ tablespoons baking soda and 3 tablespoons horticultural oil (e.g. Sunspray® or summer weight oil) in 1 gallon water is effective against powdery mildew. If you prefer, several commercial fungicides are labeled for powdery mildew prevention if applied every 7 to 14 days from bud break until humid weather ends. Read and follow label directions. For your safety fungicides used on food crops must have the plant you are spraying and the disease listed on the label. Strictly adhere to the number of days between application and harvest.

At the end of the growing season, remove and destroy infected plants and plant debris to reduce number of spores next year. Usually I recommend planting resistant varieties, but only a few cultivars of zucchini, yellow crookneck and yellow straightneck squash are resistant to powdery mildew. Yellow Scalloppini is the only patty pan squash reported to have some tolerance, but not resistance, to the disease.


Barb Larson is horticulture educator for Kenosha County University of Wisconsin Extension. Barb has a Master’s of Science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email Larson at barbara.larson@kenoshacounty.org  or call her at 262-857-1942.