Powdery Mildew on Zucchini and Cucumbers

Powdery mildew usually begins as pale spots, then forms white to gray powder on leaf surfaces. It can also show up on petioles and stems. Check older, shaded leaves as well as the undersides. On zucchini and other squash, rub a suspicious patch with your thumb. Mildew is superficial and leaves a powdery residue. Natural silver markings are part of the leaf tissue and will not rub off. Cucumber leaves do not have the same silver pattern.

On cucumbers, downy mildew makes yellow patches boxed in by the leaf veins. Dark gray to purplish fuzz may appear underneath. That pattern can move quickly, as shown in Penn State's powdery and downy mildew guide.

A little mildew on an old leaf near the end of the season is mostly a nuisance. It matters more when it reaches clean leaves or the canopy starts dying back. Heavy infection weakens the plant and can leave fruit exposed to direct sun. That sudden exposure can cause sunscald, with bleached or blistered skin and poor-quality fruit.

Powdery mildew does not need wet foliage to infect. Spores travel on dry air, yet the disease can grow quickly where humid air stays trapped inside a crowded canopy. That often happens overnight around dense leaves. Give zucchini room. Keep weeds from closing in. A cucumber trellis helps where it suits the bed. An open canopy will not erase disease already on the leaves, but it makes new growth easier to inspect and slows the damp, stagnant conditions mildew likes.

Maryland Extension notes that powdery mildew is unlike many plant diseases because wet foliage does not encourage it. UC IPM also notes that it can develop under warm, dry conditions.

When the infection is still confined to a few leaves, cut off the worst ones if the plant has plenty of healthy foliage left. Bag them for municipal green waste if your area accepts diseased plants. Otherwise, put them in the trash. Do not leave the leaves in a pile beside the bed, and do not strip a productive zucchini or cucumber down to bare stems just to make it look clean.

Oregon State Extension recommends taking infected plant material out of the garden instead of relying on a cool home compost pile to kill spores.

Treatment makes the most sense when a light infection is spreading on a plant that still has a useful harvest ahead. These sprays protect clean foliage and can slow new infections. They will not restore a leaf that is brown or badly covered. Products for home gardeners may contain potassium bicarbonate, wettable sulfur, horticultural oil, or a biofungicide. Choose one currently labeled for edible cucurbits and powdery mildew. Sulfur can scorch foliage in hot weather and may conflict with oil applications, so follow the product label rather than using a fixed temperature rule. Skip homemade baking-soda mixes. A labeled potassium bicarbonate product gives you crop-specific directions and an appropriate strength.

University of Minnesota Extension advises starting treatment when disease is first found. Maryland Extension notes that horticultural oil products are labeled for powdery mildew on cucurbits.

Start with a resistant variety when one fits your crop. Leave zucchini enough room and keep weeds from closing the canopy. For cucumbers, a trellis makes leaves easier to inspect. At season's end, clear diseased vines from the bed. Crop rotation can diversify a small garden, but wind-borne spores mean it is not a stand-alone powdery mildew cure.

University of Minnesota Extension supports resistant varieties and an open, well-weeded canopy as home-garden measures.

• University of Minnesota Extension: Powdery mildew of cucurbits
• University of Maryland Extension: Powdery mildew on vegetables
• UC IPM: Powdery mildew on cucurbits
• Penn State Extension: Downy and powdery mildew in the home garden
• Oregon State Extension: Powdery mildew cleanup