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Common Squash Problems

As usual, if there's a particular pest you'd like me to address, just drop me a line at shell@michellesgarden.com.

Squash are frost-tender annuals grown for their fleshy fruit. Summer squash, such as zucchini, are eaten before the seeds and rinds harden. Winter squash are harvested after the fruit is mature.


Squash seeds need 60 degree soil to germinate. Cover plants with floating row cover to protect them from insects and late cold snaps. Remove row cover when plants begin to flower so insects can pollinate the blossoms, or you will not get any fruit.

Squash do best in well-drained, loose-textured soils with lots of organic matter. They will grow in soild with a pH between 5.5 and 6.8, but prefer a pH above 6.0. Squash need lots of water, but don't let soil become saturated. Prevent disease problems by keeping the leaves dry. Mulch squash to help conserve water. Black plastic is a good choice for northern areas, but in the extremely warm areas it can warm the soil too much. Organic mulches are good, too, but may provide shelter for pests like squash bugs. Foil mulches help prevent aphid problems. To prevent rot, support fruit on scraps of wood.

Rotate crops so that no member of the cucurbit family (cucumbers, melons, and squash) is grown in the same place more often than every 4 years.

Squash leaves are easily burned by insecticidal soap and copper sprays. Use the most dilute spray recommended and use sparingly. Do not spray plants in direct sun or if temperatures are above 80 degrees, and don't spray drought-stressed plants.

Leaf and Vine Problems

Leaves with chewed holes. Cause: Cucumber beetles. Adults are 1/4" long, greenish yellow beetles with black stripes or spots. Larvae chew on roots. For an illustration of this pest, see the picture below. They attack young leaves and should be controlled immediately as they can spread bacterial wilt or viral diseases. Treat infested plants with a commercial pyrethrin spray or dust. Reduce problems by planting cultivars such as 'Bennings Green Tint', 'Blue Hubbard', 'Early Butternut Hybrid', 'Seneca', and 'Table King'. These cultivars tolerate beetles.

Leaves with pale green patches; afflicted leaves wilt and blacken. Cause: Squash bugs. Adults are brownish black, 1/2" long bugs. Immature bugs are whitish green with dark heads and legs. Eggs are bright orange and laid on undersides of leaves. For an illustration of this pest, see the picture below. Handpick adults and eggs. Trap bugs by laying a board near plants. Squash bugs will hide underneath it and can be destroyed each morning. To reduce problems, plant cultivars that tolerate squash bugs such as 'Early Prolific', 'Early Summer', 'Royal Acorn', and 'Table Queen'.

Leaves with yellow patches; older leaves mottled and distorted. Cause: Mosaic. Several types of mosaic viruses are found on squash. Besides affecting leaves, mosaics may also cause deformed fruit that is mottled with yellow and green. For an illustration of this disease, see the picture below. Remove and destroy diseased plants. Control aphids and cucumber beetles that spread it. Reduce problems by planting cultivars such as 'Multipik', 'Napolini', 'Superpik', and 'Superset'. These cultivars tolerate mosaic.

Leaves yellow, curled, and wilted. Cause: Aphids. Look for small, green, pink, gray, black, or white fluffy-coated, soft-bodied insects feeding on plants. Aphids can also transmit viral diseases. Control aphids by knocking them off the plants with a strong blast of water. Prevent problems by using a foil mulch or by planting silver-leaved cultivars such as 'Cocozelle' that confuse or don't attract aphids.

Leaves yellow and puckered, becoming bronzed. Cause: Mites. These tiny, red, yellow, or green, spiderlike creatures are worst in dry, hot weather. In severe cases, leaves dry out and drop off. There may be a fine webbing on the undersides of leaves. Spray plants with a weak insecticidal soap spray to control.

Leaves mottled yellow between veins; purple spots on leaf undersides. Cause: Downy Mildew. For an illustration of this disease, see the picture below. As the disease progresses, spots enlarge, older leaves turn brown and die, and younger leaves become infected. Treat plants with a dilute solution of copper spray to reduce the spread of the disease. Prevent problems by planting tolerant cultivars such as 'Super Select' and 'Zucchini Select'.

Leaves with spots, blotches, or brown areas. Causes: Powdery mildew; angular leaf spot; scab; Alternaria leaf blight; other fungal and bacterial diseases. Various diseases attack squash. Reduce problems by keeping foliage dry when watering and by not touching plants when wet. Spray infected plants with a dilute solution of copper spray to reduce the spread of the disease.

Powdery white spots, especially on upper leaf surfaces, are caused by powdery mildew. As the disease progresses, leaves turn brown and dry, and plants may die. Prevent problems by planting resistant cultivars such as 'Multipik' and 'Zucchini Select'.

Water-soaked spots that turn gray, die, and drop out leaving shotholes are caused by angular leaf spot. Fruit infected with angular leaf spot has small, cracked, white spots. For an illustration of this disease, see the picture below. Water-soaked spots can also be caused by scab. Scab causes sunken, brown spots with gummy ooze on fruit; damage is worst in cool, moist weather.

Dark brown spots with concentric rings, usually appearing on older leaves first, are caused by Alternaria leaf blight. As the disease progresses, spots enlarge and erge, and leaves curl down and eventually drop off. Infected fruit has dark, concentrically ringed, sunken spots. For an illustration of this disease, see the picture below.

Vines wilt suddenly. Cause: Squash vine borers. These fat, white, 1" long larvae burrow into the stems and exude masses of yellow-green, sawdustlike excrement. For an illustration of this pest, see the picture below. Slit stems lengthwise above injury with a sharp knife and kill larvae. Cover cut stems with moist soil so they will form new roots. Injecting stems with BTK or parasitic nematodes may also control borers. To reduce problems, plant the cultivar 'Sweet Mama Hybrid', which is resistant to vine borers. Or spray base of stems with BTK once a week in late spring and early summer.

Vines wilt at midday, starting with younger leaves; leaves remain green. Cause: Bacterial Wilt. For an illustration of this disease, see the picture below. As the disease progresses, leaves fail to recover, and die. Cut wilted stems and press out drops of sap. If it is milky, sticky, and astringent, your plant is infected. Destroy infected plants immediately. Control cucumber beetles since they spread the disease.

Fruit Problems

Flowers appear but no fruit develops. Causes: Male flower; lack of pollination. Male flowers open a week or more before female flowers and don't form fruit. See the picture below for an illustration of male and female squash flowers. If female flowers fail to set fruit, or if small fruit turns black and rots starting at blossom end, they haven't been pollinated. Pollinate open female flowers by hand.

Fruit misshapen. Causes: Diseases; poor pollination. Many diseases cause misshapen fruit; use leaf symptoms (if any) to diagnose the problem. If leaves are healthy, high temperatures may have damaged pollen, or bees may not have been active.

Fruit with spots; flesh may rot. Causes: Angular leaf spot; Alternaria blight; scab. Several fungal and bacterial diseases cause thse symptoms on squash fruit. For complete controls, see "Leaves with spots, blotches or brown areas" above.

Fruit tunnels. Cause: Pickleworms. Larvae are pale green and black and up to 3/4" long. Keep fruit off ground or mulch, since worms feed at soil level.

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