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Zucchini Growing Problems: Troubleshooting

Zucchini on vine

Squash yellow with blossom

Zucchini in North America and Australia; courgette in France, England, Ireland, and New Zealand; zucchini in Italy: it’s the summer squash with the shape of a cucumber. Yellow, green, or light green, it is one of the easiest vegetables to grow–all it needs is warm weather.

There are at least 50 popular varieties of zucchini. If you have bees to take care of the pollination, you are likely to have a bumper crop.

That is not to say zucchini is problem free: there are a few. For zucchini growing tips see Zucchini Growing Success Tips at the bottom of this post.

Here are common zucchini growing problems with cures and controls:

Seed fails to germinate. Some squash seeds are “hard”–that is naturally resistant to uptake of water which results in sprouting. To overcome “hard” seed, increase germination, and slightly decrease germination time, soak seed in tepid water for 24 hours before sowing. Dry the seed on a paper towel before planting.

Plants are eaten or cut off near soil level. Cutworms are gray grubs ½- to ¾-inch long that can be found curled under the soil. They chew stems, roots, and leaves. Place a 3-inch paper collar around the stem of the plant. Keep the garden free of weeds; sprinkle wood ash around base of plants.

Leaves have yellow specks that turn brown, then black and crisp; vines wilt from point of attack. Squash bug is a flat, shield-shaped black or brownish bug with a triangle on its back; it sucks juices from plants. Trap adults beneath boards in spring, hand pick and destroy. Look under leaves for bugs.

Runners wilt suddenly; holes in stems near base of plant. Squash vine borer is a fat, white caterpillar with a brown head that emerges in late spring. It bores into stems to feed causing plants to wilt. Look for entrance holes where frass may accumulate; slit vine with knife and remove borer; bury runner at that point to re-root. Exclude adult moth with floating row covers. Time planting to avoid insect growth cycle. Plant resistant varieties.

Leaves curl under and become deformed and yellowish. Aphids are tiny, oval, and yellowish to greenish pear-shaped insects that colonize on the undersides of leaves. They leave behind sticky excrement called honeydew which can turn into a black sooty mold. Use insecticidal soap.

Mottled, distorted leaves. Mosaic virus causes leaves to become thickened, brittle, easily broken from plant; plants are stunted and yields are poor. The virus is spread from plant to plant by aphids and leafhoppers. Remove diseased plants. Remove broadleaf weeds that serve as virus reservoir.

Leaves turn pale green, yellow, or brown; dusty silver webs on undersides of leaves and between vines. Spider mites suck plant juices causing stippling. Spray with water or use insecticidal soap or rotenone. Ladybugs and lacewings eat mites.

Round white powdery spots and coating on leaves. Powdery mildew is caused by fungal spores. Spores germinate on dry leaf surfaces when the humidity is high; spores do not germinate on wet leaves. Common in late summer or fall but does not result in loss of plant. Avoid water stress. Pick off infected leaves.

Few fruits form even though plants are flowering. Not enough bees. The more bees the more flowers that will be pollinated and likely to set fruit. The average size of a squash is increased when the vine is pollinated by many bees. Use chemical sprays sparingly being careful that pollinators are not harmed.

Holes chewed in leaves, leaves skeletonized; runners and young fruit scarred. Spotted cucumber beetle is greenish, yellowish, ¼ inch (7mm) long with black spots and black head. Striped cucumber beetle has wide black stripes on wing covers. Hand pick; mulch around plants; plant resistant varieties; dust with wood ashes. Cultivate before planting to disrupt insect life cycle.

Holes in leaves and flowers; tunnels in vines and fruits. Pickle worms are the larvae of night-flying moths. Moths lay eggs on squash plants. Caterpillars feed on leaves and inside vines and fruits. Pupae may be found inside rolled leaves. Exclude moths with floating row covers. Plant fast-maturing varieties to promote strong growth before pickleworms attack. Plant a few squash as trap crops. Keep garden clean.

Water-soaked spots on leaves; spot become circular with gray centers. Leaf spot or Septoria leaf spot is a fungus disease. Plant resistant varieties. Rotate crops. Keep garden free of plant debris. Apply copper dust or liquid copper spray every 7 to 10 days.

Water soaked spots on leaves, stems and fruits become covered with cottony mold. Bacterial wilt clogs the circulatory system of plants. It is spread by cucumber beetles and is seen often where the soil stays moist. Remove and destroy infected plants before the disease spreads. Make sure soil is well drained. Control cucumber beetles with rotenone or sabadilla. Rotate crops.

Round to angular spots on leaves, reddish brown to black; sunken water-soaked areas on fruit; fruit shrivels and become watery. Anthracnose is a fungus disease that spreads in high humidity and rainfall. Leaves may wither and fall. Plant may die back. Generally found in eastern North America. Spray or dust with a fixed copper- or sulfur-based fungicide every 7 to 10 days. Remove and discard infected plants. Avoid working in the garden when it is wet which can result in spread of spores. Keep tools clean.

Vines wilt suddenly and die starting with one or two leaves. Bacterial wilt clogs the circulatory system of plants. It is caused by bacteria that live in cucumber beetles and is seen often where the soil stays moist. Remove and destroy infected plants before the disease spreads. Control cucumber beetles with rotenone or sabadilla. Wash hands and clean tools with a bleach solution.

Plants are stunted and yellow; runners gradually die. Fusarium wilt is a fungal disease which infects plant vascular tissues. Fungal spores live in the soil and can be carried by cucumber beetles. Plant disease-resistant varieties. Rotate crops. Remove and destroy infected plants. Fungicides are not effective.

Stems on older plants appear water soaked and turn into cracked brown cankers; fruits become water soaked. Gummy stem blight and black rot are fungus diseases. Infections can girdle stems can cause collapse. Remove and destroy infected vines. Rotate crops where fungus can persist. Grow powdery mildew resistant plants.

Dark, leathery areas appear on the blossom end of fruit. Blossom end rot is caused when there is too little moisture in the soil, particularly when temperatures are greater than 90°F. Sometimes there is a calcium deficiency in the soil which keeps roots from taking up water. Mulch planting beds to keep soil moisture even; water regularly. Test soil for calcium deficiency.

Dense white mold on blossoms or small fruits. Choanephora fruit rot is a fungus that grows on blossoms and developing fruit. Remove and destroy infected blossoms and fruits. Keep the garden clean of debris that can harbor fungus. Rotate crops.

Water-soaked or pale green spot on leaves that turn white; fruit cracks. Scab is caused by soilborne bacterium. Disease can be cosmetic. Plant resistant varieties. If scab occurs, change varieties next year. Sulfur may be worked into soil to make it slightly acid and reduce disease.

Zucchini Growing Success Tips:

Planting. Grow squash in full sun. Squash prefers well-drained soil. Sow squash in hills or raised beds; this will ensure good drainage and a warm growing bed. Add aged compost to the planting hole before sowing. Give squash plenty of space; be sure to set plants at least 3 feet apart and more depending upon the variety.

Planting time. Sow squash in the garden as early as 2 weeks after the last average frost date in spring. To get a head start on the season sow squash indoors about 3 weeks before you transplant it into the garden. Sow succession crops every 2 to 4 weeks to extend the harvest and to protect against crops loss to insects or disease. Time all plantings so that squash comes to harvest before the first frost in fall.

Care. Squash is often attacked early by cucumber beetles. Protect seedlings with floating row covers until they begin to flower. Squash grows on short vines; to improve air circulation and keep fruit clean, train vines to stakes using horticultural tape or cloth ties.

Harvest. Pick zucchini and all summer squash when it is young and tender. Don’t wait for squash to get big; it will be woody and tasteless. Use a knife or garden shear to cut zucchini from the vine.

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. has more than 10 million visitors each year.

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