Zucchini Problems: What Causes Bumps On Zucchini

By: Kristi Waterworth

Those big, beautiful leaves of zucchini plants protect their bounty of fruit from the elements, allowing for what seems like a never-ending supply of straight, smooth-skinned zucchinis. For most gardeners, the question of how to get rid of so many fruits is foremost in their minds, but when a gardener has bumpy zucchini fruit, disposing of excess fruit becomes a secondary issue to correcting the deformed zucchinis. Let’s learn more about what to do for bumpy zucchini fruit.

Bumps on Zucchini

Although there may be an open-pollinated zucchini variety with bumpy skin, bumps on zucchini are not typical. Usually, bumps are considered a sign of one of the more serious zucchini problems, caused by one of many incurable plant viruses. Cucumber mosaic virus, watermelon mosaic virus, papaya ringspot virus, squash mosaic virus, and zucchini yellow mosaic virus can all cause these bumpy, deformed fruits.

Symptoms of the many viruses in zucchini may bear close resemblance to one another, with common symptoms like scattered yellow patches on young or maturing leaves, leaf deformation, and irregular bumps or yellow spots on zucchini fruits. Stunting of plants often occurs, especially if the zucchini plant contracted the virus early in life or the seed itself was infected.

A less common cause can be due to rapid growth or an excess of calcium in the soil.

Preventing Viral Related Zucchini Problems

There is no way to treat zucchinis once they’re infected with viruses, but there are many preventative measures you can take at planting time, especially if you have lost crops to viruses before. Many viral pathogens are transmitted by sucking insects, like cucumber beetles or aphids, but they can also spread through infected seeds that mature into infected plants.

Do not save zucchini seeds if there’s any question as to the viral status of the mother plant. Instead, order certified virus-free seeds from a reputable supplier. If you direct seed your zucchinis, take some extra time to lay down reflective mulch and row covers to protect your zucchinis from virus-vectoring sucking pests. Transplants raised in a greenhouse need to be carefully monitored for insect pests.

You can slow the spread of zucchini viruses in your garden by keeping the grass and weeds in your yard closely trimmed, since weedy and overgrown areas are very attractive to the bugs. When viral infections become obvious, remove the infected plants immediately to reduce the chance of spreading disease further. Always work with disease-free plants before moving to the diseased ones, as some plant viruses can be passed on dirty tools or clothing, especially when grafting or pruning zucchinis.

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What's wrong with my zucchini?

Take a gander at my zuke fruit:

These are about 4-5 inches long. And this is what's happening to most of my zucchini. I have three plants planted in a hill. The foliage is green and doing great. Here it is a week ago:

The fruit get to a certain size, then turn yellow and die on the end. What's going on? There's no visible insect pests on the whole plant, nor signs of SVB around the base. The soil is great - a bit acidic, but otherwise fine. They've been watered regularly.

I called the local nursery and they said to fertlize more, but they weren't really sure what was causing it. I've had similar problems in past years with curcubits, which I always blamed on powdery mildew.

So what's going on here? How do I get good fruit?

What would cause my zucchinni fruits to be deformed like this?

This is my first year gardening and a brand new garden location. I am full organic and am using semi-raised beds (5.5" tall with 2' into native soil). My soil is very rich, actually too rich according to my soil report but things are growing and producing well!

I have two Black Beauty zucchini plants that are huge and producing nice zucchini frequently. However, I also frequently get fruits that are like the one on the right. I have a drip system and think I am watering properly and I spray seaweed often. I also spray BT with the seaweed and have soaked the base of the zucchini plant and soil with spinosad to hopefully prevent the dreaded SVB. So far, I am having very little pest problems. I have found and killed 3 cucumber beetles and see and SVB moth that I killed. I also have some home made yellow tangle foot traps up that have not caught much of anything bad. Everything looks great and healthy and the squash grows fast.

We did get a 3.75" rain last weekend after the one on the right was half way ready. Could that have done it which may indicate I am not watering enough previously? I water 1.5 hours once a week and each plant has a 2 gallon per hour dripper at the base.

Any suggestions? The fruits taste just fine so it is not really an issue but, like many, I would like to grow pretty fruits as well!

Thanks for any and all replies and tips/suggestions!

UNH Extension

There are a number of reasons your zucchini plants may not be producing much fruit. To start, it’s important to understand that zucchini and other squash plants are monoecious, meaning they produce separate male and female flowers on the same plant. While these flowers may look very similar at first glance, there are some distinct differences once you take a closer look. The most obvious differences are the small immature fruits at the bases of female flowers and the long thin stems of male flowers (pictured above). Early in the growing season, squash plants tend to produce more male than female flowers. While you may have tons of flowers, in order to produce fruit you must have both male and female flowers at the same time.

Bees and other pollinators are usually responsible for transferring pollen from the male flowers to the female flowers, which ultimately leads to fruit development. If there are few bees in your garden, you’ll likely have poor pollination and fruit set. Bees are sometimes few and far between in urban areas. If you think this is the case in your own garden, you can try playing the role of a bee yourself by hand pollinating the flowers. The pollen of squash plants is very sticky and is formed in the center of the male flowers. You can try using a small paint brush to move some of the pollen from the male flower to the stigma of the female flower. If that sounds too tedious, you can also just remove the male flower and gently roll its pollen onto the stigma of the female flower. It’s best to try hand-pollination early in the morning as squash flowers open early and only last for one day. Also keep in mind that squashes can only be fertilized by their same species. A zucchini cannot be pollinated by a winter squash and vice versa.

Zucchini Growing Problems: Troubleshooting

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.

Examine the Inside and Outside

Slice the zucchini lengthwise if the spoiled area extends deeply into the squash. Examine the interior for streaks of discoloration or soft, watery-looking areas. If the damaged sections are localized, cut them away and use the rest. Alternatively, if they run though much of the squash's length, discard the zucchini.

Squeeze the zucchini gently if it has no visible damage or spoilage. If it's slightly rubbery, it is still sound but is losing its freshness. Do not serve it raw. Instead, cook it in soups, stews or fritters, or shred it for use in baked goods. Alternatively, if it's still plump, firm and rigid, with skin that is still glossy rather than matte, it's in excellent condition and can be used in any of your favorite preparations.

Watch the video: VERTICAL GARDENING SMALL SPACE. growing zucchini vertically. Save garden space!

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