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15 Problems and Pests that Plague Zucchini and Squash

Zucchini and squash can be relatively easy plants to grow. But like most crops, there are a number of things that can go wrong.

If you’re having issues with your zucchini or squash plants, you might be making one of these mistakes. But in this article, we’ll delve a little deeper to look at some of the problems and pests that plague zucchini and squash. And if you want to be sure of your heftiest zucchini harvest yet, you’ll want to follow these six important tips.

Which problems you encounter will depend not only on your gardening efforts but also on where you live.

Developmental Problems in Zucchini and Squash

The first category of problems we’ll take a look at are developmental problems. These problems can have a range of root causes – many of them environmental. Read on to discover what might be causing any issues, and what you can do about it.

1. A Lack of Female Flowers

All zucchini and squash develop both female and male flowers. The female plants can be identified by the fruits forming at their base. But sometimes, things can go wrong. 

If you cannot see female flowers on your plants, this may simply be because it is still early in the season. Immature plants will tend to produce only male flowers early in the growing season, then progress to develop both types as the season moves on. 

If, however, you continue to note a lack of female flowers as the season progresses, this could be due to a problem with the environmental conditions in which they are growing. 

Often, unseasonably low temperatures, a lack of sunshine can be to blame for a lack of female flowers. There is not much that can be done when it comes to the temperature and a lack of sunshine. You can’t change the weather. 

However, there are several things that you can do to help prevent this problem. For example:

  • Consider growing zucchini and squash under cover if you live in a colder climate zone.
  • Don’t sow/ plant out your zucchini and squash too early. Wait until all risk of frost has passed where you live.
  • Protect plants grown outdoors with cloches or protective covers in chilly conditions.
  • Make sure you place your zucchini and squash where they will get as much sunlight as possible. Avoid shady spots and overcrowding that could reduce light levels and put more stress on plants. 

A lack of female flowers is usually due to temperature and sunlight issues. But it can also be caused by plant stress.

So keeping plants as healthy and well-fed and well-watered as possible will keep them strong and promote healthy development. Feeding and watering are especially important if you’re growing zucchini or summer squash in containers.

2. A Lack of Male Flowers

Later in the year, it’s also possible to see all female flowers on your plants, and no male flowers at all. Of course, this is also a problem, since you need both flower types for pollination and fruit set to occur. 

High temperatures during hot weather, or in a poorly ventilated undercover growing area can cause this problem. To avoid this issue you should:

  • Avoid planting out transplants too late (after midsummer).
  • Cool the environment around your plants with careful companion planting.
  • Make sure plants are not overcrowded and there is good air flow.
  • Improve ventilation if plants are grown under cover.
  • Cool a greenhouse or polytunnel by damping down hard surfaces.
  • Add thermal mass inside your covered growing area to temper extremes of temperature.

(Thermal mass – materials like water-filled containers, stone, brick, clay etc. catch and store heat energy during the day, and release it slowly when temperatures fall at night. So they can help keep temperatures more even.)

Remember, however, that it’s natural for plants to produce fewer male flowers as fall approaches, and for fruit production to drop off. 

3. Plants Have Male and Female Flowers But Fail to Set Fruits

If you do have both male and female flowers but fruits fail to form or drop off, this can have a number of causes. 

Your zucchini and squash plants may:

  • Simply be too young and small still to sustain fruits.
  • May be stressed due to unsuitable growing conditions, poor watering or inadequate nutrition (or have a disease problem that makes them lack in vigor – as we’ll discuss below). 
  • Be self-regulating. If some mature fruits have developed, but you have not harvested them regularly enough, the plant may drop immature fruits. It does this to regulate the number of developing fruits it has according to the number it can effectively support. 

However, the most common reason that fruits fail to set is poor or non-existent pollination. 

Zucchini and squash are pollinated by insects such as honey bees. A lack of pollination is often due to a lack of insects to provide their pollination services.

There may simply be few insects around due to poor or cold weather. But there may also be a problem because you have not done enough to entice these pollinators to your garden. 

You should choose appropriate companion plants for your zucchini and squash to attract pollinators. And garden organically to ensure a pollinator-friendly environment in your garden. 

If you’re growing your zucchini and squash under cover, there may be a problem with pollinators when it comes to gaining access to your crops. Leave doors/vents open on warm days to allow them access. And again, choose companion plants to draw them in. 

In poor/cold weather, or when growing indoors, however, it may be necessary to hand pollinate. Check out this article for more information on hand pollination. 

4. Fruits Start Rotting or Fails to Mature

If fruits form successfully but start rotting or fail to mature, environmental problems are often to blame. The first step is to look at watering and fertility to check if there are any problems in these areas.

If they’re not well-fed and well-watered, the plants may simply not be strong enough to maintain fruit production. 

There may also be a problem with the soil or growing medium. It might be too acidic, or too alkaline, which will cause problems with uptake of nutrients. 

If the ends of your squash or zucchini turn dark, this is an indicator of blossom end rot. Blossom end rot is not a disease but is caused by a lack of calcium. Either there is not enough calcium in the soil, or the soil pH is low, and the plant cannot absorb the calcium that is available. 

Try adding egg shells and other calcium rich materials to a compost, and using this as mulch around your plants. If this is a recurring problem, liming the soil may help. But this should only be carried out after a soil test to see whether or not this is really required.

Excessive nitrogen or excess potassium may also be to blame. Make sure you are watering evenly and not over-fertilizing.

Of course, fruits rotting and a lack of vigor may also be caused by certain diseases. Some common diseases of zucchini and squash are outlined below.

Diseases of Zucchini and Squash

There are several different categories of disease that can affect zucchini and squash. The most common are fungal diseases. The other categories involve diseases caused by bacteria, and by viruses. 

5. Powdery Mildew 

Powdery mildew (and other mildews) are caused by fungal infection of your zucchini and courgette plants. All cucurbits (the family to which zucchini and squash below) are prone to this problem. It can be identified by a white, dusty coating on the leaves, stems and flowers. 

Don’t confuse the normal white patination on the leaves for this problem though. It is normal to see whitish markings on the leaves.

Powdery mildew will often stunt the growth and affect the yield of your zucchini and squash plants, though it will rarely kill your plants. 

Cut off affected leaves if there are just a few, to prevent the problem from spreading.

Make sure you do not add them to your compost heap or spread the problem to other cucurbit plants in your garden. 

To reduce the spread of fungal infections, create a homemade fungicide, which can help tackle milder infections. 

Add 10g baking soda to 1-liter water and spray this onto the leaves of your zucchini and squash plants (though not on a hot, sunny day).

However, prevention is always better than cure. To reduce the likelihood of a fungal infection taking hold:

  • Make sure you keep plants well watered, and water soil rather than leaves.
  • Reduce moisture loss with good quality organic mulch. (But keep the area around the stem clear.)
  • Make sure there is good air flow around plants, and avoid overcrowding.

If you keep having issues – choose resistant zucchini or squash cultivars.

6. Anthracnose

This is another upsetting fungal disease caused by Colletotrichum phomoides. It attacks the leaves and the fruits of zucchini and squash. At first, it can be identified by yellowish and watery spots.

Later, these spots expand and turn brown and then black. Eventually, these areas can dry up and leave holes in the leaves. On fruits, infection will display as cankers sunk into the surface of the fruit.

Again, it’s important to dispose of any infected material immediately if this problem appears. 

You can reduce the chances of seeing this disease if you:

  • Buy your seeds from a reputable company.
  • Rotate your crops so you don’t grow them in the same soil year after year. 

And as usual, making sure plants are as healthy as possible will reduce the likelihood that they will succumb to disease. 

7. Verticillium Wilt

This problem is due to a soil-borne fungus called Verticillium albo-atrum. If your plant begins to develop darkness and rotting at the base of the stem, and upper parts of your zucchini or squash plant begin to die back then this fungal infection may be to blame. 

Sadly, if this is the problem, there is nothing that can be done to save your plant.

Remove and dispose of your plants as soon as you spot a problem – including the roots and as much of the soil or compost from around them as possible. 

Planting on slight mounds to keep the stem base dry can help to reduce the likelihood that this, or other similar problems will develop. 

8. Bacterial Wilts

There are also a number of bacterial wilts that can effect zucchini and squash plants. Many of these go hand in hand with pest species. For example, Erwinia tracheiphila is spread by the cucumber beetle (more on this below). If plants suddenly wilt and die, bacterial infections can be to blame.

Tackling the example above will involve taking steps to get rid of the pests that are spreading the disease. And again, keeping plants as healthy and strong as possible with good care will reduce the chances of diseases taking hold. 

9. Mosaic Virus

One disease that is neither fungal nor bacterial is the mosaic virus. There are two strains of mosaic virus that can infect zucchini and squash – the squash mosaic virus and the zucchini yellow mosaic virus.

Plants infected with a mosaic virus will have stunted growth. Their leaves will be mottled, crinkled, or turn pale green in patches, and the fruits infected will be irregular in shape, with mottled coloration and a warty appearance. 

Pests like aphids and other sapsuckers are responsible for spreading these viruses. So again, organic pest control (more on this below) is the way to halt their spread. It is also important to grow from certified, disease-free seeds. 

Pests That Plague Zucchini and Squash

Zucchini and squash can be plagued by a number of different pests. Here are some of the most common ones gardeners encounter when growing these plants:

10. Aphids

Aphids, and other sap suckers like white flies, are a common issue in the vegetable garden. Most gardeners will encounter them at one point or another. 

The best way to combat them is by choosing companion plants that attract predatory wildlife to your garden – wildlife like ladybugs and lacewings will help keep aphid numbers down. Or you can purchase and apply ladybugs yourself.

11. Cutworms

Cutworms can cut off tender young zucchini or squash seedlings at the base. These are the larvae of certain moth species, which live in the soil. They are quite common in areas where a grass lawn has been turned into a new vegetable plot. 

Again, attracting things that eat them – birds etc. is a good way to control them. But since they stay mostly beneath the soil during the day, you can also stop them from munching at night by placing a cardboard collar around the base of the plants.

12. Flea Beetles

Flea beetles can feed on zucchini and squash as well as on other common crops. Though they will not usually kill your plants, they can reduce vigor and reduce yield. 

The best method of control is planting companion plants like radishes or nasturtiums that will act as trap crops. 

13. Cucumber Beetles

As mentioned above, cucumber beetles can spread disease. They feed on zucchini and squash, and can decimate your crops. They may eat seedlings, cause holes in foliage and vines, and leave deep marks on your fruits. 

The adult beetles are yellow and black and relatively easy to spot. Catch and remove these when you spot them, use sticky traps, and use covers and hand-pollinate where there is a problem with this pest. Plant nasturtiums as a trap crop.

14. Squash Bugs

Squash bugs are similar to stink bugs but are thinner and smaller. They can cause yellow spots that eventually turn brown, cause wilting, and ragged holes. Look out for the eggs and nymphs on the undersides of the leaves on your zucchini and squash. And pick off the adults whenever you see them. 

Practice crop rotation, and make sure you get rid of old vines on your compost heap. Cover plants for the first month or so, or delay planting a little if these are a problem where you live. Plant nasturtiums and tansy as companion plants. If these are a recurring issue, choose resistant zucchini and squash varieties. 

15. Vine Borers

If healthy zucchini or squash plants suddenly wilt, look near the base of the stem. If you see small holes that are exuding beige frass, the problem is squash vine borer feeding inside. These are the larvae of a large hummingbird moth. If multiple borers feed inside the stem, it will gradually rot and kill the plant. 

Again, covering your crop in the spring can help prevent this problem. As with cutworms, placing a collar around the base of the plants can also help. Consider choosing resistant varieties if these are prolific where you live. 

You may encounter other problems and pests, but these are the most common.

Forewarned is forearmed. Educating yourself about what might go wrong is a good place to start when trying to grow food successfully. 

As usual in the garden, it is best to try to prevent a problem rather than trying to tackle it once it has occurred. In an organic garden, making sure that environmental conditions are just right, and making the ecosystem as biodiverse as possible are key. Looking holistically at the garden can’t prevent all problems. But it can usually help to keep them in check. 

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Elizabeth Waddington

Elizabeth Waddington is a writer, permaculture designer and green living consultant. She is a practical, hands-on gardener, with a background in philosophy: (an MA in English-Philosophy from St Andrews University). She has long had an interest in ecology, gardening and sustainability and is fascinated by how thought can generate action, and ideas can generate positive change.

In 2014, she and her husband moved to their forever home in the country. She graduated from allotment gardening to organically managing 1/3 of an acre of land, including a mature fruit orchard,which she has turned into a productive forest garden. The yield from the garden is increasing year on year – rapidly approaching an annual weight in produce of almost 1 ton.

She has filled the rest of the garden with a polytunnel, a vegetable patch, a herb garden, a wildlife pond, woodland areas and more. Since moving to the property she has also rescued many chickens from factory farms, keeping them for their eggs, and moved much closer to self-sufficiency. She has made many strides in attracting local wildlife and increasing biodiversity on the site.

When she is not gardening, Elizabeth spends a lot of time working remotely on permaculture garden projects around the world. Amongst other things, she has designed private gardens in regions as diverse as Canada, Minnesota, Texas, the Arizona/California desert, and the Dominican Republic, commercial aquaponics schemes, food forests and community gardens in a wide range of global locations.

In addition to designing gardens, Elizabeth also works in a consultancy capacity, offering ongoing support and training for gardeners and growers around the globe. She has created booklets and aided in the design of Food Kits to help gardeners to cool and warm climates to grow their own food, for example. She is undertaking ongoing work for NGO Somalia Dryland Solutions and a number of other non governmental organisations, and works as an environmental consultant for several sustainable companies.

Visit her website here and follow along on her Facebook page here.