13 Zucchini Diseases: How to Identify, Prevent, and Treat Them

Are you noticing some diseases on your zucchini this season? There are many common diseases that can affect garden grown zucchini. In this article, organic gardening expert Logan Hailey walks through the 13 most common diseases you'll likely see when growing zucchini this season, as well as how to identify them, prevent them, and treat them!

zucchini diseases

Zucchini is one of the most beginner-friendly garden vegetables around. These fast growing and prolific summer squash can be planted once and yield all summer long. Although they aren’t totally immune to problems, there are many simple organic fixes to common zucchini diseases.

With their rapid growth speed (some zucchini varieties mature in as little as 45 days), growing zucchini can provide a quick reward for eager spring gardeners. As long as they have modest fertility and a steady supply of moisture, zucchini usually require very little tending other than a regular harvest.

But when wet summers, long droughts, pathogen-spreading bugs, or soil-dwelling diseases take hold, zucchini plants can face some major challenges. Some zucchini diseases can significantly reduce yields or even kill your plants. Others pose a risk of spreading to Cucurbit cousins like cucumbers, squash, and melons. Let’s dig into how to identify, prevent, and treat the 13 most common zucchini diseases in your garden.

13 Most Common Zucchini Diseases + How to Deal With Them

Like many related members of the Cucurbitaceae family, zucchini has characteristically large broad leaves and shallow roots. While this can be great for shading out weeds and rapid growth, zucchini plants also have a tendency to succumb to a few foliar and root pathogens when they are weakened.

If you’re wondering what’s wrong with your zucchini plant, use this simple guide to identify, prevent, and treat the most common zucchini pathogens.

Powdery Mildew

Powdery Mildew
Powdery mildew can appear on zucchini in humid climates.

In humid climates, a summer zucchini crop without powdery mildew would feel like a miracle. This notorious Cucurbit disease can be caused by a few different fungi including Podosphaera xanthii, Erysiphe cichoracearum, and Sphaerotheca fuliginea.

All of them cause similar symptoms and tend to take hold during mid or late summer when the weather is warm, or when there are fluctuations between dry days and moist or humid nights.

Powdery mildew (often abbreviated “PM” in seed catalogs) is said to produce more crop losses than any other plant disease. So be sure to choose PM resistant varieties, prevent disease colonization, and treat infected plants early to preserve your zucchini harvest!

How to Identify

Powdery mildew looks just like it sounds– a white or gray powdery dust that has settled on the zucchini leaves, almost as if someone dusted your plants with flour. As it progresses, you may notice larger whitish blotches, mildewy fuzzy stems, and yellow or brown dead leaves.

*Note: Don’t be confused by white patterns and markings on the leaves of certain zucchini varieties. Zucchini with natural white variegation will have a consistent pattern across leaves, only appear on the upper surface, and shouldn’t appear “fuzzy” or irregular like PM.

How to Prevent

You can easily prevent this mildew with cultural practices that ensure airflow and circulation. The number one most important preventative method is watering from the base. Do not overhead irrigate zucchini (or any squash for that matter). Powdery mildew spreads via rainfall and water splashing up from the soil, so drip irrigation or soaker hoses are key to keeping the leaves dry.

Another secret to keeping PM at bay is ensuring adequate spacing between plants. If your garden is particularly susceptible to fungal attacks, use a wider spacing of 18-24” between zucchini plants to keep plenty of air flow between them.

There are some PM resistant zucchini seed varieties such as ‘Dunja, ‘Sunglo’ and ‘Success PM’.

How to Treat

If powdery mildew takes hold of your zucchini leaves in the late summer, don’t panic! The easiest thing to do is cut off infected leaves and dispose of them in the trash, being careful not to blow the spores around. You can also use a diluted neem oil solution wiped over the leaf surface to slow the spread and prevent further infection.

Downy Mildew

Downy Mildew
To prevent the development of Downy Mildew disease, do not allow drops of water to fall on the zucchini leaves.

Unlike the dust-like powdery mildew, downy mildew has a yellow blotchy appearance. It tends to take hold at the beginning or end of the season when the weather is cool and wet.

Downy mildew is caused by a fungus-like organism called an oomycete, which are closely related to algae. They need water to spread. 

How to Identify

Yellow to pale green spots on the upper side of the leaf are the key early indicator of down mildew. The spots will be bordered by the leaf veins. Later they may turn entire leaves brown, often similar to frost damage. To distinguish from angular leaf spot, look for gray or purple fuzzy surfaces on the undersides of the leaves.

How to Prevent

Downy mildew requires moisture on the leaf surface to spread. This means that keeping water droplets off the leaves is the name of the game. Don’t overhead irrigate (instead, use drip or soaker hoses). Use wider spacing in late zucchini plantings to improve air circulation. You can also grow zucchini in greenhouses or low tunnels to try to keep rain off the leaf surface.

How to Treat

For mild infections, a diluted neem solution or organic fungicide can be used to wipe out downy mildew and prevent further infection. However, if it’s the end of the season and the plant has been heavily colonized, it’s usually best to remove the entire plant and throw it in the trash. Avoid handling in windy conditions or composting plant residues, as this can spread more pathogen spores.

Blossom End Rot

Blossom End Rot Zucchini
Blossom End Rot – as soon as you notice blackened spots on your zucchini, immediately remove the damaged fruit.

Though not technically a disease, blossom end rot is a very common problem that looks like an infection. It is actually caused by a lack of bioavailable calcium in the soil and/or erratic watering.

How to Identify

If you notice blackened, water-soaked spots on the “butts” of your zucchini, this is a sure sign of blossom end rot. It begins with decaying or mushy-looking blossoms. The rotten end can creep all the way up to the middle of the squash, resulting in brown disintegrating zucchini.

How to Prevent

Amend squash beds with ground oyster shell, crushed eggshells, dolomite lime, or gypsum according to package instructions (we love Down to Earth organic approved oyster shell fertilizer). Consistent moisture is also vital for zucchini. They don’t want to dry out and then receive big quantities of water at once. Instead, use drip lines or irrigation timers to maintain a steady soil moisture that isn’t too dry, nor too soggy. Mulch the soil with straw or leaves to keep the soil protected from hot sunny weather and drought conditions.

How to Treat

Once blossom end rot begins, all you can do is pick and remove the affected fruits. Then, improve your watering practices and amend with a zucchini friendly fertilizer as described above.

Fusarium Crown Rot

Fusarium Crown Rot
If your plant looks sluggish and dehydrated, this is most likely the first sign of Fusarium Crown Rot.

Wilted leaves or a droopy plant with plenty of soil moisture is never a good sign. This fungal disease rots zucchini plants from the base and can kill whole plants in just a few days.

This particular disease is caused by Fusarium solani, which can live in the soil and proliferate on old plant debris.

How to Identify

The first sign of Fusarium is excessive wilting. The entire plant will look dehydrated and limp even though it has water. Water-soaked lesions may appear on the leaves and then turn progressively darker until the leaves decay altogether. The base of the plant may look blackened and rotten and have a bad smell. Pinkish fungal strands can also be found at the surface of the soil near the crown.

How to Prevent

Always remove zucchini crop residues at the end of the season and avoid leaving them in the garden to decay (this can create a breeding ground for the fungus to overwinter). Crop rotation is also essential for fighting Fusarium Crown Rot. Keep Cucurbit-family crops (melons, cucumbers, pumpkins, squash) out of the same area of the garden for 1-3 years.

Lastly, ensure a healthy soil microbiome and adequate drainage by amending with generous amounts of compost or leaf litter at the beginning of every season. Use a broadfork to aerate the soil. Avoid overwatering.

How to Treat

Once Fusarium begins rotting your zucchini plants, the only thing you can do is remove the plant and dispose of it in the trash. There are no garden-approved treatments for this disease, so prevention is key.

Verticillium Wilt

Verticillium Wilt Zucchini
One of the symptoms of Verticillium Wilt is wilting, and unfortunately, there is no treatment.

This cool-weather loving fungus often appears in mid-to-late summer and can attack a range of plants, including zucchini, melons, peppers, tomatoes, and a wide range of fruit trees.

It has some funky characteristics that can make it confusing to identify. Fortunately, there are several resistant varieties available from major seed companies.

How to Identify

The Verticillium fungus initially causes zucchini leaves to wilt, turn yellow, and wither aWay. Sometimes half of the plant will look normal while the other half wilts.

How to Prevent

Select resistant varieties marked with a “V”. Sanitation of garden tools and removing crop debris is also helpful.

How to Treat

Unfortunately, there is no treatment for Verticillium Wilt. Remove and destroy infected plants by burning or throwing them away.

Alternaria Leaf Blight

Alternaria Leaf Blight
Alternaria Leaf Blight grows on wet leaf surfaces so try to avoid top watering the plant.

Another common zucchini diesease, Alternaria cucumerina is an aggressive fungus that causes problems in cucurbits as well as brassicas. It overwinters on infected crop debris and thrives on moist leaf surfaces.

The characteristic “bullseye” or “shot-hole” appearance makes it easy to recognize.

How to Identify

This leaf blight begins on older leaves and then spreads to younger ones. It is characterized by small angular yellow or brown colored spots that have a green or yellow “halo” around their perimeter. As it progresses, the splotches join together and cause the leaves to curl under and eventually die.

How to Prevent

Avoid overhead irrigation and only water plants from the base. You can try covering plants during heavy rains to prevent water from splashing on the leaves. Always remove zucchini crop debris from the garden at the end of the season. A foliar spray of diluted neem solution may also be useful to prevent infection.

How to Treat

Some organic fungicides like Equisetum (horsetail) tea, compost tea, copper sprays or bio-fungicides like Bacillus subtilis or Actinovate are useful for Alternaria Leaf Blight. Apply directly to leaves based on package instructions. It’s also best to remove infected leaves by cutting at the base and disposing of them in the trash.

Cucumber Mosaic Virus

Cucumber Mosaic Virus
Cucumber Mosaic Virus is spread by pests or garden tools.

Contrary to its name, Cucumber Mosaic Virus also affects its zucchini cousins. It is one of those pathogens that is spread by pests or contaminated garden tools.

This disease causes curling and a mosaic-like pattern in zucchini leaves, often severely stunting zucchini plants and reducing the yields. Luckily, there are lots of resistant varieties available.

How to Identify

Deformed, small zucchini fruits with rough skins or pale, distorted leaves are the first sign of infection. The leaves can look cupped, puckered, or stringy. A mottled mosaic pattern of yellow and pale green may be subtle or obvious on the leaf surface. Plants can quickly become stunted and wither away. The virus is most commonly found alongside an aphid or cucumber beetle infestation.

How to Prevent

CMV-resistant seed varieties are widely available and are the easiest form of prevention. Cultivars like ‘Green Machine’, ‘Yellow Fin’, and ‘Desert’ are reliable options. Practice good sanitation with garden tools by cleaning them with rubbing alcohol or bleach after use. Using companion plants like garlic, onions, or fragrant herbs can help keep aphids and cucumber beetles at bay.

How to Treat

Remove any infected plant parts. The virus cannot be totally eradicated, but it helps to get rid of aphids by spraying leaves with a solution of 1 1/2 teaspoons of neem oil, 1/2 teaspoon dish soap, and a quart of water. Remove cucumber beetles by hand and drown in a soapy solution.

Damping Off

Damping off
To prevent damping off, grow seedlings in a well-ventilated area with well-drained soil.

Damping off fortunately only affects zucchini seedlings. Once your plants have grown past the seedling stage, you no longer have to worry about this disease.

The fungus thrives in stagnant, moist, cool conditions of an improperly cared for greenhouse, but it can also cause failure of direct sown zucchini seeds in the garden. Environmental control is the key to prevention. 

How to Identify

A fuzzy white mold appears on the soil surface and girdles young seedlings at the base. The blackened baby zucchini stems will eventually fall over and the young cotyledons will turn yellow and die. If the tops of your seedlings simply disappear, you’re more likely to be dealing with a vole or mouse than damping off.

How to Prevent

Only grow seedlings in a well ventilated greenhouse with well-drained seed starting soil mix. Keep at least one fan running near seedlings at all times. Avoid overwatering. In the garden, prevent damping off by preventing overcrowding and lightly covering seeds with peat moss when sowing.

How to Treat

Once seedlings fall victim to damping off, they can’t be revived. Improve the circulation and re-seed.

Gummy Stem Blight

Zucchini Disease Gummy Stem Blight
Gummy Stem Blight can be seed-borne or soil-borne.

This disease is another aptly named disease that causes the zucchini stem and leaves to get all nasty, soft, and well… gummy. The fungus Didymella bryoniae is the same pathogen that causes black rot in squash, cucumbers, and melons.

It thrives in 85% humidity or higher and temperatures around 60-70°F. The wide range of foliar symptoms can make it difficult to correctly identify and can quickly cause rotten inedible squash fruit. Luckily, many of the prevention and treatment methods are identical to Powdery Mildew. 

How to Identify

Gummy Stem Blight first manifests itself in the stems of zucchini with soft brown lesions that have a brown halo. They later turn to water-soaked regions and blackened dead areas that may ooze liquid from gross textured brown beads.

How to Prevent

This fungus is both seed-borne and soil-borne, meaning disease-free seed sourcing and cultural controls are both necessary for prevention. Often Powdery Mildew Resistant (PMR) seed varieties have some resistance. Remove all crop debris or dead plant material from the garden so that the fungus can’t overwinter. Cucurbit crop rotation is a must.

How to Treat

Once Gummy Stem Blight appears, remove infected plants as soon as possible. Use a neem spray or organic bio-fungicide like Actinovate to protect nearby cucurbits.


In order to stop the spread of Anthracnose, you need to remove the infected plants and plant the new ones in another place in the garden.

The fungus Colletotrichum orbiculare can affect all above-ground zucchini parts including stems, leaves, and fruit. This pathogen typically appears in mid to late summer when there is prolonged warm, moist weather.

Spores are easily spread from splashing rainwater, irrigation, and infected tools or infected seeds, so sanitation is a must.

How to Identify

Leaves typically show irregular brown spots that may decay out and give the leaf a ragged or ripped appearance. Zucchini squash get sunken black spots up to ½” wide. They may have fluffy white mycelium and coral-pink spores on the squash surface during wet weather.

How to Prevent

Always purchase disease-free zucchini seeds from a reputable source (never save seed from infected plants). Cucurbit rotation is key to avoid spread between cucumber, squash, or melon cousins. Avoid overhead irrigation and don’t work in the zucchini patch when conditions are wet. Sanitize tools with diluted bleach and be sure to destroy all crop debris at the end of the season.

How to Treat

An organic fungicide with copper or sulfur may stop the spread, but adequate coverage of leaves and fruit is required. Often it’s best to just remove infected plants and re-grow zucchini in a different area of the garden.

Bacterial Wilt

Bacterial Wilt
Bacterial Wilt is spread by striped cucumber beetles. Use BW resistant seeds for planting or use straw mulch.

As if spotted and striped cucumber beetles weren’t annoying enough on their own, they are unfortunate spreaders of this detrimental disease.

Caused by the bacterium Erwinia tracheiphila, Bacterial Wilt tends to strike in the spring and can cause major losses in zucchini and cucumber crops.

How to Identify

The first sign is dull or pale green leaves. Wilting begins with a small part of a plant or isolated individual plants, starting at the tips and then progressing down to the center. Sometimes the plants bounce back overnight and then wilt again during the day, no matter how moist the soil is. Leaves later turn yellow or brown and wither. There may also be oozing white strands of bacteria when zucchini stems are stripped open. Striped or spotted cucumber beetles are typically present.

How to Prevent

The best prevention is planting BW resistant seeds. It is also important to maintain weeded beds because weeds can host cucumber beetles. These yellow and black beetles are the main culprits for spreading Bacterial Wilt. The E. tracheiphila bacteria overwinters in their gut and enter the plant when they feed on it. Prevent cucumber beetles with row cover, straw mulch, neem oil solution, and companion plants like nasturtiums and marigolds.

How to Treat

Once the bacteria takes hold, there isn’t much you can do. Remove any BW infected plants immediately.

Bacterial Leaf Spot

Bacterial Leaf Spot
Bacterial Leaf Spot develops in hot and humid environments.

Hot, humid midsummer conditions are the perfect conditions for Bacterial Leaf Spot. Caused by Xanthomonas campestris, this disease can be problematic in zucchini as well as turnips. Prevention is key because there aren’t any known treatments.

How to Identify

Zucchini Bacterial Leaf Spot can look a lot like some nutrient deficiencies from a distance due to the yellowing, necrotic appearance. But when you look closer there are small speckled dots with yellow margins on the leaves. The zucchini squash themselves may show beige indented spots.

How to Prevent

This disease most often attacks plants through existing wounds, so the first line of defense is trying to avoid damaging zucchini stems or leaves. Because the leaf spot pathogen is primarily seed-borne, it is also important to source reputable disease-free seed. Crop rotation of cucurbits every 2-3 years is beneficial.

How to Treat

Copper sprays and removal of infected parts may help control the spread, but heavily infected plants can’t be saved.

Septoria Leaf Spot

Septoria Leaf Spot
Septoria leaf spot is characterized by beige spots with dark brown edges.

Found mostly in the Midwest, Septoria cucurbitacearum is another foliar pathogen that affects the whole Cucurbit family as well as Solanaceous crops like tomatoes. It loves humid, slightly warm conditions around 60°F in the spring and fall, but typically goes dormant in peak summer heat. 

How to Identify

Septoria Leaf Spots are distinctly beige or white round spots with dark brown margins. They may merge together to create brown dead regions of the leaf.

How to Prevent

Always remove old crop debris from the garden and practice Cucurbit crop rotation. Provide plenty of airflow and plant zucchini away from tomatoes.

How to Treat

Prune away infected leaves and use potassium bicarbonate or on organic copper fungicide to treat nearby plants.

Final Thoughts

Once you deal with the annoying task of treating a zucchini disease, you’ll probably want to do everything in your power to prevent the same issue next season.

The secret to preventing all crop diseases is growing a healthy plant to begin with. Just like a healthy lifestyle prevents you from getting sick, a robust thriving plant is far more resilient under stress.

Plant stressors include exposure to pathogens, pests, drought, and extreme weather. While we can’t control the weather, we can do our best to nurture the healthiest plants possible by providing ideal conditions for growth.

Zucchini needs full sun, warm temperatures, loamy well-drained soil, consistent moisture, and at least 12-18” of space between plants. If the plant’s basic needs aren’t being met, it is more likely to be colonized by disease-causing microbes.

Everyone’s garden succumbs to pests or diseases at some time or another. Thankfully, there’s an abundance of tools in our organic gardening toolbox to prevent and treat almost all of these zucchini diseases:

  • Only plant robust healthy seedlings from reputable seed sources.
  • Source disease-resistant varieties when possible.
  • Provide adequate spacing between plants (at least 18-24” in all directions) to ensure proper airflow.
  • Avoid overhead irrigation at all costs.
  • Instead, use soaker hoses or drip irrigation to keep zucchini leaves free of excess water.
  • Practice crop rotation by keeping Cucurbits out of the same garden beds for 1-3 years.
  • Plant companion plants near zucchini, such as marigolds, nasturtiums, and fragrant herbs.
  • Always remove Cucurbit crop debris from the garden at the end of the season.
  • Keep weeds at bay to reduce competition and prevent pests.
  • Maintain as much airflow as possible. When in doubt, widen spacing.
  • Amend generously with biologically-rich organic compost.
  • This will help ensure proper microbial balance in the soil.

When in doubt, remember that zucchini are very forgiving, and can yield a bountiful harvest if you stick to a proper maintenance plan. There is no harm in removing a potentially infected plant and replacing it with a new one. If you lose a couple zucchini plants to a disease, don’t worry: these squash grow quickly enough to give you time to replant if necessary.

Most importantly, take photos and keep track of your zucchini growth patterns in your garden log. If you identify a disease, be sure to remember what, where, and when it arose. Those records may come in handy in future seasons.

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