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

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.

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’.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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

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