How to Fight Cucurbit Scab

By Erin Marissa Russell

The plant disease cucurbit scab is caused by the Cladosporium cucumerinum fungus, and it affects cucumbers, melons, pumpkins, summer squash, watermelons, winter squash, and zucchini. All portions of these plants that grow above the ground can be impacted by the disease, but the most significant symptoms appear on the edible fruits. The disease is at its worst when conditions are foggy, dew is heavy, rain is light, and the weather is cool.

The pathogen that causes cucurbit scab can be seedborne, but it also survives in debris from infected plants that remains in the soil for up to three years. The fungus can also grow on dead organic material, such as fallen trees, dead leaves, cow patties, and even the remains of dead insects or animals. It can spread via the movement of insects or humans that come into contact with the fungus, gardening tools that have touched infected plants, and even moist air. When temperatures are between 59 and 77 degrees Fahrenheit (15 to 25 degrees Celsius), the fungus responsible for cucurbit scab is most likely to create spores and spread.

How to Identify Cucurbit Scab

Being familiar with the symptoms of cucurbit scab is vital for gardeners who grow susceptible crops so that quick action can be taken if the symptoms appear on their plants. Those who grow vulnerable plants should inspect them regularly for the signs of cucurbit scab infection, which are listed below.

Leaves of affected plants may exhibit brown or gray lesions haloed in yellow that can gradually change to have a shot-hole look.

When lesions are severe on foliage, leaves can become deformed or twisted.

The first signs of lesions on fruit may initially resemble damage from insects—small pockmarked areas that turn sunken and gray.

Lesions may have spores that turn the centers dark green to greenish black, with a texture similar to velvet.

As lesions spread outward and increase in number, they can merge to create large areas of discoloration on foliage and fruit.

Affected tissue on foliage or fruit can start to ooze a sticky material.
The lesions that result from cucurbit scab can provide an entry point for other organisms, leading to decay that exhibits different symptoms.

With varieties that are somewhat resistant to cucurbit scab, the disease can cause raised corky spots instead of the usual lesions.

On susceptible crops, rotted areas can permeate deep into the tissue of infected fruits.

How to Prevent and Treat Cucurbit Scab

Even if your plants have never struggled with cucurbit scab, preventive measures are recommended to make sure you don’t have to battle this disease. An outbreak of cucurbit scab can drastically reduce crop yield, and for those who sell their garden’s bounty, the disease can wreak havoc on profits as well as plants. Incorporate as many of the environmental and cultural control measures listed below as you can to help keep cucurbit scab at bay.

Choose the location where you grow cucurbits carefully, keeping the factors that help discourage cucurbit scab in mind. An ideal location to avoid infection with cucurbit scab will have plenty of drainage in the soil and will offer lots of air circulation to help speed up drying of moisture on foliage and fruits. Avoid areas where there is significant tree cover or a dense canopy over where your cucurbits will grow, as the canopy will inhibit air circulation and help create conditions where cucurbit scab will thrive.

Resistant varieties of susceptible plants are widely available on the market. Choosing to grow a resistant variety is one of the best ways to ensure that your garden won’t be afflicted with cucurbit scab. Seed catalogs and websites that sell seeds will specify which varieties are resistant to cucurbit scab in their product descriptions. Choosing varieties that are recommended for gardeners in your region and are specified as resistant is the best option.

Although certified disease-free seed is not guaranteed to be 100 percent free of the disease, it is much less likely to introduce cucurbit scab to the garden because the seed has been tested and the pathogen has not been found present in the tested sample. When it is available for the variety you wish to grow, certified disease-free seed is better than other options. If you save seed from a crop to use the next season, make sure not to save seeds from diseased plants or any plants in their vicinity.

Make sure to give your plants enough space to allow for sufficient air circulation and to avoid helping the disease to spread via crowded conditions. Refer to the seed packet or planting instructions for the specific variety you are growing to determine the appropriate spacing.

When invasive weeds are present in the field with vulnerable plants or surrounding the perimeter of the field, they can provide an alternate host for the fungus behind cucurbit scab. Weed the area where your cucurbits are growing carefully, and extend your weeding routine to include the surrounding area, and you’ll ensure that the fungus that causes cucurbit scab doesn’t have a place to hide. Weeds can also harbor bugs that will help spread the disease if they are present in the field or the surrounding area.

Because the cucurbit scab fungus can persist in plant debris in the soil for up to three years, keeping the garden cleaned up is vital to prevent the disease or, if the disease is already present, to control its spread. Regularly clean up debris from cucurbits and weeds as well as the other organic materials where the fungus can grow: wood from nearby trees, dead leaves, cow patties, and dead insects or animals. After you’ve harvested the last of your crop, clean the field very well so the fungus doesn’t have a spot to spend the winter. Remove all the vines, roots, and fruit from cucurbit crops at the end of the season.

Avoid working in the field with your cucurbit crops when conditions are wet due to morning dew, rainfall, or recent irrigation. Moist conditions help to spread the disease, and your movements in the field or use of gardening tools when the weather is wet can help transport the fungus from one plant to another.

Overhead irrigation systems or watering plants from above can help foster the moist conditions that lead to cucurbit scab infection and can also help to spread the disease. Opt instead for irrigation systems that water plants at the soil level, or if you water by hand, aim the water around the base of your plants, above where the roots are growing underground. (This method is also the best way to hydrate your plants, as they are unable to absorb moisture through foliage, fruit, or anywhere other than through their root system. Additionally, water that splashes onto plants when they’re irrigated from above can cause sunscald when the water is heated by the sun or lead to other fungal diseases.)

After growing a vulnerable crop in a certain location, rotate out with plants that are not in the cucurbit family for two or three years before growing cucurbits in that spot again.

Some gardeners choose to use preventive fungicide treatments to help avoid their crops becoming infected with cucurbit scab. These sprays are normally applied only when an area has a history of infection with cucurbit scab. For fungicides to work, they must be used before susceptible plants have developed fruit. For squash plants, fungicide should be applied when plants begin to bloom. Melon crops and pumpkins should receive fungicide treatments when vines begin to run when fungicide will be used.

Additionally, fungicide treatments may not be effective if they are used when the weather is cool and wet for long periods. Even if you choose to use fungicides, we recommend employing as many environmental and cultural controls from the list above as you can to have the maximum possible protection against the disease.

Cucurbit scab is a serious problem for gardeners who grow vulnerable crops. But as you can see, there are plenty of steps you can take to drastically reduce the risk of your plants becoming infected or to treat the disease if it does appear in your garden. It can be extremely difficult to kick an active cucurbit scab infection once it has taken hold. There’s no reason to wait until you see the symptoms of cucurbit scab to take action—as they say, an ounce of prevention is worth a pound of cure.