7 Common Watermelon Diseases and How to Treat Them

If you’re not careful, bacterial- and fungal-borne diseases can infect and destroy your watermelon plants. While some diseases are more damaging than others, they can all affect fruit. Plant biologist Emily Estep lists seven watermelon diseases, explaining how to identify them, how to treat them, and how to prevent them in the first place.

Written by Emily Estep Horticulture review by Sarah Jay Last updated: August 8, 2024 | 8 min read

Closeup of a Citrullus Vulgaris sapling featuring dark and light green stripes, sitting on the dry soil, attached to fuzzy, thick stems.

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There’s nothing more refreshing than a slice of homegrown watermelon on a warm summer’s day. Growing watermelon is a fun and rewarding experience that’s not too different from growing cucumbers, pumpkins, or squash.

Unfortunately, there are a handful of diseases that you need to watch out for when cultivating watermelon. These diseases are both fungal and bacterial in nature. Some are easy to treat, but others have no cure. If you’ve invested a lot in watermelons this year, it’s crucial that you arm yourself with knowledge.

Whether you suspect your watermelon plants may have a mysterious illness and want to treat it, or you just want to know how to prevent common watermelon diseases, we’ve got you covered.

Anthracnose

Anthracnose is a foliar disease caused by the pathogen Colletotrichum obiculare. It affects all above-ground portions of watermelon plants, including the leaves, the stems, and even the fruit.

This disease is particularly harmful because it can destroy entire vines and cause fruit decay. Sometimes, the anthracnose spots on fruits are overlooked when they’re small and then harvested fruit decays and leaks unexpectedly.

Symptoms of anthracnose include:

Small, brown spots that first appear on the foliage. These spots may be circular or somewhat angular.
Brown spots that grow larger until they crack.
Similar spots and lesions on the petioles and the vines, also growing in size.
As these spots continue to appear and grow, you may discover that entire vines have died.
Small spots on melons that are mostly circular.
Growing spots on melons that crack, sink into the flesh, and cause decay.

If you discover anthracnose in its early stages, you may be able to treat it to some degree. Remove and destroy any visually affected portions of the plant. Apply fungicide every week in humid environments or every two weeks in more arid environments. Fungicides with chlorothalonil may be effective.

There are multiple steps you can take to prevent anthracnose:

Start your season with disease-resistant varieties, such as ‘Crimson Sweet’ and ‘Charleston Gray.’
Rotate crops so that melons and cucumbers are not planted in the same spot within a 3-year window.
Apply a thick layer of mulch over the soil to prevent soil-borne pathogens from splashing onto melon plants when it rains.
Provide space between each melon plant, and prune regularly to prevent the spread of anthracnose from leaf to leaf.
If you’ve had trouble with anthracnose in years prior, consider applying preventative fungicide instead of waiting to see if the disease returns.

Bacterial Fruit Blotch

Bacterial fruit blotch is a disease caused by the bacteria Acidovorax citrulli. The disease first appeared in 1989 in Florida and has been decimating harvests ever since. Though the disease does little damage to foliage, it can cause yield losses of 50 percent.

Leaves of a plant infected with Bacterial blotch, exhibiting small dark spots spread through out the leaves. — Large, off-color “blotches” form on fruit, providing a pathway for pathogens to enter.

The disease is most harmful to melons but can also damage cucumber, pumpkin, and zucchini. Large, off-color “blotches” form on fruit, providing a pathway for pathogens to enter. These secondary pathogens cause fruit decay.

Symptoms of bacterial fruit blotch include:

Small blotches may be visible on cotyledons shortly after germination, but they are easy to miss.
These blotches become brown, small spots that do not look particularly threatening and do not expand much.
When fruits first emerge, they may have small blotches on their surface.
As fruits mature, these blotches become larger and may cover half the fruits’ surface.
Infected fruits will decay as pathogens enter through these large blotches.

Once detected, bacterial fruit blotch is very difficult to treat. Gardeners should rely on preventative measures to avoid bacterial fruit blotch in the first place. However, should the disease emerge, you may have some success applying copper-based fungicide. Begin weekly application before the plant flowers and continue until fruits are harvested.

These are multiple steps you can take to prevent bacterial fruit blotch:

Seek out disease-resistant varieties.
Purchase symptom-free transplants from reliable growers.
At the end of each season, remove any watermelon debris from your garden.
Remove any watermelon seedlings that emerge during the growing season.
Rotate melon crops, keeping them as far away from where they were grown previously as possible.
Use drip-tape irrigation and avoid overhead watering as much as possible.
Preferably, remove and destroy any affected crops as soon as you suspect the disease.

Gummy Stem Blight

Gummy stem blight is a foliar disease caused by Didymella bryoniae, a fungal pathogen that causes cucurbits to rot. This blight can affect plants at any stage in their growth and can damage all parts of the plant, including the fruit.

“Gummy stem” refers to when the disease attacks the stems of the plant, inciting a gummy ooze, though this is more common with cantaloupe. Watermelon plants usually experience gummy stem blight on their leaves.

Symptoms of gummy stem blight include:

Dark leaf spots on the leaves that can be up to an inch in size. These spots usually occur around the margins of leaves.
Leaves with lots of spots will die off.
Watery, oozing stems.
You may see small, black dots on the plant’s petioles. There are the fruiting bodies of the fungus that causes gummy stem blight.

Treatment options for gummy stem blight are limited once it takes hold. You can try treating it with fungicides. Products with chlorothalonil and mancozeb may be effective. Truthfully, you should remove any watermelon plants infected with gummy stem blight as a preventative step for next year.

There are multiple additional steps you can take to prevent gummy stem blight:

Practice crop rotation. Wait at least two years before planting watermelon or other cucurbits in the same spot.
Apply fungicide preventatively if you know you’ve had issues with gummy stem blight in the past.
Alternate fungicides from year to year, as gummy stem blight is resistant to some fungicides.
Use drip-irrigation and avoid watering from overhead. Splashing can spread the disease.
Apply a thick layer of mulch on top of soil in an attempt to prevent leaves from making contact with pathogens living in the soil.

Downy Mildew

Downy mildew is a disease spread by Pseudoperonospora cubensis, a type of water mold. The disease can cause foliar loss to such a degree that watermelon yields are reduced. Loss of leaves can also leave fruits exposed to too much sun, leading to sunscald.

Many ornamental and edible plants are subject to downy mildew, one of which is watermelon. Not to be confused with powdery mildew, which looks like a gray powder, downy mildew first appears as yellow splotches that fade to brown.

Symptoms of downy mildew include:

Yellow or light green splotches on the oldest leaves of the plant.
Splotches that get bigger, turn brown, and slowly cover newer leaves.
Leaves that become consumed with downy mildew shrivel up and fall off their petioles.
The petioles and stems will remain and appear healthy, which helps differentiate downy mildew from anthracnose.
Fungal growth on the undersides of leaves, corresponding to splotches on the leaf surface.

If you suspect downy mildew on your watermelon plants, remove the infected leaves right away. If possible, use shade cloth to protect newly exposed fruits. You can also attempt to treat downy mildew with fungicides, including chlorothalonil, copper, and mancozeb.

There are a variety of steps you can take to prevent downy mildew:

Apply fungicides (such as chlorothalonil and mancozeb) preventatively, before downy mildew appears.
Water via drip-irrigation to minimize leaf wetness as much as possible.
During periods of high humidity, inspect watermelon leaves regularly for early signs of downy mildew.
Keep your eye on online downy mildew forecasts, such as the Cucurbit Downy Mildew Forecast, which tracks the disease via airborne spores.

Note: Downy mildew only survives on living plants. This means that it does not live in the soil over winter, so crop rotation will have no effect.

Fusarium Wilt

Fusarium wilt is a soilborne watermelon disease that’s caused by Fusarium oxysporum, a fungal pathogen. It is a problem for many plant species, including solanaceous crops and many ornamentals. It can spread through soil, seeds, and the air.

Gardeners need to keep a serious eye out for Fusarium wilt, because it can kill entire plants and does so before any ripe melons are ready to harvest. Individual vines suffer first, but eventually, the entire plant will die. This disease is particularly insidious since it can destroy many plant species.

Symptoms of Fusarium wilt include:

Individual vines may first appear dull or begin to wilt.
Affected vines continue to struggle, turning brown before dying off.
You may see large brown splotches near the crown of the plant, and when it rains, you may even see pink mold on these splotches.
Dead vines may have brick-red coloration when you cut into them.

There is no cure or treatment for Fusarium wilt. Since this disease will kill watermelon vines before they produce fruit, there is no reason to try and salvage or treat diseased plants. Remove them entirely and destroy them. It is difficult to manage.

These are the steps you should take to prevent Fusarium wilt:

Choose watermelon varieties that are resistant to Fusarium wilt, such as ‘Crimson Sweet’ and ‘Charleston Gray.’
Practice aggressive crop rotation. Wait at least five years before planting again in a site that had Fusarium wilt. Studies have found that planting hairy vetch as a cover crop reduces Fusarium wilt in the meantime.
Consider avoiding locations with a history of Fusarium wilt entirely.
Always remove any plants you suspect of having Fusarium wilt and sanitize your tools afterwards.

Angular Leaf Spot

Angular leaf spot is a bacterial disease caused by Pseudomonas syringae pv. lachrymans bacteria. It affects many cucurbit species, including watermelon. Angular leaf spot is not as deadly to watermelons as some of the other diseases on this list, though it does cause superficial lesions on fruit. Sometimes, these lesions develop into rot.

Additionally, the bacteria that causes angular leaf spot can not only enter the fruit but also the seed and contaminated seeds can infect fields. This is why you should never collect seeds from watermelons or other similar fruits that you suspect may have had contact with angular leaf spot.

Symptoms of angular leaf spot include:

Small, angular brown spots on leaves have a yellow ring around them. These spots will be located along the veins of the leaves.
These spots will dry out, become brown, and then fall out of the foliage, leaving holes behind.
Small, watery, brown marks on fruit that may rot.
When infected leaves get wet, they may have a white liquid on their undersides. This is the bacteria spreading.

If you suspect your watermelon plants have angular leaf spot, you can try applying a copper fungicide. This may slow the spread of disease if you start early, protecting healthy plant tissue. Once it’s progressed significantly, there’s not much you can do.

Take these steps to prevent angular leaf spot:

Seek out disease-resistant varieties, and always buy seeds from reputable sellers.
Practice crop rotation, waiting at least two years before planting cucurbit crops in the same spot.
Use drip-irrigation to avoid getting the leaves wet, and never handle the leaves when wet.
Always practice excellent sanitation with your tools and hands when dealing with watermelon, so you don’t spread the bacteria.

Verticillium Wilt

Basil seedlings planted in a garden bed, infected with Verticillium, with the seedlings' stems and leaves, turning brown, wilting. — As the vines and leaves die off, any remaining fruit may struggle with sunscald.

Verticillium wilt is caused by the fungus Verticillium dahliae. It can infect a long list of plants. The disease will kill entire watermelon vines before killing the whole plant. As the vines and leaves die off, remaining fruit may struggle with sunscald.

Verticillium wilt has a lot in common with Fusarium wilt. Symptoms of verticillium wilt include:

Yellow chunks on leaves may appear shortly after the plant has first set fruit.
Leaves will wilt and die along individual vines.
After individual vines die, the entire plant may follow.
While the interior of dead vines has a reddish tint when plants have Fusarium wilt, they will have a lighter, brown tint with verticillium wilt.

There is no treatment for verticillium wilt. Once infected, it’s only a matter of time before plants die. You should remove infected plants as an early form of prevention for next year’s crops, especially since so many species are susceptible to verticillium wilt.

Utilize these prevention methods to avoid problems with verticillium wilt:

Practice extreme crop rotation and consider avoiding areas with a history of verticillium wilt altogether.
Do not plant watermelon right after cotton, peanuts, or potatoes, which also struggle with this disease.
Consider using a plastic tarp as a mulch layer between the soil and your watermelon plants.

Key Takeaways

There are a handful of diseases that can seriously harm watermelon plants. Once you spot many of these symptoms, the best thing you can do is usually remove the plants entirely. To prevent disease, practice crop rotation, keep your tools clean, and grow resistant varieties whenever possible.