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How to Treat 7 Deadly Watermelon Diseases

Growing watermelon in your garden is an exciting way to add this summertime fruit to your diet. Unfortunately, instead of biting into a juicy slice of watermelon at the end of the season, you’re sometimes looking at wilted vines and diseased fruit. You may wonder what went wrong. 

Keep reading to learn how to identify the most common watermelon diseases and treat them. You may just save your summer garden!

1. Powdery Mildew

How to Treat Watermelon Diseases

How to Identify Powdery Mildew

Powdery mildew is one of the most common watermelon diseases. You may notice a powdery coating on the vine’s leaves, which is a major sign of this fungal infection. 

Initially, it may appear harmless, as the infection only affects the leaves, not the fruit. But, as the powdery mildew grows, the leaves will wilt and fall off. Leaves are important for producing healthy fruit. They also provide shelter from the sun, so you could end up with sunburned fruit. 

How to Treat Powdery Mildew

To treat powdery mildew, apply a fungicide to the affected areas. Fungicides work even better if applied before an infection occurs, so keep that in mind for next season if you’re experiencing repeated watermelon diseases. 

Because powdery mildew is often caused by a lack of airflow, make sure you are giving all your watermelon plants plenty of space to spread without crowding one another. This will help you avoid watermelon diseases.

2. Downy Mildew

Downy mildew

How to Identify Downy Mildew

Similar to powdery mildew, downy mildew only affects the leaves of the plant. The big difference is that downy mildew is signified by drooping leaves accompanied by brown and yellow spotting. It is less common than powdery mildew, but much more damaging.

This late-season infection spreads on the underside of the leaves in high humidity. The widespread damage can often lead to the whole plant dying, and the existing fruit getting sunburned.

How to Treat Downy Mildew

The best way to fight downy mildew is to apply a fungicide to the plant before infection. It may be too late for this season’s crop, but you’ll be ready to fight any pesky watermelon diseases next year!

3. Alternaria Leaf Spot

Alternia Leaf Spot

How to Identify Alternaria Leaf Spot

Alternaria leaf spot is another one of the watermelon diseases that solely affects the leaves. A common symptom of this fungal infection is brown spots covering the leaves. The spots will usually get larger over time, causing the leaves to fall off the vine. 

How does it spread? Similar to mildew, plants that are exposed to constant wet conditions make perfect hosts for Alternaria leaf spot. 

How to Treat Alternaria Leaf Spot

A good way to prevent Alternaria leaf spot is by using drip irrigation or watering at the soil level. Watering your plants overhead can cause the leaves to stay wet, which provides a breeding ground for several watermelon diseases. 

Rotating your crops in the garden every season is another great idea to keep your garden healthy. For plants that are already affected, apply a biweekly fungicide. 

4. Fusarium Wilt

Fusarium leaf wilt

How to Identify Fusarium Wilt

Fusarium wilt is a fungal infection that starts in the roots of the watermelon plant, then spreads to the vine tissue, causing the whole plant to wilt. You may notice your plant’s leaves looking grayish at first, then proceeding to wilt and turn yellow. 

Luckily, this is one of the watermelon diseases that only affect certain cultivars of watermelon, so you don’t have to worry about it spreading to your vegetables.

How to Treat Fusarium Wilt

To prevent Fusarium wilt from entering your garden, look for varieties of watermelon that are resistant to this disease. Crop rotation is another great preventative method, lessening the fungi spores’ chance of survival in the soil. 

You can also add lime to create a neutral pH level in your garden, which will help keep Fusarium wilt from spreading. 

5. Gummy Stem Blight

Watermelon disease

How to Identify Gummy Stem Blight

Another one of the most common watermelon diseases is gummy stem blight. A symptom of this fungal infection includes brownish-black, wrinkled spots on leaves. There will also be cankers, or lesions, on the vine stem, which will secrete a brown gummy substance. 

Gummy stem blight thrives in wet, humid conditions. Because it’s seed and soil-borne, this disease can occur at any time throughout the growing season, even as early as the seedling stage. Transplants can also be susceptible to this infection. 

How to Treat Gummy Stem Blight

Because this fungal infection spreads so easily, it’s important to discard the infected plants instead of trying to treat them.

When starting watermelon seedlings, it’s better to be safe than sorry. If any of the seeds look to be diseased or contaminated, immediately dispose of them. It’s also important to only save watermelon seeds from healthy plants in your garden. 

6. Bacterial Fruit Blotch


How to Identify Bacterial Fruit Blotch

Watermelon diseases don’t always just affect leaves. They can affect the fruit, as well. While you may notice spots on just the leaves at first, bacterial fruit blotch will create lesions on the top of the fruit right before harvest time. That’s terrible news if you were looking forward to enjoying your watermelon in the next couple of weeks!

The infection usually just affects the rind of the watermelon, not the meat. Unfortunately, it can still cause the rind to crack, allowing the whole fruit to decay. 

How to Treat Bacterial Fruit Blotch

To treat bacterial fruit blotch, apply a copper fungicide to the affected areas in your garden, following the package instructions. 

Similar to preventing gummy stem blight, it’s important to know where your seeds and transplants are coming from. Seeds should be saved from disease-free watermelons, and you should only buy transplants from disease-free greenhouses. If you’re not sure, always ask!

7. Anthracnose


How to Identify Anthracnose

Anthracnose appears first as small yellowish-brown spots on the leaves. You may then notice cankers on the stem, which can spread to fruit, causing sinking spots and fruit decay. Infected plants usually have little to no yield, as the anthracnose will hinder whatever growth has been made so far. 

How to Treat Anthracnose

Just like other watermelon diseases, using disease-free seeds and transplants is the number one way to prevent anthracnose. This fungal infection spreads rapidly, so visit your garden often, and apply fungicide as soon as you detect the disease. Also instead of using a sprinkler or spray hose, opt for drip irrigation in your garden. 

If you’re too late to save your watermelon plant, go ahead and remove the plant from your garden and dispose of it to prevent further infection in your surrounding plants. It’s a bummer this year, but you’ll be better prepared for the following seasons. 

Wrapping Up Watermelon Diseases

Once you’ve identified which of the watermelon diseases is destroying your garden, it’s time to make a prevention plan! Being knowledgeable about what causes most diseases and infections, such as high humidity and contaminated seeds, will help you create a healthy growing space in the seasons to come.

Want to learn more about this iconic summer fruit? Then visit our watermelon plants page to learn more about watermelon planting, growing, cooking, and more!