Growing Plants

Black Spots on Watermelons: Causes and Solutions

Black spots on watermelons can be a common concern for enthusiasts who eagerly await the season of juicy delights. This blog delves into the underlying causes behind these unwelcome blemishes.

By understanding the origins of these diseases of watermelon, we can equip ourselves with the knowledge to cultivate healthy fruits, ensuring a sweet and satisfying harvest for all our summertime gatherings.

What Causes Black Spots On Watermelons?

black spots on watermelon

Watermelon vines are easy to cultivate but can get sick and develop black spots. The three main reasons for these fruit lesions are:

  • A common disease called Anthracnose fungus.
  • Another problem is called blossom end rot.
  • A fruit rot phase is known as black rot.

Anthracnose Fungus

Anthracnose fungus is one of the most common watermelon diseases. It causes black and brown spots on infected leaves, stems, and fruits. It loves warm and wet conditions.

The disease build-up starts as small brown spots on the diseased leaves and then gets bigger combining with other spots and cracks. These blemishes are also initial symptoms of wilt in zucchini plants.

The stems and petioles also get tan-colored marks, and sometimes the whole vine dies. The fruit gets circular dark brown spots of ¼ to ½ inch in width.

These fruit spots crack and sink, and the inside of the fruit rots.

The fungus survives on old diseased plant debris and can stay in the field from old melon-infected seeds. It spreads to other plants through rainy weather, sprinkler water, or surface runoff.

When new dark spots appear, they produce spores that similarly spread the disease to other plants.

Blossom End Rot

Blossom end rot is a common issue that affects watermelon fruits. It occurs when there’s insufficient calcium; the plant doesn’t get enough water, or too much nitrogen in the soil.

Typically, this condition impacts the initial developing fruits during rapid plant growth.

Dark fruit spots emerge on the blossom end of the affected watermelons, which can expand, ultimately covering the entire base of the fruit.

Black Rot

Black Rot (also known as gummy stem blight) is a disease that causes spots on leaves and fruits, caused by a watermelon fungus called Didymella bryoniae.

The first fruit symptoms of this disease are small moist patches on the watermelon, which get bigger and become sunken and discolored over time.

Along the disease development, black fruiting bodies also appear, making the lesions on watermelon fruit even worse.

Gummy stem blight infection can harm the fruit quality and appearance, threatening watermelon crops and harvests.

Belly Rot

Belly bot on watermelon is a fungal infection caused by certain fungi like Pythium aphanidermatum, Rhizoctonia, or Sclerotium rolfsii.

This issue is more common during hot and wet weather or after heavy rain. It can also happen if the soil has poor drainage.

The initial symptoms of belly rot include a wet area on the bottom side of the fruit resting on the ground, and the fruit sinks and gets covered in white fungus.

When you cut open the diseased fruit, the inside rind may look brown or black. It’s essential to quickly spot and deal with this problem to protect the watermelon crop from belly rot.

How To Treat Black Spots On Watermelon Leaves And Fruits?

watermelon leaves turning black

By implementing these strategies, watermelon growers can successfully cure diseased vines and ensure healthier and more fruitful harvests.

Fungicide Applications

You may need fungicide applications with fungicides or bactericides to manage foliar diseases effectively.

For the best results, it’s typically advised to apply these fungicide sprays regularly, every 7 to 14 days, as a preventive measure.

Disease-Resistant Varieties

Disease-resistant varieties are a great way to protect your plants from anthracnose or gummy stem blight epidemics. It’s the most effective and affordable way to control these destructive diseases.

However, no resistant varieties are available for other diseases, and many plants don’t have any natural resistance.

Disease-resistant varieties are the most affordable way to control these fruit infections.

Crop Rotation

To cure black spots on watermelon plants and prevent diseases in the soil and lesions on leaves, a helpful approach is crop rotation.

This means planting non-cucurbit crops (not related to watermelon) every three to four years.

Doing this helps manage the growth of harmful fungi and bacteria that cause destructive diseases in watermelons. These disease-causing agents can survive in the soil or old plant remains.

If watermelon crops are grown in the same spot repeatedly, these harmful microorganisms can multiply and cause more damage to healthy plants.

Crop rotation breaks their cycle and keeps the watermelon plants healthier long-term.

Post-harvest Clean-up

To stop bacterial fruit blotch from spreading after harvest time, quickly remove all infected vines and debris. Please put them in bags and dispose of them safely. Don’t compost them.

They can stick around on old infected plant remains and infect more vegetables if improperly handled. So, taking these steps will help protect your crops from foliar disease.

How Do You Prevent Black Spots From Forming On Watermelon?

brown spots on watermelon

Preventing black spots from forming on watermelons involves a combination of proactive measures and vigilant care.

  • Optimal Soil and Watering: Ensure you plant them in well-drained soils and water the plants early in the day. This way, the vines have enough time to dry out before nightfall.
  • Pest Management: Watch for pests like powdery mildew, and get rid of beetles and aphids, as they can weaken the diseased vines and lead to black rot.
  • Four-year Rotation Practice: Rotate where you grow watermelons every 3 – 4 years in your garden, avoiding areas where cucurbit family plants have been recently grown.

Following these straightforward and practical steps, you can protect your crop development and enjoy healthier, spot-free fruits.


Can You Eat Diseased Watermelon With Black Spots?

It depends on the size and extent of the black spots.

Small brown lesions are usually harmless and can be cut off, but if they are large or cover a significant portion of the watermelon, it may be best to avoid consuming it.

How Long Does It Take For Anthracnose To Spread In A Watermelon Patch?

Anthracnose can spread rapidly in watermelon patches, especially in warm and humid conditions.

It can take as little as a few days for the disease to spread from one cucurbit plant to another, and it can quickly infect the entire patch if left untreated.

What Is The Best Fungicide For Watermelon?

The best fungicide for watermelon depends on the specific fungal disease affecting the crop.

Generally, an application of fungicides with different modes of action is recommended to prevent the development of resistance.

Common fungicides used in watermelon production include copper fungicides, systemic fungicides, and protectant fungicides.


Identifying the causes of black spots on watermelons empowers us to take preventive actions and protect our crop development.

Whether it’s managing fungal pathogens with timely sprays, opting for disease-resistant varieties, or practicing crop rotation, there are various ways to keep these common watermelon diseases away.

We can savor the delightful sweetness of spot-free watermelons all summer by implementing these measures and staying vigilant.

Samuel Mark

Hello I am Samuel. Samuel's Garden is a garden blog where I share my experiences in garden caring and tree growth. Hope you enjoy it!

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