What Causes Growing Watermelons to Rot?

Blossom-End Rot

• Blossom-end rot can lead to significant loss of melons. Water-soaked spots on the fruit are one of the most notable symptoms of the disease. Other diseases may invade affected tissues. Remove any watermelons showing signs of the disease as soon as possible.

  Blossom-end rot is caused by a calcium deficiency. Irregular watering practices can lead to development of the disease. Water your melons to a depth of 6 inches and offer extra water during fruit set and development, suggests the Clemson Cooperative Extension. Avoid wetting the foliage when possible by watering at ground level. Low pH and lack of soil calcium can also lead to blossom-end rot. Apply lime as an amendment to improve the soil, but do so only if you know the soil is lacking in calcium.

Gummy Stem Blight

• Gummy stem blight is a fungal disease that affects the leaves and stems of watermelons and causes black rot in fruit, according to the Clemson Cooperative Extension. Brown lesions, split stems and circular spots on the watermelon fruits may bleed an amber- to brown-colored gummy substance.

  High humidity and temperatures around 75 degrees Fahrenheit are ideal for development of the disease, notes the Cornell University Department of Plant Pathology, which links incidence of the disease to the appearance of powdery mildew infections. Combination fungicides are available to treat both. The fungus that causes gummy stem blight lingers in plant material, so affected plants and dead plant debris should be removed as soon as possible.

Fusarium Wilt

• Fusarium wilt causes discolored foliage, vascular damage and rot near the crown of the plant as it moves up through the roots of infected watermelons. Affected plants may wilt on one side. The infection can lead to death of your plants. The fungus that causes this wilt remains in the soil. Plant resistant varieties of watermelon to prevent development of the disease. Fusarium wilt is one disease that does not move between close plant relatives of watermelon, and no fungicides are available for treatment of the disease, according to the Purdue University Extension.

Prevention

• Practice crop rotation techniques. Allow three years to elapse between planting watermelon or cantaloupe, pumpkin, squash or cucumbers in the same garden space because many diseases can pass among these garden crops.

  The Clemson Cooperative Extension recommends black plastic mulch for use with watermelons because it helps prevent rot, reduces the number of weeds in competition with your plants and helps warm the soil, allowing for earlier planting. The mulch reduces the incidence of rot by keeping melons out of contact with the soil's surface.