Plant Health Problems
The disease problems on watermelon are similar to those of other Cucurbits. See Cucurbit for a detailed discussion of the other common diseases of this host.

Diseases caused by Fungi:

Fusarium wilt of watermelon, Fusarium oxysporum sp. niveum.
Normal disease progression starts with wilting of leaves. Then leaves turn yellow, starting at the crown of the plants and progressing outwards. Eventually, the entire vine turns brown and dies. Plants that survive remain stunted and rarely produce harvestable fruit.

Since the disease overwinters in the soil, crop rotation (5-7 years) reduces damage.

Insect Problems:

Melon aphid, Aphis gossypii.
The melon aphid sometimes infests watermelon. This aphid infests many different kinds of plants both in the field and in greenhouses, but it is particularly troublesome on cucurbits. It generally makes its appearance in scattered places in the field late in the summer and may become abundant on the undersides of the leaves. When aphids are abundant, they may cause a downward twisting or cupping of the affected leaves. Honeydew from the aphids may cause the fruit to be sticky, and aphids may also transmit viral pathogens. See aphid fact sheet.

Melonworm, Diaphania hyalinata.
These insects will attack watermelon on rare occasions. The slender, active greenish caterpillars have two narrow white stripes running the full length of their bodies. They feed mostly on the foliage, but may burrow into the fruit. They are rare, however, and no specific control measures have been needed.

Pickleworm, Diaphania nitidalis.
This insect occasionally causes damage to late maturing crops. The white, yellowish or greenish larvae have yellow brown heads and are about three-fifths of an inch long when mature. They burrow in the bud, blossom, vines and fruit. Control is not generally needed.

Squash beetle, Epilachna borealis.
The squash ladybeetle occasionally feeds upon the leaves of watermelon. This ladybeetle and the Mexican bean beetle are two kinds of ladybeetles that injure plants in Connecticut. Most others are beneficial and devour aphids and scale insects. The squash ladybeetle hibernates in sheltered places and emerges to lay its eggs in June on the underside of the leaves. Both adults and larvae feed on the leaves. The larvae usually appear about the middle of July. They are about 5/8" long, bright yellow with six rows of long, black, branched spines. The adult is dull yellow and marked with 12 black spots.

There is one generation each season. Control is not usually necessary.