The major production perils affecting watermelons include excessive rain, which can promote disease growth, excess heat, and in non-irrigated areas, drought.
Excessive rain, particularly when accompanied by other critical environmental factors, can affect the watermelon plant at every stage of growth. For example, germination can be reduced if excess rain is accompanied by cool weather at planting time, causing seed rot. Cool, wet weather following emergence of the watermelon plant increases the incidence of damping-off, which also reduces plant survival. Periods of warm, wet weather increase the incidence of foliar diseases such as anthracnose and downy mildew. Excessive rain as watermelons approach maturity can cause white heart (a physiological disorder), lower the melons' sugar content, and result in bursting of the fruit. Flooding, of course, can kill watermelon plants if the roots are submerged in water for a day or more.
Excessive heat and direct sunlight increase the likelihood of yield losses due to sunscald or sunburn, which causes yellowing of the rind. Normally, watermelon vines provide a protective canopy that shades the melons from direct sunlight. Any disturbance of this protective canopy, such as drought, harvesting activities, or diseases, increases the chance of sunscald damage.
Cold temperatures may reduce seed germination. If accompanied by excessive moisture, cold temperatures may cause severe plant losses due to seed rot and damping-off. Replanting may be necessary in such situations.
Drought may reduce watermelon yields by affecting plant growth, limiting the development and size of the melons. In severe situations, plants may die. Drought can also exacerbate losses due to sunscald.
Hail damages young watermelons by causing scars on the rind. Although the damage is only skin deep, a grower cannot sell scarred watermelons because brokers will not handle them if scar-free melons are available.
The most common insect pests affecting watermelons include root maggots, cutworms, cucumber beetles, aphids, thrips, and melon worms. Some cultural practices reduce the potential for economic injury by certain insects. Planting watermelons when optimum growing conditions insure rapid seedling growth, for example, minimizes the period when plants are vulnerable to injury from seedling insect pests, such as cutworms and root maggots. Spring plantings harvested by early July escape the period when many insect pests pose their greatest economic threat.