Cucumbers, Melons, Squash

John T. Middleton, Guy Weston Bohn.

Most cucurbits are perishable and are consumed shortly after harvest cucumber (Cucumis sativus), muskmelons (C. melo), summer squash (Cucurbita pepo), and watermelon (Citrullus vulgaris) are some of them.

A few, such as winter squash (Cucurbita maxima and C. moschata) and pumpkin (C. pepo, C. moschata, and C. mixta) can be stored for later use.

Others are processed, such as some varieties of cucumber and gherkin (Cucumis anguria). Some are used as ornamental gourds (mostly Cucurbita pepo and Lagenaria siceraria) and for commercial fibers, such as the luffas (Luffa aegyptiaca).

A variety of diseases affect the fruits and other parts of the plant, rotting the produce or affecting the plant in such a way that the yield is reduced.

SEED DECAY AND SEEDLING BLIGHT are diseases that reduce plant stands. Good stands of cucumber, muskmelon, squash, and watermelon are usually had by seeding in moist, warm soils in warm weather. Poor stands usually result when seeds are sown in cool, wet soils in cool weather. Poor stands are due to decay of the seed or to the death of seedlings, caused by action of such soil-inhabiting fungi as Pythium irregulars, P. ultimum, and Rhizoctonia solani.

The severity of seed decay and seedling blight is related to the relative growth rates of the attacking fungi and the cucurbit plant.

Cucurbit seeds germinate best and seedlings grow most rapidly when the soil temperatures are between 85 and 95 F. and soil moisture is moderate and not excessive. Seeds planted in cold, overly wet soils, or planted too deep, germinate and emerge slowly; they may decay before they break through the seed coat. After they break through, the slow-growing, tender seedlings are susceptible to attack by Pythium and Rhizoctonia for several days.

If the soil temperature and moisture are right, the cucurbit grows fast, the period of exposure to the causal fungi is shortened, and the more robust seedling escapes infection or is able to overcome the effects of attack.

Upper left: Scab on squash fruits; the disease affects other cucurbits, including cucumber. Upper right: Cucumber plant infected with the downy mildew fungus; the disease is important along the Atlantic coast. Lower left: Anthracnose of watermelon. Lower right: Spinach blight, caused by the common cucumber mosaic virus.

Seed decay may be controlled by dusting the seed with a protectant fungicide such as chloranil (Spergon) at the rate of 6 ounces to 100 pounds of seed, or thiram (Arasan) at the rate of 4 ounces to 100 pounds. Seed protectants do not control blight of seedlings after they break through the seed coat.

The best stands and minimum losses from seedling blight will be assured by coating the seeds with a protectant fungicide, sowing the seeds as shallow as possible consistent with adequate soil moisture supply, and planting during the warm season when soil temperatures are above 80 F.

ROOT ROT occurs on cucurbits at all stages of growth. The first symptoms are usually stunting, production of small and somewhat yellowish leaves, wilting, failure of fruit to set and mature, and sometimes collapse and death of the plants.

The roots of affected plants may appear water-soaked and flaccid, with some sunken, darkened lesions on the larger, fleshy roots. Occasionally the crown of the plant may become girdled and the top break off.

The two most common root rots are incited by fungi. One, pythium root rot, is caused by the soil inhabitors Pythium aphanidermatum, P. irregulars, and P. ultimum. The other, fusarium root rot, is caused by Fusarium (Hypomyces) solani f. cucurbitae.

Pythium root rot may affect all cultivated cucurbits, but the disease becomes important only in certain seasons on particular crops.

Cucumber, squash, and watermelon are affected by root rot incited by P. irregulars and P. ultimum only in the spring and fall when the soil temperatures are comparatively low and hence favorable for the growth of the fungi and unfavorable for the growth of the plants. Pythium root rot is most damaging in soils that are poorly drained and have been cropped to cucurbits or other plants, such as peas and spinach, which provide an increase in abundance of Pythium in the soil. Land previously cropped to alfalfa, carrots, cereals, crucifers, lettuce, and some other plants that do not support a large population of these species of Pythium in the soil will usually yield satisfactory harvests of cucumber, squash, and watermelon.

Root rot of muskmelon, incited by Pythium aphanidermatum, occurs during the warm season and not when the soil temperatures are low, for the causal fungus grows best at high temperatures and sufficiently better than the muskmelon so that root decay results. Root rot of muskmelon is usually most severe in land previously cropped to alfalfa and sugar beets, for the fungus is able to increase in numbers on those crops, providing a large fungus population for infection of the muskmelon crop. Damage from pythium root rot may be minimized by planting on land which was not sown to muskmelon at least the preceding year and which is adequately drained and by planning an appropriate rotation system that avoids alfalfa and sugar beets the preceding year.

Fusarium root rot primarily affects pumpkin and squash. It occasionally occurs on muskmelon, watermelon, and cucumber. The fungus may affect the fibrous and fleshy roots, but it is most commonly found on the main root and crown of the plant, usually girdling it at the ground level and causing the collapse of the plant. The fungus can persist in the soil for years, but is able to attack only cucurbits. Fruits on the ground are frequently rotted by the fungus. Seeds harvested from such infected fruits carry the fungus on their surface, providing a ready means for introducing it into clean, uninfested soils. Seed may be disinfested by soaking it for 5 minutes in 1 1,000 mercuric chloride and rinsing well in water. Land known to be contaminated by the fungus should not be planted to susceptible cucurbits.

FUSARIUM WILT in muskmelon and watermelon is caused by fungi that enter the root system from the soil and grow in the water-conducting vessels of the plants.

If very young plants are attacked the seedlings may rot before or after emergence or the plants may become stunted. When mature plants are attacked, the tips of the runners wilt, and gradually the entire plant wilts and dies. The woody part of the stem is discolored brown. The roots may exhibit cankers.

The disease is most severe when soil temperatures are comparatively high, about 80 F., favoring growth of the causal fungi, and sufficiently below the optimum for plant growth, about 90 F., to retard plant growth enough to render them more susceptible to damage.