Diseases Primarily of Cucurbits
Angular leaf spot (Pseudomonas lachrymans)
Angular leaf spot is a bacterial disease of cucurbits which first appears as small, water-soaked spots on the leaves. Leaf symptoms are sometimes difficult to distinguish from those of scab and downy mildew. The increase in size of the leafspots is limited by veins, giving the lesions an angular appearance. The upper surface of the spot fades to tan, and many of the diseased spots eventually drop out, leaving irregular holes. On the lower surface of the tan lesion, a white crust is formed when the sticky, milky ooze produced by the spot dries. This crust can be used to distinguish scab-infested plants from those with angular leaf spot. On stems and fruits, the lesions have a water-soaked appearance and may also become covered with crusty white exudate. Brown areas appear on the surface and brown streaks extend internally down to the seeds. Downy mildew is distinguishable from angular leafspot by chlorotic margins and purple fuzz on leaf undersides where the spores form.
The angular leaf spot pathogen survives on infected plant residue, volunteer seedlings, infected seed, and older plantings. It spreads through the field by any movement of people or equipment, especially when foliage is wet. Temperatures between 70 degrees and 80 degrees F enhance disease development. Disease-free seed and a 1 to 2 year rotation lower disease incidence. Most of the commercial pickling and slicing types of cucumbers are resistant to angular leaf spot. Copper-containing fungicides will control this disease, but high application rates of metallic copper will cause injury and reduce yields. Tender, young, rapidly growing plants are the most susceptible to copper injury.
Anthracnose (Colletotrichum lagenarium)
Anthracnose is said to limit honeydew production in the eastern United States, and is more of a problem on honeydew, watermelon and cucumber than on muskmelons. Anthracnose is a fungal disease infecting young cucurbit plant tissue. It originates on leaf veins as small, circular, yellow spots which gradually become brown and enlarge to 0.25 inch. In severe cases, the spots coalesce to cover the entire leaf. In watermelons, the lesions are black. In cucumbers, large lesions may resemble those resulting from gummy stem blight infections. Very young leaves may be distorted. The fungus also invades the fruits, making them vulnerable to opportunistic infection by soft rot organisms.
Anthracnose is spread by the presence of infected plant residues and volunteer cucurbits, by use of infected seed, and by infection from older infected plantings nearby. It is also spread by people, machinery, animals, or insects moving through the fields, especially when the foliage is wet. Fall crops are more likely to be infected than summer crops because the disease spreads most rapidly in warm, wet weather. Crop rotation and the use of resistant cultivars help control anthracnose in the spring or early summer crops when disease pressure is less, but may be inadequate for control in the fall.) The anthracnose fungus lives in the soil for up to two years.
Belly rot (Rhizoctonia solani)
Belly rot of cucumber appears as irregular scabby lesions on the underside of the fruit. It develops most rapidly at temperatures over 82 degrees F but is inhibited by wet weather. Belly rot develops rapidly when conditions are only slightly moist. Preventing direct contact with the ground by using plastic or other mulches protects the fruit to some extent because this fungus is soil-borne. Fungicides, if used, should be applied when the vines are in transition from upright to horizontal growth (tip-over). Cultivars differ in susceptibility, but none are resistant.
Downy mildew (Pseudoperonospora cubensis)
Downy mildew, potentially a very serious problem in cucurbits, first appears as small yellowish areas on the leaves. When the leaves are wet overnight, spots may appear in the early morning as water-soaked, somewhat angular areas on the lower sides of leaves. The lesions change from yellow to brown as they enlarge, but retain an angular pattern of development. On the lower sides of leaves, blackish-purple spore masses appear over the yellow-brown lesions, and severely infected leaves may die in 10 to 14 days. Downy mildew develops on older leaves first and does not affect stems or fruit.(38) Downy mildew can overwinter in some areas, but usually moves northward from subtropical areas. Unlike powdery mildew, which grows rapidly in dry weather, downy mildew development is promoted by warm to hot, wet weather.The most practical method of control is the use of resistant cultivars. If fungicides are to be used, they must be applied with high pressure sprayers to assure complete coverage of foliage undersides. Good air drainage and growing in fields isolated from earlier plantings will also lessen disease severity. Rotation and reduced planting density to promote more open canopies should also be considered in areas with high incidence of this disease.
Fruit decay of squash (Choanephora cucurbitarum)
When the fungus, Choanephora, becomes established in senescent blossoms, it invades the young squash fruits, causing them to decay. Infected fruits look like pin cushions covered with small, black headed pins. Wet periods promote prolific growth which restricts air circulation around the base of the plant and increases the incidence of this disease. The fungus is spread by insects, wind and splashing water. No controls are available.
Gummy stem blight (Didymella bryoniae)
Gummy stem blight is a warm, wet weather, fungal disease of cucurbits which primarily affects older tissue. It is the most serious disease of melons in Georgia and the most destructive foliar disease of watermelon and cucumber in the southeastern states. In cucumber, necrotic areas appear on the margins of the leaves. Circular tan lesions girdle the stem in advanced cases. During prolonged rainy periods this lesion-girdling process can occur so rapidly that foliage and vines appear to suddenly collapse. When present, fruit lesions appear as small water-soaked areas, similar to the red-brown lesions caused by anthracnose, but enlarging to indefinite size. A brown streak may appear near the blossom end of the infected fruit. Early stages of gummy stem blight may resemble late stages of anthracnose, but lesions are larger, often originating at leaf margins.
In melons, symptoms progress from the center of the plant outward until the entire plant is affected. Elongated water-soaked spots on leaves, petioles and stems turn light brown to gray. Stem spots appear first at the nodes, elongating into streaks. Gum oozes from near these stem streaks; leaves on affected vines turn yellow and die.
Sources of gummy stem blight infection include seed, older plantings, plant residues, and volunteer seedlings. Movement of workers or equipment spreads the infection. When possible, it is best to use long rotations with crops other than cucurbits. Other control measures include using disease-free seed and applica-tions of extra potassium at fruit formation.
Conventional control is based on fungicide applications beginning when the vines begin to run, or at the first signs of the pathogen, but control is often inadequate. At least six applications of fungicides were necessary under the heavy disease pressure conditions of South Carolina to prevent loss of marketable yield. Resistant cultivars exist, but no currently available curcubit cultivars have commercially acceptable levels of resistance.
Powdery mildew (Erysiphe cichoracearum and Sphaerotheca fuliginea)
Powdery mildew is widely distributed and destructive. In cucurbits, the powdery mildew fungus first appears on old leaves as tiny, white superficial spots, followed by infection of stems and young leaves. As spots enlarge, they become covered with powdery spore masses. Under ideal conditions for disease development, including hot, dry weather, the entire leaf surface becomes covered with the spore masses and the leaves drop prematurely. Fruits are not infected but may have poor quality and texture because of the limited photosynthetic capacity of the reduced leaf area. Powdery mildew is spread by wind and survives on weeds, and other crops, especially cucurbits. Resistant cucumber and muskmelon cultivars are commercially available.
In watermelon and muskmelon, powdery mildew can be controlled by sulfur and sulfur-based fungicides, but fungicide-insensitive pathogen strains have also been reported control of cucumber powdery mildew with single sprays of potassium and phosphates in dilute water solutions with a surfactant. Sulfur should be used on muskmelons very carefully because some cultivars will be damaged by this chemical.
Cottony leak (Pythium spp.) or Pythium fruit rot
This common fungal disease of cucumber starts as a small water-soaked spot, usually appearing where fruits touch the ground, as in belly rot infections. These spots enlarge into raised, water-filled blisters about 0.25-inch in diameter. Subsequent disease development is very rapid, with the blisters coalescing and the entire infected area of the fruit becoming soft, watery, brown and covered with a fluffy white mat. Transition from spot to spoilage may occur within 48 hours under conditions of high humidity and standing water. In cucumber, cottony leak symptoms are often confused with Phytophthora fruit rot.
Suggested controls are:
Rind necrosis of melon causes light brown discolored areas in the rind which are generally firm and dry to the touch. It rarely extends in to the flesh. The exact cause is unknown, but a bacterium may be involved.
Scab (spot rot) of cucumber (Cladosporium cucumerinum)
Scab is a fungal disease of cucumber first appearing on leaves as small, circular to angular, water-soaked brownish spots. These spots resemble angular leaf spot except that the undersides of scab lesions are not covered with a crusty white exudate. Under high humidity conditions, scab spots produce a velvety mat of mycelium and spores. On fruit, a gummy brown exudate occurs on the surface of the water-soaked areas and, as the spots spread, the tissue sinks and dries to form a tan scab which expands up to one-half inch in diameter. Brown streaks do not extend into the flesh, also distinguishing scab from angular leaf spot. Symptoms first appear on the leaves, petioles, or stems but the most noticeable damage is on the fruit. This disease survives on plant residue, older plantings, and volunteer seedlings. Unlike most other cucurbit diseases, scab is not seedborne and does not require splashing rain to disseminate the spores. Spores are airborne and can enter a leaf that is merely wet by dew, making the disease difficult to control in areas with cool, wet nights, such as in the Appalachians. Good air and soil drainage will lessen scab severity. Most cucumber varieties are resistant to scab.
bridgesj@unity.ncsu.edu