Gummy Stem Blight and Phoma Blight on Cucurbits
Vegetable Disease Information Note 8 (VDIN-008)
Charles W Averre, Extension Plant Pathologist

Gummy stem blight or black rot and Phoma blight are stem and foliage diseases of cucumber, cantaloupe, pumpkin, and watermelon. They may also result in fruit rots on these and other cucurbits. The diseases are caused by two different fungi and occur in NC wherever cucurbits are grown, but are less or a problem in the mountains of NC. In recent years gummy stem blight and Phoma blight are often the most serious foliar diseases on cucurbits.

Symptoms of gummy stem blight on cantaloupe and cucumber leaves are expressed by irregularly circular tan spots. On watermelon leaves, the cucurbit stems near the crown, dark spots are produced with sunken streaks or cracks accompanied by a gummy ooze. The disease usually spreads from the center of the crown outward. The disease on fruits is called black rot. Lesions start out as water-soaked areas which enlarge to an indefinite size. Gummy exudates may appear on fruit and stem lesions, and the black fruiting bodies of the fungus may become conspicuous.

The spots on cucurbit leaves produced by the Phoma blight are usually more irregular in shape and most often are on the margin of the leaves. The spots are light brown on cantaloupe and cucumber and dark brown on watermelon. Fewer lesions are produced on stems but symptoms also tend to spread from the crown of the plant, giving the plant a blighted appearance. Fruit are generally not affected.

Management

The gummy stem blight fungus may be seed-borne and may survive on cucurbit plant debris in the soil from year to year. The Phoma blight fungus may also attack other host plants such as green beans, okra, tomato, and tobacco, and may overwinter on the refuse from these crops. The spores of the fungi germinate and infect young seedlings through the hypocotyl or stem. Cotyledons and young leaves of cantaloupe and watermelon are more susceptible than those of cucumber and pumpkin.

Cucurbit seed should be purchased, if possible, from those companies that produce seed in arid regions such as the interior valleys of California. Seed should also be treated by the seedsman with an approved fungicide. A fungicide such as benomyl (Benlate), chlorothalonil (Bravo), copper, maneb (Manzate D or Dithane M-22 Special), mancozeb (Dithane M-45 or Manzate 200) or Dikar (Polyram) may be used to control the two diseases in the field. It is important to start the spray schedule as soon as the plants emerge and spray at 7-10 day intervals, covering all plant surfaces. High pressure (400 psi), high volume (100 gal/acre for a mature crop) spray equipment provides most effective coverage. Crop rotations with nonhost plants such as small grain, corn, of two or more years are effective in reducing the incidence of these diseases, if disease-free seed are used. Currently no varieties are resistant to these two diseases.

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Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies.