Scab of cucurbits or vine crops is primarily caused by the fungus Cladosporium cucumerinum. Scab is primarily a disease of cucumber, cantaloupe, honeydew melon, muskmelon, summer and winter squash, true and other pumpkin types, and gourds. Watermelon is highly resistant. Susceptible cucumber varieties have sustained 50 percent or greater yield loss during prolonged cool, moist weather.    

Symptoms

Leaf symptoms appear as numerous small, water-soaked or pale green spots. These spots gradually turn white to gray and become angular. Often, lesions are surrounded by a yellowish halo, and the lesion center tears away, leaving ragged holes in the leaves. On fruit, small (1/8-inch), gray, slightly sunken, oozing, gummy spots develop that resemble insect "stings". Later, the spots enlarge and finally become distinct sunken cavities. Affected fruit are often invaded later by soft-rotting bacteria that produce a mushy, foul-smelling decay. On highly resistant fruits, especially on certain squashes and pumpkins, irregular, knoblike formations may develop.  

Life Cycle

The scab fungus overwinters on the seed and in crop debris (especially squash and pumpkin). Temperatures between 59 and 77 F, accompanied by moist weather, with frequent fogs, heavy dews, and light rains, are most favorable for scab development. Under these conditions, spores are produced and spread locally by splashing rain and by equipment or workers moving through the field, or long distance by moist air currents. Secondary disease cycles can occur as long as the weather remains favorable.  

Management

Management: Crop rotation and clean tillage (for example, plowing) help reduce the risk of disease by reducing the amount of primary inoculum (spores) in the immediate area. Wherever feasible, grow scab-resistant cucumber varieties. Apply recommended fungicides when conditions favor disease.