Scab, caused by Cladosporium cucumerinum is a significant problem for summer and winter squash, pumpkin, melon, and watermelon. Resistant cultivars of cucumber are widely available. The fungus can infect all above ground portions of the plant, but is most serious when it occurs on fruit.
On leaves, symptoms consist of gray to brown lesions with a yellow halo that may become shot-holed in appearance. Numerous lesions may cause leaf twisting and deformation. Fruit lesions at first resemble insect stings and appear as small, sunken, gray areas. Dark green sporulation may occur in the lesions. A sticky substance may ooze from infected tissue and secondary decay organisms may invade.
The pathogen survives in the soil on infected crop debris for up to three years, may be seedborne, and is capable of saprophytic growth. The disease is favored by fog, heavy dew, light rains, and cool temperatures. The pathogen spreads on moist air, on insects, equipment, and humans.
Cultural Controls & Prevention:
- Rotate with non-cucurbit crops for 2-3 years.
- Plant resistant cultivars if available.
- Use seed that does not have significant levels of the pathogen.
- Select sites with well-drained soil and good air movement for rapid drying of foliage and fruit.
- Avoid overhead irrigation and dense plant canopies.
- Protectant fungicides are recommended for control. Fungicide sprays may not be effective during extended cool, wet weather due to the short disease cycle of this pathogen.
- To be effective, fungicides must be applied before fruit formation.
Chemical Controls & Pesticides:
For current information on disease recommendations in specific crops including information on chemical control & pesticide management, please visit the New England Vegetable Management Guide website.