Back to Diseases of cucurbits
Scab is caused by the fungus Cladosporium cucumerinum. The fungus infects cucumbers, melons, summer squash, pumpkins, and winter squash. Watermelon is very resistant to the disease and many varieties of cucumber that have resistance to scab are now available.
- Leaf spots are white to gray and may have a yellow halo.
- As the infection ages, the center of leaf spot falls out. Leaves look shot holed or ragged.
- Cucumber, summer squash, zucchini, & pumpkin have sunken fruit spots that are covered with greenish black velvety fungal growth.
- Somewhat resistant winter squash have raised corky areas on infected fruit.
- The extent of fruit rot varies depending on resistance of variety.
Scab can be introduced into the field on infected seed or as spores carried on moist air currents. Fungal spores are then spread through the field by wind, insects, tools, and workers. Scab survives the winter in plant debris and once introduced, can reoccur season after season if management strategies are not implemented. Disease development is favored by cool (around 70F), moist weather.
Summer squash, pumpkins and cucumbers (varieties not specifically bred for disease resistance) are considered very susceptible to the disease. Fruit infections on these crops will profusely sporulate and rot will extend deep into the fruit.
Gourds and some winter squash (blue hubbard, buttercup) are moderately susceptible and will show symptoms on both leaves and fruit. Fruit infections, will not extend deep into the fruit however. Moderately resistant squash, like acorn and butternut, have few leaf spots and fruit infections will be raised tan corky spots instead of sunken craters. Fruit spots in these crops rarely produce fungal spores.
- Purchase clean seed from a reputable source. Do not save seed from infected plants.
- Resistant varieties of cucumber are available and are the best management option for that crop.
- Rotate vegetables so 2 or more years go by before planting any member of the squash family in the same location.
- Use drip irrigation instead of overhead sprinklers if possible.
- Do not work in plants when wet.
- Remove and destroy infected fruit and vines at the end of the season in small gardens.
- In large fields till under crop residue at the end of the season to speed up decomposition.
- Several fungicides are registered for use against Scab. Preventative sprays are effective but are only necessary in fields with a history of scab. Squash sprays should start at bloom. Melon and pumpkin sprays should start when the vines begin to run.
- Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.