Cucurbit Scab

Posted: September 30, 2014

Cucurbit scab is caused by the fungus Cladosporium cucumerinum and can cause a dramatic decrease in fruit quality of various cucurbit crops, including summer and winter squash, pumpkin, melon, and watermelon.

Severe Cucumber Scab
Summer squash fruit with severe cucurbit scab symptoms.

Disease Symptoms

Cucurbit scab symptoms appear on leaves and fruit. On leaves, symptoms are gray to brown lesions with a yellow halo. Severe foliar infections can cause leaf deformation. Fruit lesions first appear as small pockmarks, resembling insect damage.

Progression of Cucurbit Scab
Progression of cucurbit scab symptoms on summer squash fruit.

These lesions develop into pussy indentations approximately ¼ inch in diameter.

Puss of Cucurbit Scab
Exudate from cucurbit scab.

Multiple lesions will eventually grow together. The lesions can become sites for secondary decay organisms to take hold. Symptoms on fruit can have substantial economic impacts.

Pathogen Life Cycle

This pathogen overwinters on infected crop debris for up to 3 years. It may also spread through seeds. Spores develop under moist conditions when temperatures are between 59° and 77°F. Spores are spread by insects, farm equipment, and workers brushing against infected plants. In crops such as summer squash that are harvested multiple times per week throughout the season, spreading of the spores once the infection occurs is inevitable.

Prevention and Management

Disease prevention is always the best management strategy. Once the disease has appeared in the field, management is very difficult. Prevention can be accomplished by the use of crop rotation. Fields should be planted with non-cucurbit crops for 2-3 years between cucurbit plantings.

Many scab resistant cultivars are available and should be favored. Seed catalogs will specify resistant cultivars. Seed should also be sourced from reputable seed sources that can ensure pathogen free seed.

Another strategy is to minimize moisture in the field when infection is most likely. Some options are selecting planting sites based on good soil drainage and air flow, selecting drip irrigation over overhead irrigation, managing weeds, and using appropriate plant spacing. Check the Penn State Commercial Vegetable Production Recommendations guide or seed catalogues for appropriate plant spacing for various crops and cultivars.

Protectant fungicides can be applied on a weekly schedule as recommended by the Penn State Commercial Vegetable Production Recommendations guide. Applications should begin well before the onset of fruiting.

References and Sources for More Information

Scab of Cucurbits. Report on Plant Disease. September 1990. University of Illinois Extension. Accessed 9/14/2014.

Cucurbit Scab. Agriculture & Landscape Program. Vegetable Program. 2014. UMass Amherst Extension. Accessed 9/14/2014.

Commercial Vegetable Production Recommendations (Pennsylvania). 2014. The Pennsylvania State University Extension. Accessed 9/14/2014.

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Cucurbit Scab

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Contact Information

Jason Lilley
  • Graduate Student
Elsa Sánchez
  • Associate Professor of Horticultural Systems Management
Phone: 814-863-2433