Field Scouting Guide: Scab of Cucurbits

leaf symptoms of scab of cucurbits

Although scab of cucurbits leaf lesions start out pale green with yellow margins, the lesions turn brown and grow larger in the more advanced stage.
Photo by Bruce Watt

This month’s Field Scouting Guide concentrates on scab of cucurbits. Beth Krueger Gugino, Ph.D., Pennsylvania State University, shares tips about how to spot and treat this disease.

BASICS

  • Scientific name: Cladosporium cucumerinum
  • Common name: Scab of cucurbits
  • Geographical range: Widespread in North America and Europe — worldwide
  • Crops affected: Summer and winter squash, pumpkin, cantaloupe, melon — watermelon is considered more resistant, and many cucumber cultivars have resistance.

DISEASE IMPACT

In the Mid-Atlantic region, scab is an occasional problem when we have cool, below normal, and moist, wet weather later in the season (heavy dew). Outbreaks tend to be limited to the field scale.

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The economic impact comes as a result of the fruit symptoms. Scab often renders the fruit unmarketable. In a prolonged cool, wet season, symptomatic fruit infected in the field could develop symptoms post-harvest. The fruit can be affected at any growth stage, and it is not uncommon for the fruit lesions to be invaded by secondary, soft-rotting bacteria.

IDENTIFICATION

Scab spots start as small, gray, sunken spots that enlarge to become circular to oval in shape and can develop dark-green sporulation, as well as a gummy ooze.

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Foliar lesions are initially pale green and irregularly shaped with yellow margins. They then enlarge and turn brown, and the center drops out giving a shot-hole appearance.

Symptoms can vary slightly by cucurbit crop. You can easily confuse the problem with anthracnose, angular leaf spot, and Septoria.

scab symptoms on summer squash

Early fruit scab of cucurbits lesions are small, gray sunken spots.
Photo courtesy of Clemson University – USDA Cooperative Extension Slide Series via Bugwood

RECOMMENDED TREATMENT

As with many diseases, the pathogen that causes scab survives overwinter in crop debris. So crop rotation (three-plus years in non-cucurbit crops) is a key management tool, as is host resistance when available. Select high-quality, pathogen-free seed and resistant cultivars if available. Resistant cultivars will develop fewer lesions.

Disk under crop residue at the end of the season to facilitate decomposition. Implement cultural practices that minimize leaf wetness and maximize air circulation. Manage cucurbit weed hosts that may harbor the pathogen and be a source of inoculum.

Target fungicide applications for when environmental conditions are favorable for scab (cool temperatures — 70˚F to 75˚F — and wet). We recommend you use multi-site mode of action protectant fungicides that contain chlorothalonil or mancozeb (depending on the crop) on a shorter, five-day interval due to the short disease cycle.

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