Extension Logo
Extension Logo
University of Minnesota Extension
https://extension.umn.edu

Scab of cucurbits

Quick facts

  • Leaf spots are white to gray and may have a yellow halo. As the infection ages, the center of the leaf spot falls out.
  • Scab travels on infected seed or as spores carried on moist air currents.
  • Disease develops in cool (around 70° F), moist weather.
  • Rotate vegetables so two or more years go by before planting any member of the squash family in the same location.
  • Do not work in plants when wet.
  • The extent of fruit rot varies depending on the variety’s resistance.
Cucurbit fruit with black scab spots

The fungus Cladosporium cucumerinum causes scab. 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.

Cucurbits leaves infected by scab with brown spots

Identifying scab symptoms

  • Leaf spots are white to gray and may have a yellow halo.
  • As the infection ages, the center of the leaf spot falls out. Leaves look ragged.
  • Cucumber, summer squash, zucchini and pumpkin have sunken fruit spots 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 the variety’s resistance.

What causes scab

  • Scab travels on infected seed or as spores carried on moist air currents. Fungal spores 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 not treated correctly.
  • Disease develops in cool (around 70° F), moist weather.
  • Summer squash, pumpkins and cucumbers (varieties not specifically bred for disease resistance) are very susceptible to the disease.
    • Fruit infections on these crops will produce spores 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.
  • 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.

Preventing and managing the disease

  • 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 two 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 available 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.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Michelle Grabowski

Reviewed in 2018

Share this page:

© 2018 Regents of the University of Minnesota. All rights reserved. The University of Minnesota is an equal opportunity educator and employer.