Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Cucumber (Cucumis sativus) -- Scab (Gummosis)
 
Cause: A fungus, Cladosporium cucumerinum, that overwinters in cucumber debris and on seed. Spores are disseminated by insects, clothing, and tools, and are blown long distances in moist air. Host penetration occurs less than 9 hours after germination; infection is visible in 3 days. Spore production may occur on the fourth day. The fungus severely injures fruit, which can be infected at all stages of growth but is particularly susceptible while young.
Symptoms: At first, a sticky substance is exuded in drops as if the trouble were the result of an insect puncture. Spots then gradually turn gray, become slightly sunken, and enlarge to about 0.375 inch in diameter. The canker gradually grows darker, and the tissue collapses farther into the cucumber until a pronounced cavity forms, lined with a dark green, velvety layer of fruiting fungus.

The first symptom on foliage is the appearance of a slightly water-soaked, paler green area. Spots usually are numerous. Gradually they turn gray to white and become angular, often with yellow margins. The fine vein in the spots may remain brown or tan and are distinct against the white background. The dead tissue cracks, breaking away until the whole leaf has a ragged appearance.

Cultivar Resistance: Scab-resistant cultivars include 'Dasher II', 'Raider', 'Encore', 'Sprint', 'Poinsett 76', 'Turbo', 'Regal', 'Flurry', 'Calypso', 'Quest', 'Gemini', 'Marketmore', 'Pioneer', 'SMR-58', and 'SMR-18'. Commercial growers should consult processors for the resistant cultivars to grow.
Cultural control:
  1. Use a 2- to 3-year rotation with non-cucurbit crops.
  2. Plow under plant debris after harvest.
  3. Use resistant cultivars whenever available.
Chemical control:
  1. Bravo Ultrex at 1.8 to 2.7 lb/A. Bravo may be applied through overhead irrigation. 12-hr reentry.
  2. Kop-R-Spray at 2 to 3 tsp/gal water.
Content edited by: Cynthia M. Ocamb on January 1, 2010
 
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