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.
Cultural control
- Use a 2- to 3-year rotation with non-cucurbit crops.
- Plow under plant debris after harvest.
- Use resistant varieties whenever available. Scab-resistant varieties 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 varieties to grow.
Chemical control
- Actigard 50WG at 0.5 to 1 oz/A on 7-day intervals for disease suppression. Do not apply on less than a 7-day interval. May be applied the day of harvest. 12-hr reentry.
- Aprovia Top (Group 7 + 3) at 10.5 to 13.5 fl oz/A on 7 to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. May be applied the day of harvest. 12-hr reentry.
- Bravo Ultrex (Group M5) at 1.8 to 2.7 lb/A. May be applied through overhead irrigation. 12-hr reentry.
- Dexter Max (Group M3 + 11) at 2.1 to 3.2 lb/A on 7- to 10-day intervals. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 5 days. 24-hr reentry.
- Dexter Xcel (Group M3 + 11 + 3) at 72 fl oz/A on 7- to 10-day intervals. Preharvest interval is 7 days. 24-hr reentry.
- Liqui-Cop at 2 to 3 teaspoons/gal water. H
- Orondis Opti (Group M5 + 49) at 1.75 to 2.5 pints/A on 7- to 14-day intervals. Do not apply more than two (2) sequential applications. May be applied the day of harvest. 12-hr reentry.
Biological control
- Prestop WG on 3- to 6- week intervals. 4-hr reentry. O