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Cause Venturia inaequalis has been reported from WA and is the same fungus that causes scab on apple, crabapple, mountain ash, photinia, and pyracantha as well as hawthorn. It is not known if isolates from one host can equally infect all other hosts. The disease is common west of the Cascade Range in Oregon and Washington and coastal British Columbia. Wet, cool spring weather favors the disease. The fungus overwinters in diseased leaf debris on the ground. Ascospores on leaves are produced in wet spring weather. Wind carries spores to young leaves and flowers where they initiate primary infections. Infections eventually produce conidia, which splashing rain and wind spread to new leaf or fruit surfaces in the tree. Spores germinate to cause new lesions. Several secondary cycles of conidial germination, infection, and sporulation may occur during a growing season.
Symptoms Small, dark spots occur scattered on both leaves and fruits. Spots can occur on either side of leaves. Spots become dark olive brown with dark brown to blackish margins.
Cultural control
- Shred fallen leaves with a flail mower to help speed decomposition of infected leaves.
- Home gardeners may reduce disease pressure by raking and disposing of fallen leaves. If put in a compost pile, be sure leaves decompose completely before spring.
- Pruning helps increase air flow and reduces the amount of time foliage is wet from rain or dew.
- Reduce irrigation sets so leaves do not stay wet for extended periods of time. Use sprinkler heads that do not wet the foliage of the tree or use drip irrigation.
Chemical control Protect new growth in the spring. Tank-mix and/or alternate products from different groups with different modes of action to prevent the buildup of resistant fungi. Limit the use of any one group during crop production.
- Affirm WDG at 0.25 to 0.5 lb/100 gal water. Group 19 fungicide. 4-hr reentry.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Banner MAXX at 2 to 4 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Compass 50 WDG at 2 to 4 oz/100 gal water. Do not use with organosilicate products. Group 11 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.38 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Pageant at 6 to 12 oz/100 gal water. Do not use with organosilicone-based adjuvants. Group 7 + 11 fungicide. 12-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tebuject 16 is registered for tree injections. The number of capsules used is based on tree size. Group 3 fungicide.
- Thiophanate-methyl-based products. Group 1 fungicide. 12-hr reentry.
- Cleary's 3336 EG at 12 to 16 oz/100 gal water.
- OHP 6672 4.5 F at 10.75 to 20 fl oz/100 gal water.
- Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
Reference Dar, M. S., Ahmad, M., Shah, M. D., Padder, B. A., Bhat, N. A., Khan, M. A. Ahanger, F. A., Nabi, A. and Banoo, A. 2019. Venturia crataegi causing scab on Crataegus songarica: Morpho-molecular characterization and a new record from India. Applied biological Research, 21:274-282.