Cause Venturia inaequalis, the fungus that causes scab on commercial cultivars of apple. This is the most serious disease of crabapple and can almost completely defoliate susceptible trees. Wet, cool spring weather favors the disease, which affects leaves and fruit. Trees that lose many leaves due to scab may become weak and more susceptible to other pests and environmental problems.
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 millions of 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 The first visible leaf symptoms in spring are pale, chlorotic, water-soaked spots the size of a pinhead. These enlarge, becoming darker and smoky in appearance, later taking on an olive shade and ultimately a brownish black color. Spots may be any shape but frequently are circular. Young infections often show a radiating spread of fungus tissue through the leaf; such areas later appear as irregular, brown infections. Diseased leaves can be curled and distorted and often drop early, thus defoliating and weakening the tree.
- New plantings:
- If possible, plant disease-resistant trees (see Crabapple Cultivar Susceptibility). The following types did not develop scab over a 33 year evaluation period at Wooster, Ohio: M. sargentii 'Sargent', M. baccata 'Jackii', M. x 'Beverly', M. x 'Silver Moon', and M. x 'White Angel'.
- Plant trees in full sun and/or avoid too much shade.
- Existing plantings:
- Avoid wetting the leaves when irrigating.
- When possible, rake and destroy fallen leaves before they become dry and brittle.
Chemical control Do not apply if fruit is to be consumed. Apply fungicides early and thoroughly to protect new growth. The first susceptible tissues exposed in opening cluster buds are the tips of the leaves and sepals. The most critical period for scab development is from the breaking of the cluster buds until leaves are fully expanded. Use only one fungicide from any single fungicide group once a year. Always tank-mix and/or alternate products to prevent buildup of resistant pathogens.
An interesting study from Indiana found apple orchard isolates of this fungus resistant to Group 1 fungicides (such as the thiophanate-methyl-based products below) but not isolates from ornamental crabapples.
- Affirm WDG at 0.5 lb/100 gal water. Group 19 fungicide. 4-hr reentry.
- Aprovia at 5.5 to 7 fl oz/A plus another fungicide and an adjuvant. Do not use within 30 days of harvest. Group 7 fungicide. 12-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.
- Bayer Advanced Disease Control at 0.75 fl oz/gal water. H
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
- Concert at 9 to 17 fl oz/100 gal water. May cause injury to buds, blooms or tender new growth. Landscape use only. Group 3 + M5 fungicide. 12-hr reentry.
- Daconil Weather Stik at 1.4 pints/100 gal water every 7 to 14 days. Group M5 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Flint at 2 to 2.5 oz/A for edible types or Compass O 50 WDG at 2 to 4 oz/100 gal water for ornamentals. When using Compass do not use with an organosilicate surfactant. Use as a protectant fungicides and not curatively. Do not use within 14 days of harvest. Group 11 fungicide. 12-hr reentry.
- Heritage at 1 to 4 oz/100 gal water plus a non-silicone-based wetter sticker. Do not apply to 'Flame', 'Brandywine', or 'Novamac'. Phytotoxicity can be extreme on sensitive cultivars. Group 11 fungicide. 4-hr reentry.
- Insignia at 4 to 8 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventively only. Group 11 fungicide. 12-hr reentry.
- Inspire Super at 12 fl oz/A. Do not apply within 14 days of harvest. Group 3 + 9 fungicide. 12-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Dithane M45 at 3 or 6 lb/A.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Manzate Pro-Stick at 3 or 6 lb/A.
- Penncozeb 75 DF at 3 or 6 lb/A.
- Pentathlon DF at 1.5 lb/A plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Merivon at 4 to 5.5 fl oz/A. Do not use with EC or oil-based products. May be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Monterey Fungi-Fighter at 0.5 Tbsp/gal water. H
- 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.
- Orkestra at 4 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
- Pageant at 6 to 12 oz/100 gal water. The addition of a silicone-based surfactant has improved control. Group 7 + 11 fungicide. 12-hr reentry.
- Phospho-Jet is registered for tree injections and rates are based on tree size. Research indicated multiple applications are needed for scab control but only one application per year is allowed on label. Group 33 fungicide. 4-hr reentry.
- Phyton 27 at 4 fl oz/10 gal water. Group M1 fungicide. 48-hr reentry.
- Pristine at 14.5 to 18.5 oz/A. The addition of a silicone-based surfactant has improved control. Can be used day of harvest. Group 7 + 11 fungicide. 12-hr reentry.
- Propiconazole-based products. Group 3 fungicides.
- Banner MAXX at 2 to 4 fl oz/100 gal water. 12-hr reentry.
- Infuse Systemic Disease Control at 0.5 Tbsp/gal water. H
- ProCon-Z at 2 to 4 oz/100 gal water. 24-hr reentry.
- ProPensity 1.3 ME at 2 to 4 fl oz/100 gal water. 24-hr reentry.
- Systemic Fungicide (ferti-lome) at 0.25 fl oz/gal water. H
- Rubigan AS at 4 to 12 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Sovran at 3.2 to 6.4 oz/A. Use as a protectant fungicides and not curatively. Some sweet cherries, such as 'Van', may be injured if accidentally sprayed. Group 11 fungicide. 12-hr reentry.
- Spectracide Immunox at 1 fl oz/gal water. H
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
- Tebuconazole-based fungicides are registered. Do not use within 75 days of harvest. Group 3 fungicides. 5-day reentry.
- Tebucon 45 DF at 4 to 8 oz/A.
- Tebuzol 3.6F at 4 to 10 fl oz/100 gal water. Not for food. No reentry interval listed.
- Torque at 4 to 10 fl oz/100 gal water. 12-hr reentry.
- Unicorn DF at 2 to 3 lb/A. Includes sulfur in the formulation. 5-day reentry.
- Tebuject is registered for tree injection as a preventative treatment. Do not inject trees less than 2 inches in diameter or unless trees are suffering from various stresses.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Thiophanate-methyl-based products. These materials will kill earthworms, which help decompose scab infected leaves. Use in a tank-mix with other products. Group 1 fungicides. 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.
- Systec 1998 FL at 20 fl oz/100 gal water.
- Tee-Off 4.5 F at 10.75 to 20 fl oz/100 gal water plus a surfactant.
- Topguard at 13 fl oz/A plus another fungicide. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Vangard WG at 5 oz/A alone or at 3 to 5 oz/A when tank-mixed with another fungicide. May be applied day of harvest. Shorten intervals between applications when weather is warm. Group 9 fungicide. 12-hr reentry.
- Veranda O at 0.5 lb/100 gal water. Group 19 fungicide. 4-hr reentry.
- Zyban WSB at 24 oz/100 gal water. Not to be confused with the smoking cessation drug. Group 1 + M3 fungicide. 12-hr reentry.
Notes Although registered, Distinguish is unavailable for purchase or use.
Some registered products offer only suppression of this disease and thus are not recommended for use. These products include DoubleNickel 55, Ph-D, Oso, and Regalia.
References Aćimović, S.G., VanWoerkom, A.H., Garavaglia, T., Vandervoort, C., Sundin, G.W., and Wise, J.C. 2016. Seasonal and Cross-Seasonal Timing of Fungicide Trunk Injections in Apple Trees to Optimize Management of Apple Scab. Plant Disease 100:106-1616.
Beckerman, J., Chatfield, J. and Draper, E. 2009. A 33-year evaluation of resistance and pathogenicity in the apple scab-crabapples pathosystem. HortScience 44:599-608.
Quello, K.L., Chapman, K.S., and Beckerman, J.L. 2010. In situ detection of benzimidazole resistance in field isolates of Venturia inaequalis in Indiana. Plant Dis. 94:744-750.