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  2. Host and Disease Descriptions

Crabapple (Malus spp.)-Scab

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Latest revision: 
March 2024
Image related to Crabapple (Malus spp.)-Scab

Scab lesions are roughly circular spots with an irregular border, colored darker olive green with a smoky appearance.

Jay W. Pscheidt, 2000.

Scab can also occur on the fruit as raised rough areas on the fruit skin.  

Jay W. Pscheidt, 2018.

The resistant tree on the left is green and healthy looking while the tree on the right is susceptible, brown looking and has a lot of defoliation.

Jay W. Pscheidt, 2010.

Image related to Crabapple (Malus spp.)-Scab

Typical scab lesions on crabapple leaves.

OSU Extension Plant Pathology Slide Collection.

See:

Apple (Malus spp.) - Scab

Crabapple Cultivar Susceptibility

Cause Venturia inaequalis, the fungus that causes scab on commercial cultivars of apple. This is the most serious disease of crabapple and can 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.

Bud or blossom infection often leads to shedding of blossoms and no fruit production.

Cultural control

  • New plantings:
    • If possible, plant disease-resistant trees. 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. For nonbearing fruit trees only. Group 3 + 11 fungicide. 12-hr reentry.
  • Avelyo at 3 to 5 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • BioAdvanced Disease Control at 0.75 fl oz/gal water. H
  • Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. Group M5 fungicide. H
  • Broadform at 4 to 8 fl oz/100 gal water. For nonbearing fruit trees only. Group 7 + 11 fungicide. 12-hr reentry.
  • Cevya at 4 to 5 fl oz/A. Can be used on day of harvest. Group 3 fungicide. 12-hr reentry.
  • 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 1.38 pints/100 gal water every 7 to 14 days. Treated fruit must not be eaten. Group M5 fungicide. 12-hr reentry.
  • Eagle 20 EW at 6 to 12 fl oz/100 gal water. Do not use treated fruit for food. Group 3 fungicide. 24-hr reentry.
  • Flint Extra at 2.5 to 2.9 fl oz/A for edible types or Compass 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.
  • Fontelis at 16 to 20 fl oz/A. Do not use within 28 days of harvest. Group 7 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 SC at 3 to 6 fl 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.
    • Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker. Do not use treated fruit for food.
  • 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.
  • Miravis at 3.4 fl oz/A. Do not use within 30 days of harvest. Group 7 fungicide. 12-hr reentry.
  • Monsoon Turf at 4 to 10 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • 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 plus an adjuvant. 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 as a basal bark spray at 62.4 fl oz in 62.4 fl oz water plus 3 fl oz of Pentra-Bark. Spray first 5 feet of trunk including scaffold limbs until runoff. Group P7 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.
    • Alamo is registered for trunk injection, see label for details. Group 3 fungicide.
    • Banner MAXX at 2 to 4 fl oz/100 gal water. For nonbearing fruit trees only. 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.
    • Propizol is registered for trunk injection, see label for details. 24-hr reentry.
    • Strider at 2 to 4 fl oz/100 gal water. Also registered for trunk injection, see label for details. Group 3 fungicide. 12-hr reentry.
    • Systemic Fungicide (ferti-lome) at 0.25 fl oz/gal water. H
  • Rhyme at 6.5 fl oz/A. Do not use within 14 days of harvest. 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 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.
  • 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.
  • 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.
  • Tesaris at 3.5 to 4.5 fl oz/A. Do not use with oil-based products. May be used day of harvest. Group 7 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. Do not use after fruit set if applied to fruiting trees.
    • OHP 6672 4.5 F at 10.75 to 20 fl oz/100 gal water. Do not use treated fruit for food.
    • Systec 1998 FL at 20 fl oz/100 gal water. Washington Only.
  • Topguard SC at 13 fl oz/A plus another fungicide. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
  • Tourney EZ at 2 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
  • Trionic 4 SC at 8 to 16 fl oz/A. Do not apply within 14 days of harvest. 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.

Notes: Some registered products offer only suppression of this disease and thus are not recommended for use. These products include DoubleNickel 55, Ph-D, and Oso.

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 Disease 94:744-750.