Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Crabapple -- Scab
See Also: Apple -- Scab
 
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.

Typical scab lesions on crabapple leaves.

 
Cultural control:

  1. New plantings.
    1. If possible, plant disease-resistant trees (see Crabapple Cultivar Susceptibility table). 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'.
    2. Plant trees in full sun; avoid too much shade.
  2. Existing plantings:
    1. Avoid wetting the leaves when irrigating.
    2. 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.

  1. Bayer Advanced Disease Control at 0.75 fl oz/gal water.
  2. Cygnus 50 WG at 1 to 1.6 oz/100 gal water plus a non- organosilicone spreader-sticker. 12-hr reentry.
  3. Daconil Weather Stik at 1.4 pints/100 gal water every 7 to 14 days. Daconil (12.5%) can be used in home gardens. 12-hr reentry.
  4. Eagle 20 EW at 6 to 12 fl oz/100 gal water. 24-hr reentry.
  5. 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. Use as a protectant fungicides and not curatively. Rotate with other fungicides that have different modes of action. Do not make more than two consecutive applications, four applications total per season, use more than 11 oz/A/season or within 14 days of harvest. Do not use organosilicate surfactants. 12-hr reentry.
  6. Heritage at 1 to 4 oz/100 gal water plus a non-silicone based wetter sticker. Do not apply to the following cultivars: Flame, Brandywine, and Novamac. Phytotoxicity can be extreme on sensitive cultivars. 4-hr reentry.
  7. Insignia at 4 to 8 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventatively only. 12-hr reentry.
  8. Mancozeb-based products. 24-hr reentry.
    1. Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
    2. Pentathlon DF at 1 to 2 lb/A or per 100 gal water.
    3. Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
  9. Pageant at 6 to 12 oz/100 gal water. Do not use more than two (2) consecutive applications before switching to a different fungicide group. A group 7 and 11 fungicide. 24-hr reentry.
  10. Phyton 27 at 4 oz/10 gal water. 24-hr reentry.
  11. Pristine at 14.5 to 18.5 oz/A. Do not use more than two (2) consecutive applications or more than four (4) times/year. Can be used day of harvest. 12-hr reentry.
  12. Propiconazole-based products. 24-hr reentry.
    1. Banner MAXX at 2 to 4 fl oz/100 gal water.
    2. ProPensity 1.3 ME at 2 to 4 fl oz/100 gal water.
    3. Systemic Fungicde (ferti lome) at 0.25 fl oz/gal water.
  13. Rubigan AS at 4 to 12 fl oz/100 gal water. 12-hr reentry.
  14. Sovran at 3.2 to 6.4 oz/A. Use as a protectant fungicides and not curatively. Rotate with other fungicides that have different modes of action. Do not make more than two consecutive applications. Some sweet cherries, such as 'Van', may be injured if accidentally sprayed. 12-hr reentry.
  15. Spectracide Immunox at 1 fl oz/gal water.
  16. Spectro 90 WDG (chlorothalonil plus thiophanate methyl) at 1 to 2 lb/100 gal water. 12-hr reentry.
  17. Terraguard SC at 4 to 8 fl oz/100 gal water. 12-hr reentry.
  18. Thiophanate-methyl-based products. 12-hr reentry.
    1. Cleary’s 3336 at 12 to 16 oz/100 gal water.
    2. Halt (by ferti-lome) at 2.5 tsp/gal water is registered for home use. H
    3. OHP 6672 4.5 F at 10.75 to 20 fl oz/100 gal water.
    4. Systec 1998 at 20 fl oz/100 gal water.
    5. Tee-Off 4.5 F at 20 oz/100 gal water plus a surfactant.
  19. Vangard at 5 oz/A alone or at 3 to 5 oz/A when tank-mixed with another fungicide. Do not apply within 72 days of harvest or more than 22 oz/A/season. Shorten intervals between applications when weather is warm. 12-hr reentry.
  20. Veranda O at 0.5 lb/100 gal water. 4-hr reentry.
  21. Zyban WSB (mancozeb plus thiophanate methyl) at 24 oz/100 gal water. Not to be confused with the smoking cessation drug. 12-hr reentry.
Notes: Tebuject is registered for tree injection. Trees are to be injected as a preventative treatment. Do not inject trees less than 2 inches in diameter or unless trees are suffering from various stresses.
References:
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.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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