Crabapple (Malus spp.)-Powdery Mildew

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Apple (Malus spp.) - Powdery Mildew

Crabapple Cultivar Susceptibility

Cause Podosphaera leucotricha, a fungus that overwinters in terminal buds. Infected buds that survive the winter open later than healthy buds and are already covered with many conidia. Young growth is infected and supports new colonies, which continue the cycle many times during the growing season. New terminal growth is particularly susceptible. Other host plants include pear, fruiting apples, and quince. Can be a problem in nursery production when the leader is infected and stunted rather than producing more desirable rapid, long growth.

Symptoms The disease occurs on foliage, twigs, blossoms, and fruit. On the leaves, small gray or white felt-like patches of fungus develop, frequently on the undersides. Young infected leaves increase in length but not in width, and become curled and distorted. The fungus often covers entire terminals with its masses of white mycelium and powdery spores. Infected foliage is brittle and may be killed or cease to function normally, thus devitalizing the tree. Diseased blossoms are brownish, shriveled and soon die.

Cultural control

• If possible, plant disease-resistant trees.
• Plant trees in full sun; avoid too much shade.

Chemical control Do not apply if fruit is to be consumed. To minimize the development of resistant fungi, alternate or tank-mix fungicides from different groups that have different modes of action. Limit the use of any one group during the growing season.

• Aprovia at 5.5 to 7 fl oz/A plus 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
• Broadform at 2 to 4 fl oz/100 gal water. For nonbearing fruit trees only. Group 7 + 11 fungicide. 12-hr reentry.
• Cevya at 5 fl oz/A. Can be used on day of harvest. Group 3 fungicide. 12-hr reentry.
• Concert at 22 to 35 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.
• 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.
• Fontelis at 16 to 20 fl oz/A. Do not use within 28 days of harvest. Group 7 fungicide. 12-hr reentry.
• Flint Extra at 2.5 to 2.9 fl oz/A for edible types or Compass 50 WDG at 1 to 2 oz/100 gal water for ornamentals. Do not use within 14 days of harvest. Group 11 fungicide. 12-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.
• 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.
• MilStop (85% potassium bicarbonate) at 2.5 to 5 lb/A. Might supplement a normal program when powdery mildew is first observed. Do not mix with other pesticides. Thorough coverage is essential. Oregon and Washington only. 1-hr reentry. O
• 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.
• M-Pede at 1 to 2 gal/100 gal water. Good coverage is essential. Do not use within 3 days of applying sulfur. Do not mix with hard water. 12-hr reentry. O
• Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Do not use treated fruit for food. Group 3 fungicide. 24-hr reentry.
• Orkestra at 6 to 8 fl oz/100 gal water. Group 7 + 11 fungicide. 12-hr reentry.
• Oso SC at 6.5 to 13 fl oz/A. May be applied on the day of harvest. Group 19 fungicide. 4-hr reentry. O
• Pageant at 6 to 12 oz/100 gal water. The addition of a silicone-based surfactant has improved control. For nonbearing fruit trees only. Group 7 + 11 fungicide. 12-hr reentry.
• Ph-D WDG at 6.2 oz/A. May be applied on the day of harvest. Group 19 fungicide. 4-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.
• Problad Verde at 18.1 to 45.7 fl oz/A. Needs 8 to 12 hours dry time to absorb into treated tissue. Do not use within one day of harvest. Group BM01 fungicide. 4-hr reentry.
• Propiconazole-based products. Group 3 fungicides.
  • Alamo is registered for trunk injection, see label for details. Group 3 fungicide.
  • Banner MAXX at 5 to 8 fl oz/100 gal water. For nonbearing fruit trees only. 12-hr reentry.
  • Infuse Systemic Disease Control at 1.5 Tbsp/gal water. H
  • ProCon-Z at 5 to 8 oz/100 gal water. 24-hr reentry.
  • Propizol is registered for trunk injection, see label for details. 24-hr reentry.
  • Strider at 5 to 8 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 0.75 fl oz/gal water. H
• Regalia at 1 to 4 quarts/A. Use on 7-day intervals. May be used day of harvest. Does not benefit from the addition of an adjuvant. Group P5 fungicide. 4-hr reentry. O
• Rhyme at 4 to 6 fl oz/A. Can be used through a drip or microsprinkler system at higher rate. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
• Safer Garden Fungicide (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant. Do not use when the temperature is over 85°F or within a few weeks of an oil spray. H
• Sil-Matrix at 1 to 4 quarts/100 gal water plus a nonionic surfactant. Can be applied up to the day of harvest. 4-hr reentry. O
• Sovran at 4 to 6.4 oz/A. Rotate with other fungicides that have different modes of action. 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
• Tebuconazole-based fungicides are registered. Do not use within 75 days of harvest. Group 3 fungicides.
  • Tebucon 45 DF at 4 to 8 oz/A. 5-day reentry.
  • 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.
• 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.
• Topguard SC at 8 to 12 fl oz/A. Do not use within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
• Torino at 6.8 oz/A. Do not use within 14 days of harvest. Group U6 fungicide. 4-hr reentry.
• Tourney EZ at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
• Trinity at 4 to 12 fl 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.

Biological control

• Aviv (Bacillus subtilis strain IAB/BS03) at 10 to 30 fl oz/100 gal water. Unknown efficacy. Preharvest interval not specified. 4-hr reentry. O
• DoubleNickel 55 (Bacillus amyloliquefaciens strain D747) at 0.25 to 3 lb/A. Unknown efficacy. Group BM02 fungicide. 4-hr reentry. O
• Serenade OPTI (Bacillus subtilis strain QST 713) at 14 to 20 oz /A. Active ingredient is a small protein. 4-hr reentry. H O
• Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A plus an adjuvant. May be applied up to and including the day of harvest. 4-hr reentry. O
• Theia (Bacillus subtilis strain AFS032321) at 1.5 to 5 lb/A. May be applied up to and including the day of harvest. 4-hr reentry. O

Reference Moulton, G.A., Maleike, R.R. and King, J. 1996. Crabapples for Western Washington Landscapes. Washington State University Cooperative Extension bulletin EB1809.