Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Apple -- Powdery Mildew
 
Cause: Podosphaera leucotricha, a fungus that overwinters in terminal buds of apples. Temperatures less than 11oF will kill fungal mycelia in these buds. Infected buds start to die at -11oF; however, healthy buds can survive at that and even lower temperatures. Infected buds that survive 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 on apple trees is particularly susceptible. Other host plants include pear, crabapple, and quince. Pear orchards are at a higher risk of disease development if planted next to apple orchards.

Powdery mildew is a frequent problem throughout the PNW. Very susceptible apple cultivars include ‘Braeburn’, ‘Jonathan’, ‘Rome’, ‘Newtown’, ‘Granny Smith’, and ‘Gravenstein’. Fruits of ‘Jonathan’ and ‘Rome’ also may be severely affected. Moderately susceptible: ‘Winesap’. Less susceptible: ‘Golden Delicious’, ‘Red Delicious’, and ‘Delicious’ strains.
Symptoms: The disease occurs on foliage, twigs, blossoms, and fruit. Small gray or white felt-like patches of fungus develop, frequently on the undersides of leaves. 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 pow-dery spores. Infected foliage is brittle and may be killed or cease to function normally, thus devitalizing the tree. Diseased blossoms are brownish and shriveled and soon die. When fruits are infected, a net-work of lines, generally referred to as russeting, appears on the diseased area.

Note the shorter, white shoot with deformed leaves. White color due to the presence of fungal conidia.

The infected flowers on the left show strap-shaped petals with white sporulation of the fungus.

Buds infected last year open covered with conidia.

 
Cultural control:

  1. Plant resistant cultivars.
  2. Backyard growers may find it useful to prune out and destroy infected shoots as they appear early in the year. Will not be effective when the disease is severe.
Chemical control: Apply fungicides at prepink, pink, calyx, and first cover. To limit infection of next year’s buds, continue applications until terminal growth ceases. Use group 3 fungicides a maximum of four times a season. Alternate or tank mix with fungicides that have different modes of action.

  1. Bicarbonate-based products. Might be used to supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. Poor to moderate control alone.
    1. Armicarb 100 (85% potassium bicarbonate) at 2.5 to 5 lb/100 gal water. Washington and Idaho only. 4-hr reentry.
    2. Kaligreen (82% potassium bicarbonate) at 2.5 to 3 lb/A.4-hr reentry.
    3. MilStop (85% potassium bicarbonate) at 2 to 3 lb/A. 1-hr reentry.
    4. Monterey Bi-Carb is registered for home use in all states. Remedy can only be used in Washington.
  2. Cinnacure at 1 to 3 gal/A. 4-hr reentry.
  3. Flint 50 WG at 2 to 2.5 oz/A. Rotate every other application with other fungicides that have different modes of action. Do not make more than four (4) applications total per season, use more than 11 oz/A/season or within 14 days of harvest. Injury may oc-cur to Concord grapes if accidentally sprayed. 12-hr reentry.
  4. Indar 2F at 6 to 8 fl oz/A plus a wetting agent. Do not use within 14 days of harvest. 12-hr reentry.
  5. JMS Stylet Oil at 1 to 2 gal/100 gal water. Do not apply micronized sulfur within 10 days of an oil application. Do not use oil within 14 days of a sulfur application. Do not use during freezing temperatures, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. 4-hr reentry.
  6. 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.
  7. 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.
  8. Procure 480 SC at 8 to 16 fl oz/A. Do not apply within 14 days of harvest or more than 64 oz/A/year. 12-hr reentry.
  9. Rally 40 WSP at 2.5 to 10 oz/A depending on tree size. Growers have noticed less control at the 5 oz/A rate than they have gotten in the past. Use higher rates and fungicides from other chemical families. Use Eagle 20 EW at 4 to 6 fl oz/100 gal water for landscape use. Do not apply within 14 days of harvest or apply more than 5 lb/A/season. Do not use on pear. 24-hr reentry.
  10. Rubigan EC at 8 to 12 fl oz/A plus a surfactant. Do not use more than 84 oz/A/season or within 30 days of harvest. 12-hr reentry.
  11. Sovran at 4 to 6.4 oz/A. Rotate with other fungicides that have different modes of action. Do not make more than 2 consecutive applications, use more than 25.6 oz/A/season or within 30 days of harvest. Injury may occur to some sweet cherries, such as Van, if accidentally sprayed. 12-hr reentry.
  12. Spectracide Immunox at 0.5 fl oz/gal water. Do not use within 2 weeks of harvest. Do not apply more than 10 times per season.
  13. Sulfur based products. Do not apply in hot weather (more than 80°F) because sulfurs can russet fruit. Do not apply near an oil spray. 24-hr reentry.
    1. Kumulus DF (80% sulfur) at 10 to 20 lb/A.
    2. Microthiol Disperss (80% sulfur) at 10 to 20 lb/A. Do not use a spreader sticker.
    3. Thiolux Jet (80% sulfur) at 10 to 30 lb/A.
  14. Topsin 4.5 FL at 15 to 20 fl oz/A plus another fungicide. Do not use within 1 day of harvest. Do not use at any time in the orchard if your packing house uses a benzimidazole (such as Mertect or Decco Salt 19) post harvest. 12-hr reentry.
  15. Trilogy at 1% of spray volume but do not exceed 2.5 gal/A. Do not use near sulfur applications, below 45°F, above 90°F, or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand alone product. 4-hr reentry.

Biological control:

  1. Serenade MAX at 1 to 3 lb/A. Active ingredient is a protein from Bacillus subtilis strain QST 713. Variable efficacy in tests in western Oregon. 4-hr reentry.
  2. Sonata (Bacillus pumilis strain QST 2808) at 2 to 4 quarts/A. May be applied up to and including the day of harvest. 4-hr reentry.

Content edited by: Jay W. Pscheidt on January 1, 2009
 
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