Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Cherry -- Powdery Mildew
 
Cause: Podosphaera clandestina, a fungus that infects young, expanding leaves. It overwinters as a small, round, black body (chasmothecium) on dead leaves on the orchard floor or in tree crotches. Some chasmothecia may be trapped in bark crevices. In spring as buds break, irrigation or rain releases ascospores and wind spreads them to young leaves. Given the right combination of temperature and leaf wetness duration, these ascospores start the first colonies of the season. Colonies produce more (asexual) spores generally around shuck fall and continue the disease cycle. These secondary spores are favored by warm temperatures, high humidity, and dry leaves. The fungus can go from spore to spore in as little as 4-5 days under favorable conditions. Ideal conditions for the spread of the disease during the late spring and summer are high humidity and temperatures from 70 to 80oF. Young green fruit is more susceptible to infection than fruit near maturity.

The first few infected leaves may be found on the leaves of suckers, branches close to the ground, or leaves on the main scaffold branches near tree crotches. Look for the first colonies about 1 week after the first irrigation.

Disease outbreaks are infrequent but potentially destructive particularly east of the Cascades. Most sweet and sour cultivars are susceptible, but it is most severe on 'Bing'. In one trial, the cultivars 'Lambert' and 'Black Tartarian' had a moderate level of resistance while 'Chelan' was found immune. Late season cultivars such as 'Sweetheart' and 'Rainier' seem more susceptible due to the longer growing season.

Symptoms: Leaves - the first symptom is a light green, circular lesion on either leaf surface. A white cottonlike growth develops in the infected area. Severe leaf infection can result in curling and/or blistering, and leaves are covered with the characteristic white cottonlike growth. As infected leaves age, small, black bodies (chasmothecia) are formed.

Fruit - circular, slightly depressed areas develop on the surface. The white, cottonlike growth may or may not develop but can cover the entire fruit surface.

A few colonies on the underside of the cherry leaf.

Note the small black specks (cleistothecia) within the white area.

Severe powdery mildew infection of these cherries.

 
Cultural control: Start looking for the first colonies about 1 week after the first irrigation.
  1. Remove and destroy sucker shoots.
  2. Keep irrigation water off developing fruit by using low-angle irrigation nozzels. Also, keep irrigation sets as short as possible.
  3. Follow cultural practices that promote good air circulation such as pruning and judicious nitrogen management.
Chemical control: Begin at shuck fall and continue through harvest. Significant rain between budbreak and shuck fall warrants applying as soon as possible. Resistance to group 3 fungicides (such as Orbit and Rally) is highly suspected in Eastern Washington. Alternate or tank-mix products that have different modes of action. Also, group 3 fungicide applications to three (3) or fewer per year. A temperature-based computer model can help time reapplications. Do not use products on 21-day intervals.
  1. Abound at 11 to 15 fl oz/A. Alternate with other fungicides. Do not apply more than three (3) times a year or two (2) sequential sprays. May be applied the day of harvest. Sprayers used for Abound should not be used on apples such as ‘Gala’, ‘Cox’s Orange Pippin’, and ‘McIntosh’. Fair efficacy. 4-hr reentry.
  2. Bicarbonate-based products. Might supplement a normal program when powdery mildew is first observed. Do not mix with acidifying agents. Thorough coverage is essential. Poor to moderate control.
    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. Monterey Bi-Carb is registered for home use in all states. Remedy can only be used in Washington.
  3. Cabrio EG at 9.5 oz/A. Do not use more than two (2) sequential applications or more than five (5) applications per year. May be used at harvest. Excellent efficacy. 12-hr reentry.
  4. Elite 45 DF at 4 to 8 oz/A. A DMI-type product. Do not apply more than 3 lb/A per crop season. Can be applied up to and including day of harvest. Good efficacy. 12-hr reentry.
  5. Gem at 4 to 8 oz/A. Rotate with other fungicides that have different modes of action. Do not apply more than twice consecutively, exceed four (4) applications, 32 oz/A/season, or within 1 day of harvest. Do not use with ogranosilicate surfactants. ‘Concord’ grapes may be injured if accidentally sprayed. 12-hr reentry.
  6. 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. Do not use past pit hardening but may be used postharvest. Excellent efficacy. 4-hr reentry.
  7. Orius 45 DF at 4 to 8 oz/A. Can be applied through the day of harvest. 12-hr reentry.
  8. Pristine at 10.5 to 14.5 oz/A. Do not use more than 2 consecutive applications or more than 5 times/year. Can be used day of harvest. Excellent efficacy. 12-hr reentry.
  9. Procure 480 SC at 8 to 16 fl oz/A. Do not apply within 1 day of harvest or more than 96 oz/A/year. Good efficacy. 12-hr reentry.
  10. Propiconazole-based fungicides are registered. Limited to four (4) applications (2 during bloom and 2 before harvest). May be used up to and including day of harvest. In the Willamette Valley, smaller, deeper green leaves have been observed on ‘Royal Ann’ cherry trees treated with a dilute application at 3 oz/A. Severe resistance reported in Washington. Good efficacy if no resistance.
    1. Bumper 41.8 EC at 4 oz/A. Do not use on Stanley-type Plums. 24-hr reentry.
    2. Orbit at 4 oz/A. Do not use on Stanley type Plums earlier than 21 days before harvest. 12-hr reentry.
    3. PropiMax EC at 4 fl oz/A. Do not use on Stanley-type Plums earlier than 21 days before harvest. 24-hr reentry.
    4. Tilt at 4 fl oz/A. Do not use on Stanley type Plums earlier than 21 days before harvest. 12-hr reentry.
  11. Quash at 3.5 to 4 oz/A. Do not make more than 3 applications/year or within 14 days of harvest. Group 3 fungicide. 12-hr reentry.
  12. Quilt (propiconazole + azoxystrobin) at 14 fl oz/A. Do not apply more than two (2) sequential sprays. May be applied the day of harvest. 12-hr reentry.
  13. Quintec at 7 fl oz/A. Do not apply more than five (5) times/year, more than 35 fl oz/A/season, or within 7 days of harvest. Excellent efficacy. 12-hr reentry.
  14. Rally 40 WSP at 2.5 to 6 oz/A. A DMI-type product. Use Eagle 20 EW at 2 to 3 fl oz/100 gal water for landscape use. Can be applied up to the day of harvest. Resistance reported in Washington. Good efficacy if no resistance. 24-hr reentry.
  15. Rubigan EC at 6 to 12 oz/A plus a surfactant. A DMI-type product. Do not apply more than 48 oz/A/season. May be applied up to harvest. Fair efficacy. 12-hr reentry.
  16. Spectracide Immunox at 0.5 fl oz/gal water. Can be applied up to and including the day of harvest. Do not use more than 7 times per year.
  17. Sulfur-based products have good efficacy. 24-hr reentry. O
    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. Sulfur Flowable 6 (52% sulfur) at 5 gal/A.
    4. Thiolux Jet (80% sulfur) at 10 to 30 lb/A.
  18. Topsin 4.5 FL at 20 to 30 fl oz/A plus another fungicide. Rotate with other fungicides to avoid buildup of resistant fungi. Halt (by ferti-lome) is registered for home use but may not be used within 1 day of harvest. 12-hr reentry.
  19. Trilogy at 1% of spray volume. Do not use after pit hardening, 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.
Notes: Post-harvest applications of oil or lime sulfur products has been shown to restrict the development of chasmothecia. Fall application of lime sulfur products has been shown to reduce the survival of overwintering chasmothecia. It is not known if these practices will help in the overall control of the disease during the growing season.
Biological control:

  1. Serenade MAX at 1 to 3 lb/A. Active ingredient is a protein from Bacillus subtilis strain QST 713. Effectiveness as a commercial treatment is poor. 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. Efficacy tests at Prosser, WA in 2004 were inconclusive. 4-hr reentry.
References:
Ogawa, J.M., E.I. Zehr, G.W. Bird, D.F. Ritchie, K. Uriu, and J.K.Uyemoto. 1995. Compendium of Stone Fruit Diseases. St. Paul, MN: APS Press.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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