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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Whitish patches on the undersides of leaves are a symptom of powdery mildew.

Apple

Powdery Mildew

Pathogen: Podosphaera leucotricha

(Reviewed 8/06, updated 8/06)

In this Guideline:

SYMPTOMS

Powdery mildew is distinguished by superficial, white powdery growth on leaves and shoots that results in the stunting and distortion of young growth. Infected fruit are stunted and russetted, and fruit set may be reduced.

COMMENTS ON THE DISEASE

This is a major foliage disease of apples. The fungus overwinters in terminal buds that are white, flattened, and pointed. Disease development is favored by warm days and cool, moist nights.

MANAGEMENT

Powdery mildew is managed primarily by pruning infected shoots during dormancy or in early spring and by applying sprays as necessary in spring to prevent buildup of the fungus and damage to the crop. Remove infected shoot tips at pruning. Chemical control of powdery mildew is done in conjunction with controls for scab. Timing and choice of material may vary from orchard to orchard.

Organically Acceptable Methods
Pruning and treatments with lime and sulfur, sulfur alone, or certain horticultural oils can be used to treat organically certified produce.

Treatment Decisions
Preferred timing is an application at pink bud. If powdery mildew continues to be a problem in the orchard, apply additional treatments until terminal growth ceases.

Common name Amount to Use P.H.I.+
(trade name)   (days)
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy. When choosing a pesticide, also consider information relating to environmental quality.
 
A. TRIFLOXYSTROBIN
  (Flint) 2–2.5 oz/acre 14
  MODE OF ACTION: A Qol (Group 11)1 fungicide.
  COMMENTS: A strobilurin fungicide. Alternate with a sterol-inhibitor fungicide. Do not apply more than 2 consecutive applications before alternating. Do not apply more than 11 oz/acre/season.
 
B. FENARIMOL
  (Rubigan) 1EC 9–12 oz/acre 30
  MODE OF ACTION: A DMI (Group 3)1 pyrimidine fungicide.
  COMMENTS: A sterol-inhibitor fungicide. Do not apply more than 84 fl oz/acre/season.
 
C. MYCLOBUTANIL
  (Rally) 40WP 1.25–2.5 oz/100 gal water/acre 14
    (dilute application)  
  MODE OF ACTION: A DMI (Group 3)1 triazole fungicide.
  COMMENTS: A sterol-inhibitor fungicide. Continue applications through the second cover spray at 7- to 10-day intervals. Use high label rate if disease was present in previous years. For application by ground equipment only.
 
D. TRIFLUMIZOLE
  (Procure) 50WS 8–16 oz/acre 14
  MODE OF ACTION: A DMI (Group 3)1 imidazole fungicide.
  COMMENTS: A sterol-inhibitor fungicide.
 
E. MICRONIZED SULFUR# 10–20 lb/acre  
  MODE OF ACTION: A multi-site contact (Group M2)1 inorganic fungicide.
  COMMENTS: May be used after bloom. Some russetting may occur in sensitive varieties if temperatures exceed 80°F.
 
F. LIQUID LIME SULFUR# 2 gal/100 gal dilute spray  
  . . . PLUS . . .
  WETTABLE SULFUR# 4–5 lb/100 gal  
  MODE OF ACTION: A multi-site contact (Group M2)1 inorganic fungicide.
  COMMENTS: Do not use after bloom begins. Lime sulfur is incompatible with most other pesticides. Check before use.
 
G. TRIADIMEFON
  (Bayleton) 50DF 2–8 oz/acre 45
  MODE OF ACTION: A DMI (Group 3)1 triazole fungicide.
  COMMENTS: A sterol-inhibitor fungicide. Do not apply more than 24 oz/acre/season. Do not graze livestock in treated orchards.
 
H. THIOPHANATE METHYL
  (Topsin Topsin-M) 70WP 1–1.5 lb/acre  
  MODE OF ACTION: A MBC (Group 1)1 thiophanate fungicide.
  COMMENTS: Resistance to thiophanate methyl may develop if this material is used repeatedly. It is important to alternate this material with materials of a different chemistry. Do not apply more than 4 lb product/season.
 
I. LIQUID LIME SULFUR# 2–3 gal/100 gal dilute  
  MODE OF ACTION: A multi-site contact (Group M2)1 inorganic fungicide.
  COMMENTS: Lime sulfur is incompatible with most other pesticides. Check before use. An in-season application eradicates powdery mildew.
 
J. HORTICULTURAL OIL#
  (Organic JMS Stylet Oil) 1–2 gal  
  MODE OF ACTION: Unknown
  COMMENTS: Use higher rate and/or shorter spray interval when disease conditions are severe. Check with your certifier to determine other appropriate oils to use on an organically certified crop.
 
 
+  Preharvest interval. Do not apply within this many days of harvest.
# Acceptable for use on organically grown produce.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different Group number are suitable to alternate in a resistance management program. For more information, see http://www.frac.info/.

More information on apple fungicides

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Apple
UC ANR Publication 3432
Diseases
W. D. Gubler, Plant Pathology, UC Davis
Acknowledgment for contributions to the diseases section:
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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