Apple Powdery Mildew

Utah Pest Fact Sheet

Published by USU Extension and UPPDL

Utah Plant Disease Control No. 16

Revised February 1999


 

Apple Powdery Mildew

Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
 
 

 
Aspen leaf spot lesions.

Overwintering apple powdery mildew deforms the buds creating a feathered appearance (right) rather than a smooth round healthy bud (left).

Early season leaf spots.

Powdery mildew symptoms on apple foliage (right), notice upward curling of leaves and white appearance. Healthy foliage (left).

Powdery mildew affected fruit (left), healthy apple fruit (right). Fruit symptoms may differ in severity depending on age of fruit at time of infection and cultivar.

Primary powdery mildew infection of a blossom cluster and associated leaves (left), healthy blossom cluster (right).

Apple Powdery Mildew

Powdery Mildew is the most common disease of apple trees in Utah. It is caused by the fungus Podosphaera leucotricha. This fungus frequently infects new vegetative growth, causing reduced vigor, leaf malformation, and reduced viability of buds. Early infection of apple fruitlets results in a weblike russetting on the mature fruit. It may also reduce the vigor of trees and reduce return bloom. Young trees and vigorously growing shoots are the most susceptible.


DISEASE CYCLE
The fungus overwinters as fungal strands (mycelium) in leaf or fruit buds which were infected the previous season. Infected buds are not as plump as healthy buds, and they often don't seal properly, resulting in a "feathered" appearance. The buds are more sensitive to cold temperatures and will often die if the temperatures drop below -18F. However, even at lower temperatures some of the mildew survives in infected buds and inoculum is always available. The fungus also produces small black structures called cleistothecia, but the spores from cleistothecia do not play a major role in new infections.

New infections occur in the spring when infected buds open and the dormant fungus resumes growth on the developing tissues. These primary infections result in a white, powdery mass of spores which are easily spread by wind and splashing rain. Secondary infections result when these spores infect developing terminals. Late summer infection of buds results in overwintering inoculum inside the buds.


CONTROL
Plant varieties of apple with resistance to powdery mildew. Cultivars such as Jonathan, Idared, Rome, and Gala are susceptible whereas Red and Golden Delicious are more resistant. Pruning infected buds during the dormant season has not proven to be an effective cultural practice and is not recommended in commercial orchards. However, it might be practical for home owners to remove infected shoots as they appear early in the spring. Chemical control with fungicides is necessary when growing susceptible varieties.

Early spring applications of a fungicide are necessary to prevent secondary spread of the mildew in apples. Neglecting control early in the year will result in poor control during the season. Start at the pre-pink and repeat every two weeks until insecticide sprays begin. At that time, combine the fungicide with insecticide used for codling moth control and repeat as needed for insect control about every two to three weeks. Be vigilant with fungicide applications when the buds are setting in July to reduce the amount of overwintering infected buds. A good mildew control program in the current year reduces the amount of overwintering inoculum and subsequent fungicide applications.

FUNGICIDES

APPLES

Fungicide

Rate

Notes

Bayleton 50WP 2-8oz/A Repeat every 7-14 days with a maximum of 24 oz. per season. May be applied up to harvest.
Rally 40 WP 1.25-2.5 oz /100 gal Repeat every 7-14 days. Do not apply within 14 days of harvest. Do not apply more than 5 lbs/season/acre.
Rubigan 12 8-12 oz/A Repeat every 7-10 days. Do not apply within 30 days of harvest.
Funginex 18.2EC 10 oz/100 gal, 36-40 oz/A Repeat applications every 7-10 days. Do not apply after petal fall. Do not exceed five applications/season.
Benlate 50WP 2-3 oz/100 gal Repeat applications every 7-14 days. Do not apply within 14 days of harvest. Use the high rate and add summer oil at 1qt/100 gal on susceptible varieties.
Sulfurs Follow label instructions Repeat applications every 5-7 days. Do not apply when temperatures exceed 90 F. Reentry after 24 hours.
Procure 50WS 2-4 oz/100gal water Initiate at first sign of mildew, usually at 1/2" green. Continue at 7-14 day intervals until terminal growth stops.
Sulforix 2qt/100gal water Lime sulfur may injure Delicious varieties during hot weather.
Topsin M 70W 1-1 1/2 lbs/A Apply at 5-10 day intervals from green tip through petal fall. Continue at 7-14 day intervals in coversprays.
Armicarb 2.5-5 lbs/A Acceptable organic powdery mildew control pesticide. Should not be mixed with other pesticides.


Precautionary Statement: All pesticides have benefits and risks, however following the label will maximize the benefits and reduce risks. Pay attention to the directions for use and follow precautionary statements. Pesticide labels are considered legal documents containing instructions and limitations. Inconsistent use of the product or disregarding the label is a violation of both federal and state laws. The pesticide applicator is legally responsible for proper use. This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Vice President for Extension and Agriculture, Utah State University.