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Powdery Mildew of Apple and Crabapple

Ned Tisserat  Extension Specialist, Plant Pathology


Powdery mildew on apple leaves

Russetting of apple caused by powdery mildew

Figure 1.  Powdery mildew on apple leaves.

Figure 2.  Russetting of apple
caused by powdery mildew.

 

Powdery mildew can be a problem on susceptible apple and flowering crabapple cultivars in Kansas. The disease is especially severe on Jonathan apple and can result in shoot dieback, death of flower buds, and russetting of fruit.

The disease first appears in the spring on fruit buds and new shoot growth. Low winter temperatures may kill infected flower buds. Those that do develop typically break bud several days later than normal and are covered with a white dusty growth of the fungus. The leaves on infected shoots also develop white patches and become distorted. Diseased leaves are narrow, folded longitudinally, and become stiff and brittle with age. On Jonathan apple, the fruit skin becomes rough and cracked, and fruit size is reduced.

 

Cause

The fungus (Podosphaera leucotricha) overwinters on the tree in infected buds. In the spring, the fungus colonizes developing leaves and begins to produce spores on the leaf surface. These spores are disseminated by wind and can reinfect other leaves.

A high relative humidity and temperatures between 60 and 80 F favor spore germination and infection. Free water on the leaf surface is not necessary for infection and, in fact, inhibits spore germination.

 

Control

Control of powdery mildew generally is not required except on highly susceptible cultivars. Where the disease has been a problem, include a mildewcide in the spray program for scab and rust.

Several fungicides are effective in controlling powdery mildew (see table). Be sure to read the fungicide label for more detailed information on proper application.

Best control of powdery mildew is obtained when the first fungicide spray is applied when the flower buds are just beginning to turn pink. Sprays should be continued every 10- to 14-days until terminal growth has stopped.

 

Table 1. The flowering crabapples listed below usually are free of or are only slightly affected by apple scab, powdery mildew and fireblight during a normal year in Kansas

Cultivar

Cultivar
Adams Molten Lava
Adirondack Ormiston Roy
baccata ‘Jackii’ Prairie Fire
Callaway Professor Sprenger
Centurion Profusion
Coral Burst Red Baron
David Red Jewel
Dolgo Robinson
Donald Wyman sargentii ‘Satin Cloud’
floribunda Selkirk
Golden Galaxy Snowdrift
Golden Raindrops Sugartime
Harvest Gold Tina
Henry Kohankie White Angel
Indian Summer White Cascade
Jewelberry Winter Gold
King Arthur Yellow Jewel
Lancelot zumi ‘Calocarpa’
Liset  
Mary Potter  

 

Table 2. Apple varieties for the home garden with relatively high levels of resistance against several diseases. A complete listing of varietal resistance to apple diseases can be viewed at http://www.ca.uky.edu/agc/pubs/id/id93/ch_1.pdf

Cultivar

Comments
Enterprise Excellent scab, mildew, rust and fireblight resistance
Freedom Susceptible to fireblight, but resistant to scab, rust and mildew
Jonafree Highly susceptible to rust; will require fungicide applications
Liberty Excellent scab, mildew, rust and fireblight resistance
Priscilla Moderate scab and rust
Prima Highly susceptible to rust; will require fungicide applications
Redfree Good scab and rust resistance, susceptible to fireblight
Williams Pride Excellent scab, mildew, rust and fireblight resistance

 

Table 3.  Fungicides for the control of scab, cedar apple rust and powdery mildew on apple and flowering crabapple.

Fungicide

Examples of Trade Names

Comments

benomyl

 Benalte

Homeowner: benomyl

 Good against scab, but not rust

chlorothalonil

 Daconil, Bravo, Thalonil, Echo, others

Homeowner: Many trade names including Daconil

 For control of scab on flowering crabapple. Cannot be used on apple.

Coppers (many formulations)

 Bordeaux, Kocide, others Weak against rust, but many of these compounds may be used by organic growers

fenarimol

 Rubigan Commercial use for scab, rust and mildew control

ferbam

 Ferbam

Homeowner: Ferbam

 Good for scab, rust, but not mildew

myclobutanil

 Nova, Eagle, Systhane

Homeowner: Immunox

 Broad spectrum activity against scab, rust and mildew

propiconazole

 Banner MAXX Commercial use for scab, rust and mildew control on flowering crabapples only

sulfur

 Many names Control of scab, but may cause leaf burning during high temperatures.

 

Reference to products in this publication is not intended to be an endorsement to the exclusion of others which may be similar. Persons using such products assume responsibility for their use in accordance with current label directions of the manufacturer.
 

 

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Web updated 9/01/06
Web comments to jpierz@ksu.edu