General Information.  This fungal disease occurs wherever apples are grown and is particularly severe in the arid regions of the western United States. Powdery mildew can be particularly problematic in nurseries. Severity of the disease depends upon present and past weather conditions, cultivar, and control procedures. Although the disease does not kill trees per se, losses can be severe if fruit russetting occurs. Red delicious is relatively resistant to mildew. Some of the newer apple varieties (Braeburn, Fuji, Gala, Jonagold) are extremely susceptible to mildew and may require repeated fungicide applications for effective control. Pear powdery mildew is also caused by this fungus

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Symptoms.  The fungus can infect apple leaves, blossoms, and fruit. Whitish, feltlike patches of fungal mycelium on the lower surface are often the first indication of infection. The lesions may appear as chlorotic spots on the upper leaf surface. Eventually they can spread and cover the entire leaf with a white, mealy coating. Leaf curling may occur. Infected terminals are often stunted and covered with a white mealy coating. Internodal shortening is often pronounced. As the mildew colonies age they eventually turn gray or brown. Minute brown fruit bodies known as cleistothecia can form on the aging colonies

Photo: Infection of a terminal. 

Photo: Infection of a Granny Smith pedicel

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Photo: Infection results in a the production of a net-like pattern on the fruit surface. Fruit is most susceptible during the period around petal fall.

Epidemiology.   The fungus overwinters as mycelium in dormant infected buds. Conidia produced on the overwintering mycelium are liberated upon exposure to the air (i.e. at bud burst and later) and dispersed by wind currents to emerging leaves and shoots where primary infections are initiated. Colonies established from the primary infections produce more conidia (secondary inoculum) that initiate fruit, leaf, and shoot infections (secondary infections). Leaves are susceptible for only a few days after emergence. The optimum temperature for germination of conidia is about 70-75 F and is promoted by humidities of 96-100%. Early-season secondary infections are limited mostly by low temperatures. The amount of overwintering inoculum is proportional to the severity of the winter, i.e. less inoculum survives a severe winter. Temperatures of about -12 C can kill the fungus but not the buds, allowing them to produce healthy leaves. Healthy buds can survive temperatures 2-10 C lower than infected buds. Conidia are very sensitive to temperature. At low temperatures (about 5 -10 C) germination requires between 16 and 24 hours of incubation. The optimum temperature for germination of conidia, and for mycelial growth, is about 21 C (73 F). The inoculum for pear fruit comes from infested apple orchards. Infested apple orchards must be within 200 yards of the pear orchard in order to serve as a source of inoculum.

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Management.   Cultural practices should promote air circulation and the lowering of orchard humidities. Dormant pruning to remove overwintering inoculum may be feasible in orchards with low amounts of overwintering inoculum. This may result in fewer fungicide applications. However, the practice is labor intensive and may interfere with training for tree structure and production. Fungicides registered for mildew control in Washington include sulfur, fenarimol, myclobutanil, triforine, dinocap, triadimefon and benomyl. Applications should be made from the tight cluster stage until terminal shoot growth ceases in midsummer. The interval between sprays is usually 7 days during periods of rapid leaf development and 12-14 days during the postbloom period. It is critical that fruit be protected during the bloom period. Fruit susceptibility decreases after petal fall.

Online References

Clarke, Andrea 1988. Powdery Mildew of Apple. Ontario Ministry of Agriculture, Food and Rural Affairs Factsheet 88-124.

Ellis, M.A. 1994. Apple Powdery Mildew. The Ohio State University Extension Factsheet HYG-3001-94.

Hickey, K.D., Yoder, K.S., and Biggs, A.R. 1997.  Apple Powdery Mildew. West Virginia University Kearneysville Web Site.

http://www.cas.psu.edu/docs/CASDEPT/PLANT/fpath/Apple/POWMILfact.html

Ritchie, D.F., Sutton, T.B., and Sorensen, K.A. 1997. Disease and Insect Management in the Home Orchard. North Carolina State University Cooperative Extension.

Teviotdale, B.L.,  and Gubler, W.D. 1997. Apple Powdery Mildew. UC Pest Management Guidelines, University of California Statewide Integrated Pest Management Project.