Powdery Mildews
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Importance
Powdery mildews cause only minor damage to ashes in the Eastern United States. On shade and ornamental trees and seedlings, progression of the disease can reduce vigor, but it does not substantially reduce annual growth. Damage to forest trees is negligible.
Identifying the Disease
Severe infections can cause distortion of tender, late season shoots, as well as chlorosis, foliar browning, and premature defoliation.
Identifying the Fungi
Powdery mildews are easily identified by the white or light-colored, powdery fungal growth that develops on upper or lower leaf surfaces. Identification of species is based on microscopic examination of cleistothecia, conidia, and conidiophores. Cleistothecia are minute, yellow to black, sexual fruiting structures usually found late in the season on the lower surface of leaflets. Cleistothecia of P. guttata have straight appendages with a bulbous base and asci that contain two spores.
Biology
Powdery midews are mose prevalent where cool to warm, humid weather persists with little rainfall. Primary infection of ashes occurs by ascospores released from overwintered cleistothecia on fallen leaves. Conidia can cause repeated infections during the growing season.
Control
No control is usually needed for this disease.
References
Solomon, J.D.; Leininger, T.D.; Wilson, A.D.; Anderson, R.L.; Thompson, L.C.; McCracken, F.I. 1993. Ash pests: A guide to major insects, diseases, air pollution injury and chemical injury. Gen. Tech. Rep. SO-96. New Orleans, LA: U.S. Department of Agriculture, Forest Service, Southern Forest Experiment Station. 45 p.
