Powdery Mildews, Microsphaera penicillata (Wallr.:Fr.) Lev. or Phyllactinia guttata (Wallr.:Fr.) Lev.

Leininger, T.D; Solomon, J.D.; Wilson, A. Dan; Schiff, N.M. 1999. A Guide to Major Insects, Diseases, Air Pollution Injury, and Chemical Injury of Sycamore. Gen. Tech. Rep. SRS-28. Asheville, NC: USDA Forest Service, Southern Research Station. 44 p.

Importance.-Powdery mildews are common throughout the range of sycamore trees and are most damaging on younger, smaller trees in ornamental, nursery, or orchard settings. They do not cause significant damage in forests.

Identifying the Disease.-These fungi are named for the powdery white or, as they age, gray mycelia they produce on upper and lower leaf surfaces (fig. 23a). Infected leaves are often stunted, buckled, or cupped (fig. 23b). Mycelial mats may entirely cover young leaves or may occur as irregularly-shaped blotches (0.5 to several cm) on larger leaves.

Figure 23a. - White and gray mycelial mats of powdery mildew fungus on leaves in foreground; uninfected leaves in background.

Identifying the Fungi.-Young mycelial mats will have only the conidial stage of the fungus, which produces colorless spores. Older colonies may have spherical, brown-black cleistothecia (0.1 to 0.2 mm) with distinctive appendages that aid in identification. In warmer areas, cleistothecia may not form.

Biology-Initial infections are from spores that overwintered on fallen leaves or in buds. Infections begin on young shoots and expanding leaves but rarely on mature leaves. Conidia are spread on air currents and can germinate on dry plant surfaces. Disease buildup is favored by warm, dry days and cool nights.

Figure 23b. - Leaf distortion and cupping caused by powdery mildew.

Control.-Because powdery mildews grow superficially on leaves, they are easily controlled on ornamental trees with organic contact fungicides or sulfur dust. Disease occurrence may increase with activities that stimulate or prolong the occurrence of new growth; e.g., fertilizing and cultivating. Control in forests usually is not needed.