Powdery Mildew on Deciduous Azaleas
Reprinted from the The Rhodo Rooter, the newsletter of the Great Lakes Chapter
Powdery mildew is not a serious disease on azaleas but on susceptible cultivars can be unsightly. Experiments have shown a reduction of azalea growth and at times flowering over a period of two to three years if the disease is not controlled. Deciduous azaleas are generally more susceptible to mildew than other azaleas, and will tend to lose their leaves sooner if appropriate fungicides are not applied. Young plants especially grown in the greenhouse may be seriously affected if a fungicide program is not followed.
Disease symptoms include the development of white powdery spots of mycelial growth on the leaf surface. Depending on the susceptibility of the cultivar and with favorable weather the mycelium can completely cover the leaf. In some azalea cultivars, the mildew may occur on the lower leaf surface only. After the mycelial covering has developed on the leaf, single-celled conidia form in chains on the mycelium. These are produced by the thousands and are carried by the wind to adjacent plants where they incite new secondary infections. As the summer progresses, small black fungus fruiting bodies containing ascospores form on the mycelium. The fungus apparently survives the winter and dry periods in this form or as dormant mycelium on old leaves. The following spring, in most growing areas, the ascospores are released from the cleistothecia or condidia are formed on dormant mycelium, and these spores are carried by the wind to young developing leaves where they germinate and cause the initial primary infection. The exact nature of the overwintering process has not been clearly demonstrated.
The disease is favored by warm temperatures, but can progress and be serious at lower temperatures. Young leaves are generally more susceptible to the disease than older leaves. There are several azaleas that are powdery mildew resistant. Selection and planting of these resistant cultivars can offer the best means of disease control.
When desired, mildew can be controlled with protective fungicides such as benomyl (Benlate), triadimefon (Bayleton), Triforine, or dodemorph acetate (Milban) applied at weekly to 10-day intervals.