QBARS - v22n4 Azalea Petal Blight

Azalea Petal Blight
R. H. Gruenhagen, Extension Plant Pathologist
Virginia Polytechnic Institute - Blacksburg, Virginia
Reprinted with permission, from Nurserymen's Notebook,
a publication of the Extension Service, Virginia Polytechnic Institute

This disfiguring disease of cultivated azaleas was first discovered in South Carolina in 1931. It has since spread through Texas, Florida, the Carolinas, Virginia, and into Maryland. It was first reported in Virginia in 1947.
The causal fungus attacks the flowers only. The first symptoms of the disease are small spots on the petals about the size of a pin head. The spots are pale or whitish on colored petals, and a rusty brown color on white petals. Although the spots are round at first, they enlarge rapidly, and soon affect the entire petal. The infected petal tissue becomes soft and limp and eventually the whole flower collapses.
The collapsed flowers frequently cling to the twigs for a considerable period of time. Hard black objects are produced on the old diseased flowers. These bodies are known as sclerotic; and provide the way in which the fungus lives through the period when flowers are not present. Sclerotic may be formed on the flowers while they are still hanging on the twigs or after they have fallen to the ground.
Cause :
The petal blight disease of azalea is caused by the fungus Ovulinia azalea . Early in the spring the sclerotic which are resting on, or slightly below the soil surface produce small brown, saucer-shaped bodies which are called apothecia. The apothecia are raised above the soil on slender stalks. The saucer-shaped body itself is seldom more than 1/16 of an inch in diameter. Initial infection of the flowers is caused by microscopic spores (seeds of the fungus) which are developed in the apothecia and are shot into the air and are distributed by wind currents. Secondary infections are caused by spores which are produced on the infected petal tissue, and are blown about by the wind to cause infection of the entire plant.
The size of the planting and the intensity of the disease will help in determining the most effective control measures to be used. Sanitation is a reasonably effective control where only a few plants are involved. Diseased flowers should be carefully removed and burned to prevent continued spread of the infection. This method is not practical in nurseries and in home or other plantings where large numbers of azaleas are involved. A pesticide program may be necessary in these large scale plantings.
Soil Treatment:
This approach is used to control the over wintering, or sclerotial stage of the fungus. Any of the following fungicides should be applied to the soil surface around the plants from 1 to 3 weeks prior to appearance of the first blooms.  Calcium cynamid is injurious when applied directly on azaleas but is not likely to cause injury if the application is only on the soil around the plants.
Bloom Sprays and Dusts:
This phase of the control program is protective in nature and must be initiated before blooms become infected. Spray or dust applications should start when the first blooms are opening and be continued twice a week during the blooming period. The number of applications should be increased to three times a week during wet and humid weather. Any of the following materials may be used:

Material Amount for 100 sq. ft.
Acti-dione RZ, wettable powder 11 oz. (1¾ cups)
Calcium cynnamid, granular ½ 1b.
PCNB, 75% wettable powder ⅔ lb. (2¾ cups)
PCNB, 20% dust 2½ lb.
Material Amount per Gallon
Acti-dione RZ, wettable powder 1 level teaspoon
thiram, 65% wettable powder 2 level teaspoons
zineb, 65% wettable powder 2 level teaspoons
zineb, 6% dust Cover buds and blooms evenly