Ovulinia Petal Blight Disease and Its Control
R. C. Lambe, Blacksburg, Virginia, Extension Specialist
Plant Pathology, Dept. of Plant Pathology & Physiology
Virginia Polytechnic Institute and State University, Blacksburg, VA
Although the Indian and Kurume type of azaleas grown in Virginia have been most susceptible to Ovulinia petal blight or flower blight as it is sometimes called, other groups of azaleas and rhododendrons are also susceptible. If the temperatures are below normal and fairly evenly distributed during the flowering period the disease is usually damaging.
The disease is first noticed as spots on the petals about the size of a pinhead. They are pale or whitish on colored flowers, and rust-colored on white flowers. The spots are at first circular and eventually the entire corolla collapses. Affected petals are somewhat slimy and fall apart if rubbed gently between the fingers. Diseased flowers dry and cling to the plants for some time, presenting an unsightly appearance; whereas normal flowers of Indian azaleas fall from the plant while still displaying color and normal shape.
In the blighted flowers, the fungus produces hard, black objects, known as sclerotia, which over winter in the soil. Fungus fruiting bodies develop from them in the spring, bearing spores that may infect relatively few flowers. Secondary spores are produced in large numbers on the initial infections. The secondary spores are responsible for widespread, destructive outbreaks of flower spot.
Formerly petal blight occurred primarily on azaleas grown out-of-doors in the warm climate of the southern states, but it has been introduced into the North in shipments of plants from the South. Under greenhouse environment the fungus produces an abundance of fungus spores. The spores are produced by the sclerotia in old flower tissue and by free sclerotia that drop to the ground and remain undetected. Unsold plants, forced 1 year and held over for forcing again the following year, also are sources of fungus inoculum. The home gardener can reduce the number of sclerotia that will be produced by picking and destroying affected flowers, and replacing the surface litter about infected plants with uncontaminated material. Most fungicide disease preventative programs have been based on zineb or maneb plus zinc. Zineb should be used at the rate of 1 lb./100 gal. of water (2 teaspoons/gal of water) and maneb plus zinc at the rate of 1½ 1b/100 gal water (1½ tablespoons/1 gal water). However, it is necessary to apply these fungicides starting when the first indication of flower color appears in the susceptible variety making sure that the flowers are thoroughly covered. These fungicides do not prevent infected flowers from producing sclerotia.
With the testing and registration of benomyl (Benlate) for flower blight control a new dimension has been added. Comparisons made in 1973 by Drs. J. L. Peterson and S. H. Davis, Jr., plant pathologists at Rutgers University in New Jersey, with benomyl and other fungicides indicate that if Benlate is applied to the flower buds just as the color was beginning to appear in the buds, and repeated at weekly intervals, good control of the petal blight resulted and a reduction of from 58% to 0% production of sclerotia in the infected flowers occurred. The optimum flower blight control was obtained if the first application of benomyl can be timed to occur just prior to a rainfall rather than after rainfall. Earlier recommendations of benomyl on a 2-week spray schedule have been much less affective than we expected, but this recent research clarifies the situation.