QBARS - v32n2 Petal Blight - A Threat to Us All

Petal Blight - A Threat to Us All
by G. David Lewis, Colts Neck, N.J.
Professor of Plant Pathology, Rutgers University.

Petal blight, caused by a fungus called Ovulinia , has been present in epidemic proportions in many rhododendron and azalea gardens in Pennsylvania, New Jersey, New York, and Connecticut for the past two to four years. This disease is perhaps the worst of all since it causes a wet, slimy rot of the flowers of midseason and late-blooming plants. Although no other parts of the plants are affected, what good are they if we are to see no flowers? If this disease continues to spread and occur with such severity, we may eventually find ourselves devoting more time to daffodils, iris, African violets, or just crab grass. What happened? We knew that petal blight was a southern problem, and with the exception of a few local outbreaks many years ago, it did not seem to be adapted to northern areas and presented no real threat to our region. Although I certainly do not claim to have all the answers, I think that some light can be shed on the problem and some suggestions for control can be given.
Disease Build-up: Petal blight does not sweep through a garden and destroy with the rapidity of the Irish potato blight. The first year only a few flowers on a few plants will be affected. Usually, the second year shows scattered fairly severe attacks in various areas, but by the third year total devastation of mid- and late-season blooms is possible. These observations would suggest that the disease spreads fairly slowly within the garden.
Effect of Temperature: Petal blight does not attack the early bloomers. Are they resistant? I doubt it, since it would be unlikely that the genes for early bloom in all the plants we grow are closely linked to genes for virtual immunity to this disease. It seems apparent that the fungus does not grow well until fairly warm weather arrives. The first attacks are consistently observed just after our earliest evergreen azaleas have bloomed.
Effects of Seasonal Weather Patterns: Prior to the past spring (1977) many gardens in our area experienced three successive years of severe petal blight. Each of these springs was characterized by frequent rains, and was preceded by an abnormally warm winter. This past spring was variable with respect to rainfall, and was preceded by a record-setting cold winter. Petal blight was severe in areas of frequent rainfall (Philadelphia and Long Island) and far less severe in less rainy areas (central New Jersey). This strongly suggests that wet springs are more important than warm winters in favoring disease outbreaks.
Resistance: I have looked at many hundreds of different rhododendrons and azaleas during the past three years with the hope of observing resistant hybrids and species that could be grown in problem areas. I have yet to find the first resistant plant, even mountain laurel is affected! All mid- or late-season bloomers are either highly susceptible or very highly susceptible.
Garden Environment: Petal blight is much more likely to be severe in gardens that are densely planted, shady, and well protected from the wind. Why? Because these gardens dry out more slowly and the Ovulinia spores can germinate and infect only when the flowers remain wet for a number of hours (the exact time necessary is not known and undoubtedly is affected by temperature). The faster your blooms can dry, the less possibility that infection can take place.
Fungicide Sprays: Many people have tried various fungicide sprays often with very little success. Drs. Peterson and Davis, working at Rutgers, have reported progress using materials such as Benlate and Daconil. Is partial control the best we can hope for? I don't think so - based on two experiences. First, the Princeton Chapter had severe petal blight in its display garden in 1974. In the spring of 1975 a commercial tree spraying concern was employed to apply five weekly high pressure sprays using a mixture of Benlate and Daconil starting in the first week of May. Control was excellent, while a nearby unsprayed garden was devastated. The Daconil left a visible residue on the foliage. Second, petal blight broke out in our own garden in 1975. My wife and I picked every infected flower we could find and sent them off with the garbage (the fungus over winters in the dead flowers). The following spring we had agreed to a garden tour from the 1976 national convention of the ARS. In a mood of frantic desperation, I sprayed the garden approximately twice weekly with Benlate applied with a back-pack mist blower sprayer. When the tour arrived in the last week of May I could find no petal blight in our garden of over 1000 plants, but I was very tired! This spring I did not spray, and a few infected plants (about 4) were observed, so I plan to start spraying again next year.
Conclusions: Petal blight is a major threat to the genus Rhododendron , and we must learn how to control it. We need competent scientific research if we are to succeed. I think the ARS should support such research. Considerable control can be achieved by using a combination of control measures:

Sanitation - the removal and destruction of diseased flowers from your garden. Cultural Methods - the judicious pruning of your shrubs and trees to allow reasonable ventilation of your garden to promote rapid drying.
Chemical Control - at least weekly sprays (more in rainy periods - spray before the rain when possible) using spray equipment that will achieve thorough coverage of the buds and flowers with a recommended fungicide (check with your county agent or state experiment station) that is legally registered for use. Start spraying before the disease appears.
It will also help if you can arrange to have a dry spring.

Reprinted from The Rosebay , Massachusetts Chapter Newsletter.