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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 25, Number 3
July 1971

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A New Look at Azalea Petal Blight Control
J. L. Peterson and S. H. Davis, Jr.
Department of Plant Pathology. Rutgers University

Black Sclerotia of the Petal Blight
   Fig. 56. Black sclerotia of the Petal Blight
                fungus growing on rotting petals two
                weeks after the blossom stage.

        Petal blight-caused by the fungus Ovulinia azaleae Weiss was first reported in South Carolina in 1931. The disease spread rapidly throughout the southeastern states during the '30's, and was found in Texas in 1938, and California by 1940. It was not reported in mid-Atlantic states until the mid '50's, and then only in greenhouses on stock brought from the south. For a time it was thought the petal blight fungus could not over winter on outdoor plants in New Jersey. However, during the late '60's it was established in several areas of the state in outdoor plantings.
        Control of this devastating blight of azalea and rhododendron flowers was first obtained by Dr. Cynthia Westcott in 1945, using nabam plus zinc sulfate or one of the zineb materials such as Parzate or Dithane Z-78. Sprays had to be applied about three times a week during the flowering period.
        Testing has been conducted since 1945 with other fungicides and with various timing studies, but the "zineb three times a week" still remains as the recommended control for outdoor plants. Termil, a fungicide used as a vaporized dust does give good control of the disease in the confines of a greenhouse.
        A serious problem with the Ovulinia petal blight was reported during 1969 in a large privately-owned rhododendron garden in central New Jersey. Many varieties of azalea and rhododendron were blighted although the owner considered zineb to give "fairly good control of the blight." The tests reported here were started in the spring of 1970 using the more susceptible varieties as designated by the owner.
        Sprays were applied by the writers to tagged sections of eight varieties of rhododendron and azalea ('Caroline Gable', catawbiense (Gable's), Glendale 'Epicure', Glendale hybrid, 'Fastuosum Flore-Pleno', 'Roseum Elegans', 'catawbiense Album', Glass and fortunei.) Applications were made on May 19 and 26, using hand sprayers. Varieties ranged from full-bud to early-flower stage at time of first spray.
        Fungicides used were: Benlate 50W from duPont at 8 oz./100 gal. ( tbl./ gal,), Mertect-160 from Merck at 8 oz./ 100 gal. ( tbl./gal.), Daconil 2787 from Diamond Shamrock at 2 lb./100 gal. (1/tbl./gal.), and Parzate C from duPont at 1 lb./100 gal. (1 tbl./ gal.). The Daconil 2787 is the same chemical as Termil referred to previously, except it is a wettable powder and can be used as a spray.
        On June 2 during mid- to late-flowering stage, three observers individually rated flower blight for all treatments. No sclerotic (the small, gray-to-black, seed-like objects that develop on infected flowers, drop to the ground where they spend the winter, and start again the life cycle of petal blight when spores are produced the following spring) were evident at that time. On June 30 the same observers individually recorded presence or absence of the sclerotic on dead flowers.
        The Daconil 2787 treated sections blight than the other three treatments of bushes had noticeably less flower and markedly less then unsprayed portions of the bushes on June 2. However, on June 30 Sclerotia were as abundant on Daconil 2787 treated sections as on the unsprayed dead flowers. On the other hand, Sclerotia were almost non-existent on Benlate treated portions on this date.
        These results lead to two interesting possibilities for the future. First, if an entire rhododendron-azalea garden were sprayed with Benlate at weekly intervals, petal blight might be present but sclerotial production could be so inhibited that little or no disease would be expected the following year. Second, if Daconil 2787 and Benlate were combined in weekly spraying, petal blight might be completely controlled. We hope to obtain this type of information during the coming year.

Treatment rate per 100 gallons Flower blight index1 Sclerotia index2

Benlate 50W           8.0 oz

63 0.3

Mertect 160W        8.0 oz

72 1.7

Parzate C 75W      1 lb

66 2.0

Daconil 2787 75W  21b

43 2.6
Check 100 2.6
1Flower blight index: percent disease in relation to check-treatment readings set at 100%.
2Sclerotia index:  0 = none, 1 = few, 2 = moderate, 3 = abundant.

Volume 25, Number 3
July 1971

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