First Report of Powdery Mildew on Azalea Caused by Erysiphe azaleae in Louisiana
Authors: Holcomb G. E. , Ferrin D. M. 
Published: May 2007

A group of five cv. Red Formosa azaleas (also known as cv. Dixie Beauty, an evergreen indica type of Rhododendron indicum) was observed with powdery mildew symptoms in Baton Rouge, LA in early March 2006. Symptoms included leaf distortion, purple leaf pigmentation at infection sites, and irregular necrotic areas. White, sparse, superficial fungal mycelium was present on both leaf surfaces. There was no active conidia production at this time and no cleistothecia were found on the old infected leaves. Infected plants were 5 years old and growing in heavy shade beneath a large deciduous oak tree. Two other groups of powdery mildew-free cv. Red Formosa, containing 6 and 11 plants each, were growing in mostly shade-free areas within 30 and 140 m, respectively, of the infected plants. Of 5,091 azaleas surveyed in and around Baton Rouge, only three other infected plants were found beside the original five. Among the plants surveyed were 167 additional cv. Red Formosa azaleas. The three additional infected azaleas, all the same unidentified indica type cultivar and growing in dense shade, showed severe leaf distortion and leaf drop. Conidia were produced abundantly on these plants in April and early May, but the teleomorph was not found. Pathogenicity tests were performed by rubbing leaves with sporulating powdery mildew from infected cv. Red Formosa plants on terminal leaves of two branches each of three 1-gallon-container plants of the same cultivar. A clear plastic bag containing a damp paper towel was placed over each inoculated branch for 48 h and then removed. Noninoculated branches on the same plants served as controls. Plants were maintained in a greenhouse (21 to 27°C). After 5 days, the first symptoms and signs of infection appeared on inoculated leaves in the form of purple pigment formation and white sporulating mycelia. After 12 days, terminal leaves on all inoculated branches showed symptoms and signs of powdery mildew, and after 18 days, leaf puckering and irregular necrotic spotting was common; purple pigmentation often outlined infected areas in which white mycelia and conidia production occurred. Some sporulation was present on petioles and some leaf drop occurred. The foliage on noninoculated branches remained disease free. Conidia from original infected plants were produced singly, ellipsoid to cylindric, lacked fibrosin bodies, and measured 27 to 54 μm long (mean = 37.4, standard error (SE) = 0.19, n = 102) × 15 to 23 μm wide (mean = 17.4, SE = 0.13, n = 102). Conidia sometimes formed short chains of two to four on inoculated plants held in the greenhouse. Conidiophores measured 77 to 123 μm long (mean = 106, SE = 1.1, n = 12) × 7 to 10 μm wide (mean = 8.9, SE = 0.26, n = 12) and had curved or twisted bases. Appressoria were multilobed. On the basis of these characters, the anamorph of the azalea powdery mildew was identified as Oidium ericinum Erikss. = Erysiphe (Microsphaera) azaleae (U. Braun) U. Braun & S. Takam. (1,2,4). To our knowledge, this is the first report on the occurrence of azalea powdery mildew in Louisiana. Powdery mildew is more common on deciduous than on evergreen azaleas and is more common in northern parts of the United States, especially in the Pacific Northwest (3).

References: (1) U. Braun. Nova Hedwigia Suppl. 89:1, 1987. (2) U. Braun and S. Takamatsu. Schlechtendalia 4:5, 2000. (3) M. L. Daughtrey and D. M. Benson. Rhododendron diseases. Page 339 in: Diseases of Woody Ornamentals and Trees in Nurseries. R. K. Jones and D. M. Benson, eds. The American Phytopathological Society. St. Paul, MN, 2001. (4) A. J. Inman et al. J. Phytopathol. 148:17, 2000.