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Powdery Mildew of Eustoma Caused by Leveillula taurica in California. S. T. Koike, University of California Cooperative Extension, Salinas 93901. S. A. Tjosvold, and ID. Greene. University of California Cooperative Extension, Salinas 93901. Plant Dis. 79:426. Accepted for publication 17 March 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0426E.

In October 1994, commercial greenhouse plantings of the cutflower Eustoma (Eustoma grandiflorum Raf. = Lisianthus russellianus Hook) in Monterey County, CA, were found to be extensively colonized by a powdery mildew fungus. Lesions associated with infections became chlorotic and in some cases caused the leaf to twist slightly. Conidia and conidiophores of Leveillula taurica (LeV ) G. Arnaud (anamorph: Oidiop-sis taurica (Lev.) E. S. Salmon) (1) emerged from stomata on both abax-ial and adaxial surfaces of the leaves. Conidia were borne singly, rarely catenulate, and consisted of two morphological types. Pyriform-to-navicular conidia measured 54.5-57.9 x 17.6-19.1 μm. Cylindrical conidia measured 50.5-54.5 x 17.8-19.2 μm. The L. taurica teleomoph was not found on the plants. Surveys conducted in and around the greenhouses failed to detect other plants infected with L taurica. This is the first report of L. taurica on any species of Eustoma and the first report of powdery mildew on commercially produced Eustoma cultivars. This is also the first occurrence of L. taurica on any plant in the Gentianaceae. The extensive powdery mildew growth resulted in severe disease symptoms on numerous cultivars, forcing growers to discard large numbers of flowers.

Reference: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980.