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Disease Note

First Report of Puccinia jaceae var. diffusa on Diffuse Knapweed in the United States. M. E. Palm, USDA, APHIS, Systematic Botany & Mycology Laboratory, BARC-West, Beltsville, MD 20705-2350. R. D. Richard, and P. Parker. USDA, APHIS, Biocontrol Facility, Forest Science Laboratory, Montana State University, Bozeman 59717-0278; and USDA, APHIS, Mission Biological Control Laboratory, Moore Air Base, Mission, TX 78572. Plant Dis. 76:972. Accepted for publication 5 March 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0972D.

Diffuse knapweed (Centaurea diffusa Lam.) is an introduced weed that is an increasing problem in the semiarid grasslands of the northwestern United States and western Canada (1). Puccinia jaceae Otth var. diffusa Savile was found for the first time in the United States on leaves and stems of this weed throughout Stevens and Spokane counties in Washington. Identification was based mainly on urediniospore characters, most importantly the two superequatorial germ pores on slightly flattened spore walls. P. j. diffusa occurs in eastern Europe and was first reported in North America from British Columbia (2). The rust has potential as one agent in a biocontrol program (1). Safflower (Carthamus tinctorius L.), grown in some midwestern and western states and provinces, is susceptible in the cotyledon stage to this fungus. However, production is unlikely to be affected because adult plants are resistant (1,2), the ecological range of the two hosts in North America differs, no increase in pathogenicity was observed after 10 generations on safflower, and the rust could not overwinter on infected safflower debris in European field tests (1,2). Voucher specimens have been deposited in the U.S. National Fungus Collections (BPI), Beltsville, Maryland.

References: (1) K. Mortensen et al. Can. J. Plant Pathol. 13:71, 1991. (2) K. Mortensen et al. Can. J. Plant Pathol. 11:322, 1989.