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Disease Note.

Russian Knapweed Rust Caused by Puccinia acroptili in New Mexico. M. E. Palm, USDA, APHIS, Systematic Botany & Mycology Laboratory, BARC-West, Beltsville, MD 20705. S. G. Vesper, USDA, APHIS, Albuquerque, NM 87102. Plant Dis. 75:1075. Accepted for publication 15 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1075E.

Russian knapweed (Centaurea repens L.) was introduced into North America in 1898 from Eurasia (1,2), and this persistent, aggressive weed is an increasing problem in much of the western half of the continent. During a noxious weed survey in New Mexico in September 1990, four specimens of Russian knapweed heavily infected by a rust were collected from a moderately dense stand in San Juan County. The fungus was identified as Puccinia acroptili P. Syd. & Syd. based on the uniformly echinulate urediniospores with conspicuous caps on the three equatorial germ pores and closely punctate-verrucose teliospores with a hyaline, usually persistent pedicel (2). This apparently host-specific fungus, which has been reported (1) to inflict considerable stress on heavily infected plants, may have potential as a biological control agent. P. acroptili has been reported in North America from British Columbia, Saskatchewan, California, and southern Arizona, but this is the first report from New Mexico and suggests natural eastward spread of the fungus along with its host. A voucher specimen has been placed in the U.S. National Fungus Collections (BPI), Beltsville, Maryland.

References: (1) K. Mortensen and M. M. Molloy. Can. Plant Dis. Surv. 69:143, 1989. (2) D. B. O. Savile. Can. J. Bot. 48:1567, 1970.