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Incidence and Pathogenicity of Colletotrichum orbiculare and a Phomopsis sp. on Xanthium spp.. A. Nikandrow, Agricultural Research and Veterinary Centre, Forest Road, Orange, New South Wales 2800, Australia. G. J. Weidemann, and B. A. Auld. Department of Plant Pathology, University of Arkansas, Fayetteville, 72701, and Agricultural Research and Veterinary Centre, Orange, New South Wales. Plant Dis. 74:796-799. Accepted for publication 25 April 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0796.

Two diseases of Xanthium spinosum, caused by Colletotrichum orbiculare and an unidentified Phomopsis sp., were widely distributed in New South Wales. Phomopsis sp. occurred less frequently on X. occidentale, X. italicum, and X. orientale than on X. spinosum. Isolates of C. orbiculare were highly virulent on X. spinosum but were only weakly virulent to other Xanthium spp. in NSW and in the United States. Isolates of Phomopsis sp. were pathogenic to all Xanthium spp. However, isolates of Phomopsis sp. from X. spinosum, X. occidentale, and X. italicum were more virulent to X. spinosum than to the other Xanthium spp.

Keyword(s): biological control, Macrophomina phaseolina, mycoherbicide, Sclerotinia sclerotiorum, Verticillium dahliae, Xanthium strumarium.