Incidence and Pathogenicity of Colletotrichum orbiculare and a Phomopsis sp. on Xanthium spp.

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Abstract
Two diseases of Xanthium spinosum, caused by Colletotrichum orbiculare and an unidentified Phomopsis sp., were widely distributed in New South Wales [Australia]. Phomopsis sp. occurred less frequently on X. occidentale, X. italicum, and X. orientale than on X. spinosum. Isolates of C. orbicular 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.