The Distribution of Downy and Powdery Mildews and of Rusts over Tribes of Compositae (Asteraceae)

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Abstract
The distribution of downy mildews, powdery mildews and rusts over the genera of the 15 tribes of Compositae was surveyed. The proportion of host genera attacked by each of these groups of pathogens is highest in the Lactuceae, but also high in the Cynareae and Heliantheae. The tribe Mutisieae has very few genera with powdery mildews or rusts, and none with downy mildews; the taxonomic position of this tribe appears to warrant re-examination. Of the 269 genera of Compositae recorded as attacked by one or more of the above pathogen groups, many more have rusts alone (99 genera) than powdery mildews alone (32 genera), and 64 are affected by all 3 groups of pathogens. If a host genus is affected by a downy mildew, it also is affected by a rust and/or powdery mildew. Downy mildews are thus much less likely to be found on species of Compositae that are not hosts of rusts or powdery mildews. Hosts of Bremia are mostly found in tribes in which hosts of Plasmopara are comparatively rare or absent, and vice versa. Hosts of Peronospora are limited to Anthemideae and Lactuceae, and hosts of Basidiophora to Astereae and Heliantheae. No downy mildew hosts have been recorded in genera of Liabeae and Tageteae. Distribution of Puccinia is general, over all tribes. Species of Uromyces are absent from hosts in Lactuceae. Hosts in genera of Senecionea, Cynareae and Lactuceae are most likely to be affected by species of more than 1 genus of Erysiphaceae. Implications of the distribution of the 3 groups of pathogens for the taxonomic study of their hosts are discussed.

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