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Effects of Infection by Compatible Species or Injection of Tissue Extracts on the Susceptibility of Nonhost Plants to Rust Fungi. Michèle C. Heath, Associate professor, Botany Department, University of Toronto, Toronto, Ontario, Canada M5S 1A1; Phytopathology 70:356-360. Accepted for publication 14 September 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-356.

The close proximity of established colonies of Uromyces phaseoli var. typica (the bean rust fungus), U. phaseoli var. vignae (the cowpea rust fungus), and Puccinia helianthi (the sunflower rust fungus) in their respective hosts increased the number of infection sites at which incompatible rust fungi produced haustoria from first-formed haustorial mother cells. Injection of extracts from susceptible rust-infected leaves of French bean had no effect on subsequent haustorium formation by the sunflower rust fungus in bean or cowpea or by the cowpea rust fungus in sunflower. However these extracts did increase haustorium production by the bean rust fungus in cowpea and by the cowpea rust fungus in bean; the effective component was of low molecular weight and partially heat labile. In both double-inoculation and injection experiments, only in the cowpea rust fungus-bean interaction did fungal growth continue beyond the formation of the first haustorium; such growth also was elicited by extracts from susceptible rust-infected cowpea leaves and by the close proximity of already growing colonies of the cowpea rust fungus induced to develop in bean leaves by a preinoculation heat shock of the nonhost tissue. It is suggested that in susceptible rusted French bean and cowpea leaves, and in interactions of French bean with the cowpea rust fungus if fungal colonies are permitted to develop by giving the tissue a heat shock, a similar or identical factor (or factors) is produced which, in French bean plants, specifically inhibits the expression of some, but not all, defense mechanisms which can prevent incompatible rust fungi from forming haustoria. However, growth of rust fungi in any plant beyond the formation of the first haustorium seems to depend on additional interactions between the two organisms. Significant differences in the response of different cultivars of French bean towards the cowpea rust fungus also were observed during this investigation.