Interactions of the nonhost French bean plant (Phaseolus vulgaris) with parasitic and saprophytic fungi. IV. Effect of preinoculation with the bean rust fungus on growth of parasitic fungi nonpathogenic on beans

摘要

Bean leaves inoculated 24ensp;h previously with the bean rust fungus were inoculated with spores ofCochliobolus heterostrophus,Stemphylium sarcinaeforme,Stemphylium botryosum, orCladosporium fulvum. For all species exceptC.fulvum, hyphal growth resulting from stomatal penetrations was greater than that in leaves that were not rust-infected but did not continue for more than about 24ensp;h. The incidence of direct penetrations for these three fungi also was increased by prior rust infection, and the incidence of epidermal wall appositions was reduced. Growth ofC.fulvumin rust-infected leaves only exceeded that in control leaves when spores were injected into the intercellular spaces of the mesophyll tissue. Rust infection either had little effect on the incidence of cell death, normally induced by all of the tested fungi exceptC.fulvum, or it enhanced this response in association with greater fungal growth. From this and previous studies, it seems that successful rust infection increases the growth of a wider array of fungi nonparasitic to beans than treatments with growth regulators or intercellular washing fluids from rusted tissue. Its effect is most closely mimicked by preinoculation treatments with heat or protein synthesis inhibitors, but it does not induce indiscriminate susceptibility. Its effect may, in part, be due to the suppression of defenses involving wall modifications.Key words:Uromyces appendiculatus, induced susceptibility, nonhost resistance.