Uromyces appendiculatus Infection in BTH-Treated Bean Plants: Ultrastructural Details of a Lost Fight

The mechanisms of BTH [benzo-(1,2,3)- thiadiazole-7-carbothioic acid S-methyl ester]-induced resistance against bean rust caused by Uromyces appendiculatus have been explored in Phaseolus vulgaris by light and transmission electron microscopy, following the infection progression in plants challenged 7 days after treatment. While BTH did not affect uredospore germination and fungal penetration in the substomatal cavity, a first impairment to the colonization appeared evident about 48–96 h after inoculation, with alterations of infection hypha structure and reduction in mycelium expansion. No differences were found in this phase regarding the formation and ultrastructure of haustoria in untreated andBTH-treated plants, except for the deposition of electron-opaque material in the extrahaustorial matrix of the latter. A second and decisive impairment in fungal progression was observed at 7–10 days after inoculation when host cell penetrated, or in close contact with the fungal hyphae, were impregnated by phenolic compounds.