Acibenzolar-S-methyl-induced resistance to sunflower rust (Puccinia helianthi) is associated with an enhancement of coumarins on foliar surface

Exogeneous applications of acibenzolar-S-methyl (ASM) induced resistance to rust infection in sunflower, characterized by reduced infection frequency with no effect on latency period or pustule size, and no increase in host cell necrosis. Cytological studies showed that the reduced frequency of infection was due to a reduction in germination and appressorium formation, while stoma penetration, growth of infection hyphae and haustorium frmatin remained unaffected. Germination and germtube growth were not hampered by the direct application of ASM on urediospores. Their data suggested that ASM had an effect of the production and secretion of fungitoxic compounds to the leaf surface that hamper rust urediospore germination adn appressorium formation. This hypotheses is supported by the gollowing experimental results: (i) an increase in the amount of accumulated and excreted coumarins and other phenolic compounds in ASM-treated plants, and (ii) a reduction of germination and of appressorium formation when ayapin, scopoletin, leaf exudates collected from ASM-treated plants were applied exogenously. (C) 2002 Elsevier Science Ltd.