Abstract
Exogenous 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 formation remained unaffected. Germination and germtube growth were not hampered by the direct application of ASM on urediospores. The data suggested that ASM had an effect on the production and secretion of fungitoxic compounds to the leaf surface that hamper rust urediospore germination and appressorium formation. This hypothesis is supported by the following 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, and leaf exudates collected from ASM-treated plants were applied exogenously.
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