EVIDENCE FOR THE INVOLVEMENT OF ASCOCHITINE IN PHOMA LEAFSPOT-WILT DISEASE OF CLEMATIS

Ascochitine, a phytotoxic metabolite, was purified from cultures of Ph oma clematidina and identified by H-1 and C-13 nuclear magnetic resona nce spectroscopy and electron impact and chemical ionization mass spec troscopy. This toxin-induced electrolyte leakage from leaf discs of Cl ematis cultivars that were susceptible to fungal infection: while ther e was no significant electrolyte leakage from leaf discs of the cultiv ar most resistant to fungal infection. The level of ascochitine produc tion in vitro by P. clematidina isolates was related to isolate virule nce. The Fungal isolates could be characterized into two groups: (1) h igh virulence, high ascochitine production, and (2) low virulence, low ascochitine production. Ascochitine was isolated from P. clematidina- infected leaf discs, indicating the toxin is produced in vivo. Leaf ti ssues exposed to ascochitine solutions showed black flecking in propor tion to the concentration of ascochitine. Scanning electron and light microscopy of infected leaves indicated that the fungal hyphae were we ll behind the necrotic zone in leaf spots, while transmission electron and light microscopy suggested that mitochondria and chloroplasts wer e the major organelles affected by ascochitine, although extensive cel lular damage was evident. These observations suggest that ascochitine may be involved in the pathogenesis of P. clematidina against Clematis , by killing plant cells prior to hyphae ramification through the necr otic tissue.