The production of ascochitine by seven isolates of Ascochyta fabae acc
ounted for the toxicity of culture filtrates of the fungus to cells is
olated from leaves of Vicia faba. The LD(50) value for cells from cult
ivars that were susceptible, tolerant or resistant to the fungus was s
imilar i.e. 3.0 x 10(-5) M, 3.8 x 10(-5) M and 2.9 x 10(-5) M, respect
ively. Ascochitine affected neither the germination of seeds nor the g
rowth of mature plants at 5.17 x 10(-4) M but caused necrosis and wilt
ing of plant cuttings at 2.5 x 10(-4) M and 5 x 10(-4) M. There was no
association between virulence of 16 isolates of A. fabae for three cu
ltivars of V.faba and the production of ascochitine in vitro. One isol
ate produced no ascochitine in vitro and yet was the most virulent for
two of three cultivars. The toxin could not be extracted from infecte
d plants.