PERSISTENCE OF REDUCED PENETRATION BY COLLETOTRICHUM-LAGENARIUM INTO CUCUMBER LEAVES WITH INDUCED SYSTEMIC RESISTANCE AND ITS RELATION TO ENHANCED PEROXIDASE AND CHITINASE ACTIVITIES

Penetration of Colletotrichum lagenarium was reduced into leaves 2 and 3 of cucumber plants with systemic resistance induced by inoculating leaf 1 with the fungus. The reduction was evident in induced plants challenged at 7 and 14 days after induction of leaf 1 but was not observed when leaves were challenged at 2 1 and 28 days after induction. Germination of conidia and appressorial formation were not significantly different on the induced and non-induced plants. Removal of inducer leaf 1, 7 days after inoculation with C. lagenarium, did not significantly reduce the inhibition of penetration or persistence of inhibition as compared to plants with the inducer leaf attached. High peroxidase and chitinase activities were evident in the induced leaves 21 and 28 days after the induction, although no restriction of fungal penetration was apparent. Removal of inducer leaf 1 at 7 days after inoculation reduced the activities of peroxidase peroxidase and chitinase at 14, 21 and 28 days after induction. Though peroxidase and chitinase activities increased in leaves 2 and 3 between 7 and 28 days after the induction of leaf 1, the induced resistance against the fungus decreased. The data from this investigation suggest that the reduction of fungal development in induced plants results from at least two different mechanisms. One reduces penetration into the plant and the other restricts development of the fungus once it has penetrated. The reduction of induced resistance al times when peroxidase and chitinase activities are high also suggests that the activities of these enzymes at the time of challenge are not major factors for the restriction of C. lagenarium in induced cucumber plants. (C) 2995 Academic Press Limited