Source Year: 1998

Inoculation of leaves of resistant and susceptible Hevea brasiliensis clones with Corynespors cassiicola induced foliar necrosis and biosynthesis of scopoletin (Scp), considered a Hevea phytoalexin. Foliar symptoms appeared, as soon as 24 h after infection and preceded Scp accumulation in inoculum droplets (peak: 48 h after inoculation). Scp concentration was 5-fold higher in the susceptible than in the resistant clone 48 h after infection. Nevertheless, a fungitoxic effect of Scp on spore germination and on mycelium growth was shown in bioassays, but the efficient concentrations were greater than those found with other foliar pathogenic fungi of rubber trees such as Microcyclus ulei and Colletotrichum gloeosporioides. Bioassay showed that the low Scp fungitoxicity to C. cassiicola could also be related to the ability of fungus to detoxify Scp.Isoelestric focusing analysis of peroxidase activity in Hevea infected leaflets showed an increase in acidic and basic isoperoxidases that are able to use Scp as substrate. The rapid increase of Scp-oxidase activity(as soon as 16 h after inoculation) was higher in the resistant than in the susceptible clone. In vitro testing of Scp peroxidation products showed a toxic effect on C. cassiicola (conidia germination and mycelium growth), but not significantly higher than Scp. It appears that the level of Scp accumulation was a balance between its synthesis and its degradation by the pathogen or/and by foliar Scp-peroxidases. It is concluded that Scp cannot be considered as a major defence mechanism of Hevea towards C. cassiicola disease.