Biosynthesis of scopoletin in Hevea brasiliensis leaves inoculated with Phytophthora palmivora

Inoculation of resistant (R) and susceptible (S) Hevea brasiliensis leaves with Phytophthora palmivora induced foliar necrosis and biosynthesis of scopoletin (Scp), considered as a Hevea phytoalexin. The degree of resistance of four clones, as classified by the necrotic lesions, was related to the rapidity and intensity of Sep production. The resistant BPM-24, and marked partially resistant clone PB-235, displayed an early secretion of scopoletin that intensified and lasted longer than the weak partially resistant RRIT251, and susceptible clone RRIM600. The lesion size and amount of Sop after infection were positively correlated to the concentration of spores applied to Hevea leaves. In addition, in leaflets inoculated with high spore concentration, Sep reached the highest level earlier than those with low spore concentration. A fungitoxic effect of Sep on mycelium growth was shown in bioassays; the I-50 value tested on Phytophthora palmivora was relatively the same as on Phytophthora botryosa, but much lower than those found on other leaf pathogens of rubber tree, Corynespora cas-siicola and Colletotrichum gloeosporioides. The lesion size and level of Sep upon spore inoculation may be appropriate for classifying Hevea clones according to their R/S with respect to P. palmivora.