Chemical and structural barriers to Microcyclus ulei, the agent of South American leaf blight, in Hevea spp.

Young leaves of different rubber-tree clones were inoculated with Microcyclus ulei under controlled conditions. Under ultraviolet light, intense blue fluorescent light Nas emitted from the penetration site 6 h after inoculation. The speed of the appearance of this fluorescence was related to the degree of resistance of the clones: 12-36 h in totally resistant clones, 36-120 h in marked partially resistant clones, and 120 h in weak partially resistant clones. The source of this fluorescence may have been scopoletin. Strong accumulation of this coumarin may inhibit pathogen invasion in a very early phase. Four days after inoculation, intense accumulation of lignins in only a few cells ar the penetration sires was observed in totally resistant clones. Incense lignin accumulation around lesions may also stop pathogen development in some partially resistant clones.