Resistance by Corsican Pine to Attack by Heterobasidion annosum

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Abstract
Heterobasidion annosum infections in Corsican pine are accompanied by extensive resinosis; the resinous materials are derived from oleoresin, and consist mainly of resin acids. In sealed agar cultures, volatile components of these materials reduce fungal growth. Resin acids are also fungitoxic, but oleoresin had no effect on growth in culture, contrary to earlier reports. This lack of toxicity was confirmed in long-term decay experiments: it appears that resin acids are unavailable to the fungus in their naturally-occurring form. Resinosis appears to offer a mainly mechanical inhibition to fungal hyphae. Three types of naturally-occurring infection are found in Corsican pine roots. Two of these contain pinosylvins (pinosylvin and pinosylvin monomethyl ether) concentrated in a reaction zone. Pinosylvins were detected in inoculated roots after 1 week, and thereafter the concentration increased slowly. After 20 months the amount of pinosylvins found in inoculated roots had declined, suggesting destruction by the fungus. A significant correlation was shown between the extent of infection and pinosylvin concentration, and it is suggested that the pinosylvins are the most important factor determining whether an infection progresses or becomes stabilized.