Resistance by Corsican Pine to Attack by Heterobasidion annosum

Heterobasidion annosum infections in Corsican pine are accompaniedby extensive resinosis; the resinous materials are derived fromoleoresin, and consist mainly of resin acids. In sealed agarcultures, volatile components of these materials reduce fungalgrowth. Resin acids are also fungitoxic, but oleoresin had noeffect on growth in culture, contrary to earlier reports. Thislack of toxicity was confirmed in long-term decay experiments:it appears that resin acids are unavailable to the fungus intheir naturally-occurring form. Resinosis appears to offer amainly mechanical inhibition to fungal hyphae. Three types of naturally-occurring infection are found in Corsicanpine roots. Two of these contain pinosylvins (pinosylvin andpinosylvin monomethyl ether) concentrated in a reaction zone.Pinosylvins were detected in inoculated roots after 1 week,and thereafter the concentration increased slowly. After 20months the amount of pinosylvins found in inoculated roots haddeclined, suggesting destruction by the fungus. A significantcorrelation was shown between the extent of infection and pinosylvinconcentration, and it is suggested that the pinosylvins arethe most important factor determining whether an infection progressesor becomes stabilized.