Evidence of resistance to the pitch canker pathogen (Fusarium circinatum) in native stands of monterey pine (Pinus radiata)

Resistance of Monterey pine, Pinus radiata, to pitch canker, caused by Fusa rium circinatum, was investigated in a naturally regenerated native stand a nd a planted stand on the Monterey Peninsula, California. A dose of 125 spo res was selected to test resistance as it produced significant differences in mean lesion length among trees. The resistance ranking of trees remained comparable when inoculated three times over a period of seven months, but the mean lesion length decreased with each inoculation. Lesion lengths were positively correlated with symptom development. In the naturally regenerat ed stand, 30% of the trees formed lesions less than 5 mm long and were cons idered resistant to the pathogen. In contrast, only 5% of trees in the plan ted stand formed lesions less than 5 mm long. The use of prescribed burning of Monterey pine stands to augment natural regeneration allows a wider exp ression of genetic diversity in the new host populations on which natural s election in favor of resistant genotypes can be imposed by the pathogen.