Environmentally induced changes in the distribution of disease susceptibility phenotypes in Pinus radiata

Abstract

AbstractPinus radiata (Monterey pine) is highly susceptible to Fusarium circinatum, the cause of pitch canker, but heritable variation in resistance to this disease has been documented in P. radiata. In this study, the distribution of susceptibility phenotypes (=lesion lengths) was assessed by inoculating P. radiata trees with a spore suspension of F. circinatum. The results show the distribution of susceptibility phenotypes in naïve seedlings to be normally distributed, both in a breeding population and in a population established from seed collected in a native forest. This pattern aligns with expectations for a quantitatively inherited trait. However, after 2.5 years of growth in a forest, variation in susceptibility is no longer normally distributed but rather is positively skewed. Thus, exposure to the forest environment results in a distribution that is enriched for relatively resistant individuals. A similarly skewed distribution is observed in standing trees in a native population of P. radiata. The observed change in distribution cannot be attributed to ontological effects on resistance to pitch canker, because tree age accounts for almost none of the variation in susceptibility. We hypothesize that the change in distribution of susceptibility phenotypes is due primarily to infections and infection attempts by the microbiota resident in the forest.