Natural and artificial inoculation of radiata pine boles and seedlings with Phytophthora ramorum, causal agent of sudden oak death, reveals low host potential

Abstract

AbstractRadiata pine is an important plantation tree; more than 4 million ha are planted globally, and 90% of planted forests in Aotearoa New Zealand are radiata pine. It is susceptible to several root and foliar diseases caused by Phytophthora species, and is potentially susceptible to Phytophthora ramorum, the pathogen responsible for sudden oak death in the United States and Europe. A series of experiments were conducted to determine the potential risk of P. ramorum infection to radiata pine logs and seedlings in the context of forest harvest and replanting. In a natural inoculation experiment, bolts of radiata pine, Douglas‐fir and tanoak were exposed to P. ramorum inoculum produced from sporulating tanoak canopies infected with either the NA1 or EU1 lineage of P. ramorum. The experiment occurred at four sites, two NA1 and two EU1, across 2 years. Four per cent of radiata pine bolts, 8% of tanoak bolts and 0% Douglas‐fir bolts tested positive for P. ramorum. Artificial inoculations of bolts of the same species revealed a significant effect of lineage (p = .0024), species (p < .0001) and their interaction (p = .0027) on lesion length. Species was the only parameter that had a significant impact on average lesion length (p < .0001) and sporulation (p = .00144) from seedlings. No sporangia were observed on radiata pine seedlings (n = 60); few were observed on tanoak and Douglas‐fir. Although radiata pine can be colonized by P. ramorum, without a sporulating host in proximity it does not appear to be at high risk from this pathogen.