Exploring variation in susceptibility to Phytophthora ramorum in Japanese larch (Larix kaempferi)

Phytophthora ramorum is an invasive pathogen responsible for extensive mortality in larches in the United Kingdom. There is great interest from the forestry industry in the possibility of selection for resistance to P. ramorum in Japanese larch in order to retain it as a commercially viable species. This study is the first to investigate variation in resistance to P. ramorum among planted populations of Japanese larch in the UK. Our study uses inoculation of excised material from putatively resistant survivor trees and known susceptible trees. We found variation in susceptibility to P. ramorum within Japanese larch stands planted in the Galloway forest of Scotland with some trees showing significantly shorter lesion development than others, from a mean lesion length of 34.7 mm in the least susceptible clone to 135 mm in the most susceptible. Although clones from the putatively resistant and known susceptible groups were not significantly different (p =.055), we propose that survivor trees include a higher proportion of resistant or low susceptibility trees and would be a useful starting point for further work investigating natural resistance in larch.