Differential Infection and Colonisation of Needle Age Classes by <scp> <i>Cronartium ribicola</i> </scp>

ABSTRACT White pine blister rust (WPBR) is a disease on North American five‐needle white pine trees caused by the non‐native fungal pathogen Cronartium ribicola that is causing widespread decline and mortality of Pinus flexilis (limber pine) in high elevation western forests. Elucidating the infection process is important for developing solutions for managing the disease. As an obligate biotroph, C. ribicola infects pine needles through stomatal pores and parasitizes needle tissue before growing into the stem, causing stem cankers. Pinus flexilis retains 8–9 years of needles on shoots, and it remains unclear whether infection and fungal colonisation, as well as the quantities of epicuticular wax, differ between needles of various age classes or between trees with and without major gene resistance to WPBR. To assess the relationship between quantity of C. ribicola DNA in P. flexilis needles, needle age, and resistance status, P. flexilis shoots from mature resistant and susceptible field trees were artificially inoculated with C. ribicola , where dispersal of fungal inoculum was applied evenly across all needle age classes. The quantity of epicuticular wax by needle age class and tree resistance status was explored. The results demonstrate that all needle age classes of P. flexilis are susceptible to C. ribicola infection. A significant positive correlation between needle age class and C. ribicola DNA quantity, regardless of tree resistance status, was noted. Additionally, epicuticular wax weight increased significantly with needle age, and wax did not appear to hinder needle colonisation by C. ribicola .