Foliage susceptibility of six eastern Canadian forest tree species to Phytophthora ramorum

Abstract

To help assess the risk of establishment of Phytophthora ramorum in eastern Canada, detached leaves/needles of six eastern native forest species were inoculated with P. ramorum and the amount of necrosis and sporulation was determined. Inoculation was also performed by plant dipping. Balsam fir (Abies balsamea), sugar maple (Acer saccharum), yellow birch (Betula alleghaniensis), white ash (Fraxinus americana), tamarack (Larix laricina) and red oak (Quercus rubra) were the species tested. Rhododendron ‘Nova Zembla’, which is putatively sensitive to the pathogen, was also included in the assays. The degree of necrosis on detached leaves was particularly high on F. americana and B. alleghaniensis. This susceptibility was also confirmed after wounding in the plant-dip experiment in which Q. rubra was also susceptible. For the coniferous species, needle necrosis was higher on A. balsamea than on L. laricina. Reisolation assays and PCR detection of P. ramorum were generally positive when necrosis occurred, except on conifer needles and especially after plant-dipping. Sporulation was particularly intense after plant-dip assays but was barely noticeable on detached leaves/needles. Sporulation was higher on leaves of F. americana and B. alleghaniensis than on leaves of A. saccharum and Q. rubra, while needles of A. balsamea supported more sporulation than needles of L. laricina. Sporulation was even observed on some asymptomatic leaves and needles. During conducive climatic conditions, F. americana, B. alleghaniensis and A. balsamea could be infected in eastern Canada by P. ramorum and sporangia could be produced on their foliage if the pathogen were introduced into eastern Canada.