Infectivity, survival and pathology of Finnish strains of Phytophthora plurivora and Ph. pini in Norway spruce

The recently described Phytophthora plurivora T. Jung & T.I. Burgess and its close relative Phytophthora pini L.H. Leonian are new arrivals in Finnish horticulture. The objective of this study was to determine whether these species pose a threat to the seedling production of Norway spruce (Picea abies L.), one of the most important tree species in Fennoscandia. Seedlings of P. abies were inoculated on a needle scar with mycelium or on healthy tissue with a suspension of zoospores. All of the Ph. plurivora and Ph. pini isolates produced 100% incidence of disease when needle scars were inoculated with mycelium. Results show that nonwounded shoots of P. abies are also susceptible to infection via zoospores. In this case, 32% of seedlings became symptomatic and signs of wilting developed 4 days post-inoculation (dpi). Histology revealed that zoospore infections of Ph. plurivora led to colonization of almost all shoot tissues including the vascular system and pith parenchyma in less than 4 days. Necrotrophic growth was observed but no latent infection was found. Finnish isolates of Ph. plurivora and Ph. pini had higher growth temperature optima than the other Phytophthora species found in Finland, i.e. Ph. ramorum De Cock & Man in’t Veld and Ph. cactorum Lebert & Cohn. All species survived 14 days at -5 °C.