Inter- and intraspecific variation in susceptibility to dothistroma needle blight in Britain. How susceptible are Pinus sylvestris and Pinus contorta?

Dothistroma needle blight (DNB), caused by Dothistroma septosporum, is a serious disease in British pine plantations. Natural infection (NI) and artificial inoculation (AI) experiments were conducted to evaluate the susceptibility of Pinus sylvestris and Pinus contorta to DNB, relative to Pinus mugo, Pinus muricata, Pinus nigra ssp. laricio, Pinus nigra ssp. nigra and Pinus ponderosa. A range of P. sylvestris and P. contorta provenances were included in experiments to simultaneously investigate intraspecific variation within these host species. In NI experiments, 20-30 12-month-old seedlings of each species/provenance were planted adjacent to infected mature trees at two Scottish sites. In the AI experiment, eighteen 16-month-old seedlings of each species/provenance were inoculated with a D. septosporum conidial suspension and maintained under optimum conditions for D. septosporum infection and development. In the NI experiments, the most DNB-susceptible species were P. muricata and P. ponderosa, while the least susceptible species were P. contorta and P. sylvestris. Pinus nigra spp. nigra and P. nigra spp. laricio had intermediate DNB susceptibility. In the AI experiment, there was no evidence for interspecific variation in DNB susceptibility. Intraspecific variation in DNB susceptibility within P. sylvestris was seen in the NI experiment, but not the AI experiment. Intraspecific variation in DNB susceptibility within P. contorta was not observed in either experiment. In a further AI experiment with 24-month-old P. sylvestris and P. contorta seedlings, the virulence of three Scottish D. septosporum isolates (haplotypes) was examined. There was no significant interaction between isolate and relative susceptibility of pine species/provenances. Furthermore, no between-isolate variation in virulence was observed.