Differences in the virulence of Sphaeropsis sapinea strains originating from Scots pine and non‐pine hosts

The virulence of 15 endophytic and pathogenic Sphaeropsis sapinea strains was tested towards Scots pine ( Pinus sylvestris ). Two‐thirds of the strains had been isolated from Scots pines with varying health status: five isolates originated from healthy tissue (endophytic fungal stage) and five from diseased tissue (pathogenic stage). One‐third of the strains were isolated from symptomatic tissues of non‐pine hosts: black alder ( Alnus glutinosa ), European beech ( Fagus sylvatica ), European larch ( Larix decidua ), Norway spruce ( Picea abies ) and Douglas fir ( Pseudotsuga menziesii ). The S .  sapinea strain isolated from black alder is the first proof that this fungus can form associations with alder species. On four‐year‐old P .  sylvestris , one isolate per plant was inoculated on three side‐shoots of seven plants in a greenhouse (21 inoculations/strain). Differences in necrosis size caused by the isolates were measured 55 days after inoculation. The pathogenic S .  sapinea isolates originating from diseased Scots pine and from non‐pine hosts were found to cause significantly longer necroses when compared to the endophytic isolates of S .  sapinea from symptomless pines.