Distance effect of co-occurring tree species on pine wilt disease incidence inPinus densiflora seedlings inoculated withBursaphelenchus xylophilus

Field-grownPinus densiflora seedlings were inoculated withBursaphelenchus xylophilus and the incidence of pine wilt disease was analyzed with respect to the spatial relationships between pine seedlings and adjacent seedlings of other tree species in a pure pine stand and three stands mixed withAlnus sieboldiana, Sarothamus scoparius or naturally associated species. The disease incidence was 60.9% in a 0–40 cm distance class from the nearestA. sieboldiana seedlings and then decreased with increasing minimum distance. The mean minimum distance between pine seedlings andA. sieboldiana was significantly shorter in diseased seedlings than in healthy ones. The highest disease incidence (53.9%) was found in a 0–40 cm distance class from the nearestS. scoparius, although the relationship with minimum distance was blurred by a high incidence in the 80-cm distance class. The mean minimum distance between pine seedlings was significantly short in diseased seedlings in a pure stand, whereas it was not so short between pine seedlings and other tree species in the two stands mixed withS. scoparius and the naturally associated species. The distance effect on disease incidence was noticeable inA. sieboldiana andP. densiflora at high density. This was not so clear inS. scoparius and was not found in the naturally associated species.