Distance effect of co-occurring tree species on pine wilt disease incidence in Pinus densiflora seedlings inoculated with Bursaphelenchus xylophilus

Field-grown Pinus densiflora seedlings were inoculated with Bursaphelenchus 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 with Alnus sieboldiana, Sarothamus scoparius or naturally associated species. The disease incidence was 60.9% in a 0-40 cm distance class from the nearest A. sieboldiana seedlings and then decreased with increasing minimum distance. The mean minimum distance between pine seedlings and A. 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 nearest S. 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 nor so short between pine seedlings and other tree species in the two stands mixed with S. scoparius and the naturally associated species. The distance effect on disease incidence was noticeable in A. sieboldiana and P. densiflora at high density. This was not so clear in S. scoparius and was not found in the naturally associated species.