Relationship between pine wilt-tolerance rankings of Pinus thunbergii trees and the number of Bursaphelenchus xylophilus passing through branch sections

Abstract

To establish a simple, time-saving method for selecting Pinus tree candidates tolerant to the infection of Bursaphelenchus xylophilus, the causative agent of pine wilt disease, three experiments were conducted. When two virulent isolates Sc-9 and T-4 were inoculated on cut ends of branch sections, more nematodes were inhibited from passing through P. taeda branch sections than P. thunbergii sections. Sc-9 tended to pass through P. thunbergii sections more easily than T-4. The probability of nematodes passing through 5-cm-long P. thunbergii branch sections was greater when 200 nematodes were inoculated. When 200 Sc-9 nematodes were inoculated on 5-, 10- or 15-cm-long branch sections of P. taeda trees and P. thunbergii clones of different tolerance classes in late August, an insignificant correlation was found between the tolerance class of P. thunbergii clones and the number of nematodes passing through 5-cm-long branch sections, although nematode dispersal ability could distinguish between tolerant and susceptible pine clones in mid-August, suggesting it was a helpful method for selecting tolerant tree candidates. Abrupt seasonal increases in tolerance level suggested that weather conditions, such as hot temperatures and drought, may have disrupted the tolerance mechanism of resistant species of P. taeda.