Experimental verification of induced resistance after pre-inoculation with avirulent pine wood nematode isolates with consideration of genetic factors in Japanese black pine

We tested for induced resistance, a phenomenon by which pine trees inhibit symptoms of pine wilt disease after pre-inoculation with avirulent isolates from pine wood nematodes (Bursaphelenchus xylophilus), in Japanese black pine (Pinus thunbergii). One-year-old seedlings from five families were pre-inoculated with avirulent isolates (OKD-1 or C14-5) or sterilized distilled water (control), and then post-inoculated with virulent isolates (Ka-4 or Shimabara) after the prescribed incubation period following pre-inoculation. Seventeen weeks after post-inoculation with virulent isolates, morbidity and mortality rates significantly depended on seedling family, pine wood nematode isolate used for post-inoculation, and seedling height, but not on pre-inoculation with avirulent isolates. In experiments using 2-to 3-year-old clonal ramets, which have uniform genetic composition, there was no significant difference in symptom development between individuals pre-inoculated with avirulent OKD-1 and controls. These results suggest that induced resistance is not a general phenomenon in pine trees pre-inoculated with avirulent isolates.