Rapidity of Disease Development Seems to Result in High Mortality – Insight from an Inoculation Test Using Hybridized Populations Between a Virulent and an Avirulent Isolates of Bursaphelenchus xylophilus

Abstract

The mortality of pine trees caused by the pinewood nematode (PWN), Bursaphelenchus xylophilus, still remains insufficiently characterized. One of the reasons is that only a few isolates have been used in previous comparative studies that attempted to describe biological characteristics of the PWN in relation to its virulence. In this study we prepared 27 hybridized populations with a variety of virulence by mixing 2 isolates, S10 (virulent) and C14-5 (avirulent), at various proportions (S10 proportion in each population was 100, 99, 90, 70, 50, 30, 10, 1 or 0%) to characterize the pattern of disease progress over seedling populations. One- and two-year-old seedlings of Japanese black pine served the inoculation test. The numbers of the dead seedlings were recorded every 2 days for 45 days after inoculation. Using these data, we calculated 3 indices, rate of mortality increase, tolerance limit and mortality durability of the seedlings. The rate of mortality increase reflects rapidity of the disease progress and the tolerance limit reflects critical value of load necessary to kill a seedling. There was no correlation between the tolerance limit and the eventual mortality of the seedlings. This may indicate that the tolerance limit reflects physiological conditions of host rather than characteristics of inocula. The eventual mortality correlated closely with the rate of mortality increase. We also found that seedling death was rather durable when the rate of mortality increase was low. On the basis of these analyses, we concluded that the mortality caused by the PWN would eventually be higher when disease progress in a seedling population is faster.