Abstract:

Objective: This study aims to provide a theoretical basis for the effective elimination of asymptomatic carrier pines by investigating the pathological process and causes of over-year death. Method: The proportion of over-year death with naturally infected Pinus koraiensis in Dahuofang Experimental Forest Farm in Fushun City was calculated. The symptoms of naturally infected and artificially inoculated P. koraiensis were observed. The number threshold of Bursaphelenchus xylophilus (pine wood nematodes, PWNs) in P. koraiensis with symptoms in the infected year was determined. The growth of the PWN population at different temperatures in the laboratory was measured. The period suitable for the effective growth of PWN population after the emergence of Monochamus saltuarius was determined, that is, population growth period. The time from the establishment of parasitic relationship to the onset of symptoms, namely incubation period, was calculated. The cause of over-year death of P. koraiensis infected by PWNs was revealed. Result: As of June, the over-year death proportion of naturally infected P. koraiensis was 41.4%. The naturally infected and low amount artificially inoculated P. koraiensis did not show significant changes in external and internal symptoms in the year of infection or inoculation, but when the secretion of resin stopped in the following year, the trunk moisture was low, and the needles turned reddish brown. Half of the P. koraiensis inoculated with high saturated dose PWNs showed the symptoms in that year. The symptoms indicated that the PWNs were able to infect the P. koraiensis normally, and reproduce effectively, but their reproduction was hindered at low temperatures. PWNs were successfully able to overwinter but the population decreased sharply after overwintering. The number threshold of PWNs in whole plant, which was able to cause P. koraiensis to show the symptoms, was St₁=163 ± 22 nematodes·g^?1 and the number threshold in local part of a plant was St₂=621 ± 20 nematodes·g^?1. The 38-year-old P. koraiensis was inoculated with 50 000 PWNs per tree. During the population growth period of the year, the number of PWNs in the whole plant was only 90 ± 8 nematodes·g^?1, which was less than that of St₁, and the maximum number of PWNs in local part of a plant was 350 ± 18 nematodes·g^?1, which was also less than that of St₂. The population growth period in Fushun (141 days) was shorter than that in Dalian (154 days) and Nanjing (173 days). According to calculation, the incubation period of PWNs in Fushun City was 146 days, which was longer than the population growth period. Conclusion: The small number of PWNs carried by M. saltuarius leads to a low initial infection. Additionally, the life cycle of the PWNs is affected by the temperature, manifested as a long non-growth period and short population growth period, which ultimately leads to the fact that the number of PWNs does not reach the threshold for exhibiting the symptoms on P. koraiensis during the population growth period. The above comprehensive effects eventually lead to the extension of the incubation period of PWD, and 41.4% of the dead P. koraiensis being over-year death.

Key words: Bursaphelenchus xylophilus, over-year death, initial infection amount, low temperature, population growth period, population growth