How different is induced host resistance against the pine wood nematode, Bursaphelenchus xylophilus, by two avirulent microbes?

Abstract

AbstractPine wilt disease, caused by the pine wood nematode (PWN), Bursaphelenchus xylophilus, has been devastating pine forests in Japan for a century but an effective method for controlling the disease is still needed. Pre-inoculation of the host plant with an avirulent pathogen can induce resistance against subsequent infection with virulent pathogens; this is called 'induced resistance'. Host pine trees have this type of resistance against pine wilt disease but the detailed mechanism is unknown. In this study, 1-year-old potted seedlings of the susceptible Japanese black pine, Pinus thunbergii, were pre-inoculated with an avirulent isolate of PWN (C14-5), as well as an avirulent fungus, Botrytis cinerea, to induce host resistance against PWN. One, 2, 3, 4 and 5 weeks after inoculation, the seedlings were challenged with a virulent isolate of PWN (S10), and the survival of these seedlings was estimated by their external symptoms. Pre-inoculation with either C14-5 or Botrytis delayed the symptom development caused by subsequent inoculation with S10, suggesting that pre-inoculation induced host resistance against PWN. The resistance induced by pre-inoculation with C14-5 lasted for at least 5 weeks, while that induced by Botrytis weakened with time. It has been reported that the PWN survives inside the host seedlings for a long period without causing any symptoms under certain conditions, but Botrytis survives only for a while in the seedlings. Thus, the duration of host resistance induced by pre-inoculation may vary with the viability of the inoculum used for pre-inoculation. In order to obtain durable induced resistance, it may be necessary to repeat inoculations of avirulent microbes to be able to control pine wilt disease.