Influence of temperature on pine wilt disease progression in Pinus thunbergii seedlings

Pine wilt disease (PWD) affects forests globally and is caused by the pinewood nematode (PWN) Bursaphelenchus xylophilus. It has been proposed that PWD development has a strong association with air temperature. Because symptom development is related to the number of PWNs and they are poikilothermic animals whose development and multiplication are affected by temperature; it appears that temperature conditions affect the PWN multiplication rate, which leads to difference in disease progression. However, limited information is available about the temperatures inside pine trees, and the understanding of how temperature affects PWN multiplication and PWD progression remains incomplete. Therefore, in the present study, we evaluated the temperature, number of PWNs, and external symptoms of PWD in Pinus thunbergii seedlings infected with PWNs at air temperatures of 20 °C, 25 °C, and 30 °C over time. We found that the seedling temperatures were slightly higher than the air temperature under each temperature condition and that the effective accumulated temperature calculated using a base temperature of 10 °C was related to the number of PWNs and symptom development in all temperature test groups. However, at 20 °C, there were very few PWNs in some seedlings and none of the seedlings showed partial or full wilting. These findings suggest that PWD progression is greatly affected by ambient temperature and facilitated by increase in PWN populations with effective accumulated temperature especially in the range of 25 °C–30 °C.