Although the pine wood nematode (Bursaphelenchus xylophilus, PWN) seriously damages the physiology of its host pine trees, as described in Chaps. 21 and 22, it does not always kill host trees even when it successfully invades them. The main reasons for this are that both the virulence of the PWN and the susceptibility of pine trees to the nematode vary. Consequently, whether or not the disease progresses largely depends upon the host-pathogen combination (see Sect. 23.2). Furthermore, host fate after infection by the PWN can also be affected by other biotic or abiotic conditions, or both, such as precipitation and temperature plus other factors including the number of nematodes in the inoculum, and the season when PWN infection occurs (reviewed by Kishi 1995).