Histological Observations on the Responses of Pine Species, Pinus strobus and P. taeda, Resistant to Bursaphelenchus xylophilus Infection.

Abstract

Bursaphelenchus xylophilus was inoculated into the branches of wilt-resistant Pinus taeda and P. strobus, and wilt-susceptible P. thunbergii. The histology of inoculated pine branches was investigated. Nematode numbers increased in whole inoculated branches of P. thunbergii. Nematode numbers decreased in those of resistant species, except in proximity to the inoculation site of P. taeda. Histological changes were delayed in the cortex and phloem tissue and cambium of P. thunbergii inoculated branches. In resistant species, necrosis and destruction of cortex and phloem tissue and cambium (P. taeda) or occlusion of cortical resin canal (P. strobus) occurred near the inoculation site rapidly after nematode inoculation. Wound periderm was formed within 3 weeks and 5 weeks after inoculation in P. taeda and P. strobus, respectively. Wound periderm formation was observed only around the cortical resin canals in P. thunbergii. Necrosis of xylem parenchyma was conspicuous in P. thunbergii, but not in resistant species. These results suggest that responses of resistant species, wound periderm formation and occlusion of cortical resin canal, trapped the nematode within damaged tissue. It is also suggested that insensitivity of xylem parenchyma to nematode infection acts as a defensive factor.