Investigations on pathogenicity, invasion biology and population dynamics of the Pine Wood Nematode Bursaphelenchus xylophilus (Steiner und Buhrer 1934) Nickle 1970 in European conifers

Abstract

The objectives of the present study were to identify potential European host trees for the Pine Wood Nematode (PWN) Bursaphelenchus xylophilus and to investigate the interrelationship between PWN, host trees and temperature. Inoculation experiments with a life stage mixture of 2400 to 4000 B. xylophilus per sapling were conducted with three to four year old saplings in greenhouse and climate chambers adjusted at 25 °C. To evaluate the development of Pine Wilt Disease (PWD), symptoms were assessed in six wilt classes  and water content of wood and needles were recorded. Nematodes were extracted from shoots and roots using a modified Baermann funnel technique. To determine the effect of B. xylophilus on mortality of Pinus sylvestris, nematodes were inoculated in 140 saplings with seven densities: 100, 300, 800, 2400, 4000, 6000, 10000. Nematodes were extracted four and twelve weeks after inoculation or at plant death if that occurred before end of experiment. Inoculum densities had no influence on mortality rate which was in general high (70-90 %). Higher population densities in saplings after four weeks were related to higher inoculum densities. After screening 13 conifer species, Pinus sylvestris, Pinus cembra, Pinus nigra, Pinus strobus, Pinus pinaster, Pinus radiata, Pinus mugo and Larix decidua could be identified as potential sensitive and susceptible hosts for B. xylophilus isolates from Portugal, China and North America. Larix kaempferii was moderately susceptible and Pinus pinea was only susceptible against the isolate from Portugal. Single plants of the species Picea abies and Pinus halepensis contained a little number of B. xylophilus four weeks after inoculation but B. xylophilus could not be isolated from dead plants. Abies alba was no hosts for B. xylophilus. Migration and population dynamics of PWN was studied on Pinus sylvestris saplings. Plants were divided into 17 segments and nematodes were extracted from each segment at nine sampling dates within a 27 day period after inoculation. The nematode population density per sapling reached three population peaks, 12, 19 and 27 days after inoculation. PWN could be isolated from all 17 segments six days after inoculation. Results indicate that B. xylophilus first migrate rapidly throughout the host before building up a high population level. Nematodes were found to develop several overlapping populations in time depending on the area of the sapling. Four consecutive stages of nematode invasion were observed: (1) Early migration, (2) Distribution and colonisation of all plant parts, (3) Population build up and (4) Retreat into the rootsystem. The effect of temperature on population dynamics of PWN and pathogenicity towards Pinus sylvestris, Larix decidua and Picea abies was studied by inoculation of sapling with 4800 nematodes. Experiments were carried out in climate chambers at 15°C, 20°C and 25°C. Nematodes were extracted from shoots and roots at seven sampling dates during a 61 day period. Temperature had a major effect on the population dynamics of B. xylophilus in both susceptible conifer species. Temperature had no influence on the pathogenicity of PWN at 20 °C and 25 °C, as maximum mortality in Pinus sylvestris and Larix decidua was reached when temperature exceeded 20°C. However no wilt symptoms were detected at 15°C in any conifer species. The population in Pinus sylvestris increased to approximately 4000 nematodes per gram dry matter in shoots at 25°C. At 20°C the maximum population density in shoots was approximately 2500 nematodes per gram dry matter. A threshold population density of B. xylophilus must be reached for induction of irreversible wilt in Pinus  sylvestris.