Effect of Watering Regime on Disease Development in Pinus sylvestris Seedlings Inoculated with Bursaphelenchus vallesianus and B. mucronatus.

Several Bursaphelenchus spp. have been detected in declining pine trees in Europe during intensive monitoring for the pine wood nematode B. xylophilus. We investigated the pathogenicity of B. vallesianus and B. mucronatus, isolated from declining Scots pine (Pinus sylvestris) forests in Valais (Switzerland), in relation to drought stress. Four isolates of B. vallesianus and two isolates of B. mucronatus were inoculated into 3-year-old P. sylvestris trees, which were subjected to different watering treatments (50, 100, 150, and 250 ml of water per pot, biweekly). Disease symptoms, plant mortality, nematode population density, and nematode distribution in dead plants were assessed. Both Bursaphelenchus spp. proved highly pathogenic to the seedlings and watering treatment affected disease development in the inoculated pine trees. With decreasing water supply, we observed faster disease progress and higher pine mortality for both Bursaphelenchus spp. The overall mortality 70 days after inoculation was 60, 92, 95, and 100% for B. vallesianus and 40, 95, 100, and 100% for B. mucronatus in the 250-, 150-, 100-, and 50-ml watering treatments, respectively. Both nematode species multiplied in the inoculated plants; however, B. mucronatus had higher population densities than B. vallesianus in all watering treatments (on average, 33,159 versus 14,702 nematodes/dead plant compared with the initial inoculum density of 6,000 nematodes/plant). The highest nematode density was found in the lower part of the stem. About 7 to 16% of the nematodes were extracted from the roots. This study demonstrated that B. vallesianus has a pathogenicity potential comparable with that of B. mucronatus and provided experimental evidence that drought stress can result in increased symptoms caused by either Bursaphelenchus sp.