Relationship between resistance to pine wilt disease and the migration or proliferation of pine wood nematodes

Abstract

To study the relationship between resistance to pine wilt disease and the migration or proliferation of pine wood nematodes (PWN) (Bursaphelenchus xylophilus), we conducted experiments using clonallypropagated Japanese black pines (Pinus thunbergii) with pre-evaluated individual resistance levels. Bark including the cortical resin canals-one of the main migration pathways of PWN-was removed by girdling, but neither the migration of PWN nor symptom development of pine wilt disease were inhibited by this treatment in non-resistant clones. Histological observations showed no significant differences in the lumen area or the number of cortical-and xylem-axial resin canals between resistant and susceptible clone groups from a half-sib family. A bioassay using methanol extracts from resistant and susceptible clones showed that extracts from both clones showed similar attractant effects to PWN, but neither had repellent effects. The resistant clones were multi-inoculated with PWN into three split points to mimic migration in the stem. The proportion of damaged plants was not significantly different from that in single-inoculated plants (control). In this experiment, the number of PWN detected from partially-damaged plants was much higher than that from non-damaged plants. An inoculation test using stem cuttings showed that the population of PWN increased in susceptible cuttings at 1-20 days after inoculation (dai), while it remained unchanged or gradually decreased in resistant cuttings. These findings suggested that the factors contributing to resistance were associated with inhibiting the proliferation of PWN, rather than inhibiting their migration.

Histological Changes in Stems of Pinus sylvestris Seedlings Infected with a Virulent Isolate of the Pinewood Nematode Bursaphelenchus xylophilus

“…Similar histological features were reported for the Asiatic susceptible pine species (e.g. P. thunbergii, P. densiflora) [2,3,4]. The wilting process of PWD has been explain, at least in part, by water deficit induced by nematodes pathological cavitation.…”

A rapid in vitro bioassay system for testing resistance factors of pine trees to Bursaphelenchus xylophilus

“…In a previous experiment using stem cuttings (Mori et al, 2008), the difference in the number of PWN between 1200 I 900 'S 600 , Bursaphelenchus xylophilus, detected in the bioassay that were co-cultured with methanol extract-loaded paper disks. Numbers of PWN were counted 3 days after inoculation.…”

“…A common observation among these studies was that significantly fewer PWN were detected in resistant pines than in susceptible ones, suggesting that resistance factors were involved in inhibiting migration or proliferation of PWN. In particular, we demonstrated that symptom development in pine trees was unaffected by inhibiting or facilitating PWN migration, and that the PWN population drastically increased in susceptible pine stem cuttings but not in resistant ones (Mori et al, 2008). We concluded that it was likely that resistance was dependent on factors that inhibited PWN proliferation rather than their migration.…”

Metabolites from Resistant and Susceptible <i>Pinus thunbergii</i> after Inoculation with Pine Wood Nematode

“…This hypothesis was based on crosssectional observations of the main migration pathways, i.e., cortical resin canals [21]. However, our previous study demonstrated that symptom development in pine trees was unaffected by mimicking inhibited or free PWN migration [22]. This suggested that resistance was independent of PWN migration.…”

“…This suggested that resistance was independent of PWN migration. The PWN population drastically increased in susceptible pine stem cuttings by 20 days after inoculation (dai), while it remained unchanged or gradually decreased in resistant cuttings [22]. Thus, it is likely that resistance depends on factors that inhibit the proliferation of PWN, rather than migration.…”