| A devastating international forestry quarantine pest,the wood-wasp,Sirex noctilio Fabricius(Hymenoptera:Siricidae)is native to Europe and North Africa.It was first discovered to enter and colonize in Northeast region of China in 2013.It has severely damaged Pinus sylvestris var.mongolica forest stands and caused great loss.Sirex noctilio has a wide host range all over the world and especially for Pinus species.Different from majority of the wood borers which damage trees by boring into the wood,constituting with its fungal mutualist-Amylostereum areolatum(Fr.)Boidin,S.noctilio has a special damaging system with multilayered biological interactions.A.areolatum is essential and indispensable for wood-wasp larvae which feed on the symbiotic fungi.However,the growth rate of A.areolatum is low and the ability of occupying the resources and niche is weaker than many fungi present in pine ecosystems,some endophytic fungi can even destroy the mutual symbiotic relationship between S.noctilio and A.areolatum in host trees.Since S.noctilio's invading in China,the resistance mechanism of the host tree species to the invasion and colonization of S.noctilio have not been clear.In this paper,endophytic fungal communities,which from the host tree species and potential host species of S.noctilio were taken as the research object.The mechanism of endophytic fungi affects S.noctilio and its symbiont A.areolatum were discussed from the community level and species level.Main conclusions are as follow:1.In total,37 species of endophytic fungi belonging to 25 genera were isolated from different health levels(healthy,weak and dead),and different heights(upper,central and base)of the trunk of Pinus sylvestris var.mongolica.Among them,9 genera and 13 species were isolated from the healthy trunks;14 genera and 23 species were isolated from the weak trunks;8 genera and 11 species were isolated from the dead trunks.The number of endophytic fungal species decreased progressively in order of weakened>healthy>dead trunk;the isolation rate of endophytic fungi increased progressively with the increase of tree weakness.At different stem heights,the species of endophytic fungal was higher in the base part than that of the central and upper part;the isolation rate of the fungi was higher in the upper part than that of the central and base part.The similarity of endophytic fungi was lower among the different health levels.A.arreolatum was only isolated in the upper part of the weak trunk by S.noctilio attack,with a isolation rate of 4.8%.2.The relationship between the death of S.noctilio larvae in host tree and endophytic fungi,major pathogenic fungi were evaluated in the labortary.A total of 12 species of 9 genera were isolated from the surfaces and tunnel of the dead larvae in the host trees.The most abundant genera were Trichoderma,Ophiostoma,and Sphaeropsis.Except for Phlebiopsis gigantea,the other fungi belonged to host endophytic fungi.The mortality rates of S.noctilio larvae were significantly higher treated with O.minus,T.atroviride,T.viride,P.gigantea,and T.harzianum.No significant difference was noted between the control group and larvae treated with A.areolatum or S.sapinea.The sample logs inoculated with O.minus displayed the highest mortality and the lowest moisture content.Additionally,some endophytic fungi(T.harzianum,T.atroviride,T.viride,P.gigantea)grew faster and completely covered and killed the colonies of A.areolatum mycelia in the dual culture assays.However,O.minus and S.sapinea displayed low antagonistic ability against A.areolatum.The adverse effects of endophytic fungi on the growth of S.noctilio larvae may be caused by an indirect mechanism involving competition with A.areolatum and the association of O.minus reducing host moisture content.3.The effect of dominant endophytic fungi on the growth of A.areolatum were evaluated.The growth rate of eight dominant endophytic fungi was more than twice to six times compared to that of A.areolatum on PDA medium.The antagonistic effects of different endophytic fungi on mycelial growth of A.areolatum were significantly different,four endophytic fungi eventually completely covered A.areolatum,thus inhibiting its growth and death;The other four fungi could form a confrontation with A.areolatum and were non-toxic to each other.The filtrate of Chaetomium globosum completely inhibited the mycelial growth of A.areolatum,the inhibitory rate was 100%.Moreover,the effect of volatiles of endophytic fungi on the selection behavior of woodwasp was carried out in the laboratory.We observed that volatiles of O.minus and Aspergillus niger(acetophenone,acetylacetone,hexadecane,phenylethyl alcohol,and isopropylmyristate)had repellent effects on adult female woodwasp.While volatiles of A.areolatum((-)-globulol,2-hexene,cycloprop[e]indene-1a,2(1 H)-dicarboxaldehyde,terpene and cyclopentanone)had a significant attractiveness to adult female woodwasp.4.The endophytic fungal diversity on four healthy conifers(P.sylvestris var.mongolica,P.koraiensis,Picea koraiensis,Larix gmelinii)in the mixed forest infested by S.noctilio was identified.Totally,35 species of endophytic fungi belonging to 22 genera from our experimental material.Among them,the number of the fungi species was highest in Picea koraiensis(20 species);and those of P.sylvestris var.mongolica and P.koraiensis was least(the same was 13 species).The isolation rate of endophytic fungi was highest in Larix gmelinii(68.27%);and those of P.sylvestris var.mongolica was least(47.2%).The diversity level of endophytic fungi varies among the four species of conifers.The diversity index(H'=3.3425),evenness index(J=1.0979)and richness index(R=3.3744)of Picea koraiensis were the highest among the four species.The similarity of endophytic fungi was lower among the four species.Four species of endophytic fungi(A.niger,F.tricinctum,T.atroviride,T.atroviride)could be isolated from all the four kinds of conifers,accounting for 11.43%of the total species,which illustrates the host-specificity of the conifer-inhabiting fungi.5.The diversities and similarities of endophytic fungal communities from the host tree species and 8 potential host species were significantly different.The diversity of the endophyte communities was influenced by tree species and region.Combining with the results of host selection experiments of S.noctilio,we found that the isolation and colonization rate of endophytic fungi of coniferous tree with more spawning were less,and those of endophytic fungi in P.tabulaeformis were significantly less than other species.There were no significant differences in the isolation and colonization rate between P.sylvestris var.mongolica and the other tree species.In addition,Trichoderma,is highly antagonistic to A.areolatum,has a very few percent in P.tabulaeformis,P.sylvestris var.mongolica,P.yunnanensis and Larix gmelinii,and P.tabulaeformis was only 4.8%among them.While Aspergillus with weak antagonistic ability to A.areolatum occupied a high percentage of P.tabulaeformis,P.sylvestris var.mongolica,P.elliottii,and P.yunnanensis isolates.Compared with other tree species,P.tabulaeformis and P..sylvestris var.mongolica are more conducive to laying eggs and colonization by S.noctilio,hence,P.tabulaeformis might be at a high risk of invasion and colonization by S.noctilio,with gradual spreading to Central and Southern China.This study comprehensively studies the influence of host-tree endophytic fungi on the growth of its fungal symbiont A.areolatum;the growth and development of S.noctilio larvae;the selection behaviors of adult female woodwasp;the comparing of endophytic fungi communities between host tree and potential host tree.Our results would lay the foundation for futher study on the resistance mechanism of host tree species to S.noctilio,providing a key scientific theoretical basis for biological control and risk management of this forest pest. |
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