Báo cáo lâm nghiệp: " Scots pine responses to number and density of inoculation points with Leptographium wingfieldii Morelet, a bark beetle-associated fungus"

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Nội dung Text: Báo cáo lâm nghiệp: " Scots pine responses to number and density of inoculation points with Leptographium wingfieldii Morelet, a bark beetle-associated fungus"

Note Scots pine responses to number and density of inoculation points with Leptographium wingfieldii Morelet, a bark beetle-associated fungus François Lieutier Erwin a Luc Croisé a Dreyer a Unité écophysiologie forestière, Inra, Centre de Nancy, 54280 Champenoux, France b Station de zoologie forestière, Inra, Centre d’Orléans, 45160 Ardon, France (Received 8 April 1997; accepted 21 August 1997) Abstract - The effects of different densities and total distribution of inoculation points with Leptographium wingfieldii, a fungus associated with the bark beetle Tomicus piniperda, were inves- tigated in Scots pine (Pinus sylvestris). During April 1993, 40 8-year-old Scots pine trees were inoculated into the trunk and until the cambium, at breast height. Inoculation points were distributed over a circular belt of 20, 40, 70 and 100 cm width, at two densities (200 and 400 m Three ). -2 months after inoculation, the average length of the induced reaction zone was recorded on the exter- nal side of the phloem. The fraction of sapwood section that appeared blue-stained, desiccated or soaked with resin was also measured. The length of the visible, induced reaction zone in the phloem was affected neither by inoculation density nor by width of the inoculation belt. Similarly, the fraction area of damaged sapwood was small and constant at 200 mIt increased significantly . -2 only at the highest density (400 m as a function of belt width, reaching 70 % at 100-cm width. ) -2 It is concluded that: i) the average length of the induced reaction zone in the phloem is insensi- tive to inoculation density, and ii) the importance of damage in the sapwood is increasing only at inoculation densities above 400 m over a sufficiently large area. This result is discussed in -2 relation to the physiological meaning of the different symptoms, and in relation to the concept of threshold of attack density to explain why pine trees are affected only when the density of bark beetle attacks overrides this limit. (© Inra/Elsevier, Paris.) Pinus sylvestris / bark beetle / Leptographium wingfieldii / inoculation density / number of inoculation points / phloem / induced reaction zone / sapwood / blue staining / dry sap- wood / resin-soaked sapwood Résumé - Réponses du pin sylvestre à la densité et au nombre de points d’inoculation réa- lisés avec Leptographium wingfieldii Morelet, un champignon associé aux scolytes. Les effets de la densité d’inoculation avec Leptographium wingfieldii (un champignon associé au scolyte Tomicus piniperda) et de l’augmentation du nombre de points d’inoculation, ont été étudiés sur * Correspondence and reprints E-mail: lieutier@orleans.inra.fr
le pin sylvestre (Pinus sylvestris). En avril 93, quarante pins sylvestres âgés de 8 ans ont été inoculés dans le tronc à 1,30 m. Les inoculations étaient réparties sur des ceintures de 20, 40, 7Q ou 100 cm de largeur, avec deux densités (200 et 400 m Trois mois après inoculation, la lon- ). -2 gueur des zones de réaction induites a été mesurée sur le côté externe du liber. Les fractions de section d’aubier bleui, desséché, ou imprégné de résine ont également été mesurées. La lon- gueur de la zone de réaction visible dans le liber n’a été affectée ni par la densité d’inoculation, ni par la largeur des ceintures d’inoculations. De la même manière, la fraction d’aubier affectée est restée faible et constante à 200 inoculations m En revanche, elle a augmenté de manière très . -2 importante en fonction de la largeur de ceinture d’inoculation pour la densité d’inoculation de 400 m 70 % de surface d’aubier étant affecté pour une ceinture de 100 cm de largeur. Deux ; -2 conclusions sont déduites de ces résultats : i) la longueur de la zone de réaction induite dans le liber ne dépend pas de la densité d’inoculation, et ii) l’importance des dégâts dans l’aubier augmente uniquement pour des densités d’inoculations au dessus de 400 m et des ceintures d’inoculations -2 suffisamment larges. Ce résultat est discuté en fonction de la signification physiologique des différents symptômes, et en fonction du concept de seuil critique de densité d’attaque, pour ten- ter d’expliquer pourquoi la survie des pins est affectée uniquement quand la densité d’attaques de scolytes dépasse cette limite. (© Inra/Elsevier, Paris.) Pinus sylvestris / scolyte / Leptographium wingfieldii / densité d’inoculations / nombre d’inoc- ulations / liber / zone de réaction induite / aubier / bleuissement / dessèchement d’aubier / imprégnation de résine 1. INTRODUCTION When inoculated into the bole of Scots pine (Pinus sylvestris), the fungus Lep- tographium wingfieldii, associated with Bark beetles generally induce signifi- the bark beetle Tomicus piniperda, is able damages only when their attacks cant to induce important reaction zones in occur at a rather high density on a single which large accumulations of monoter- tree. This fact led Berryman [2] to define penes [10], resinic acids [19] and phenols a threshold of bark beetle attack density [22, 23] occur. The effects of massive above which trees are severely affected inoculation with this fungus have been and may die. Fungi associated with bark investigated in Scots pine by Solheim et al. beetles may mimic this behaviour when [30], who found that blue staining inoculated directly into the trunk [4, 17, occurred in the sapwood only above a 27, 30]. In the phloem, they induce an threshold of 400-800 inoculation points elliptical reaction zone surrounding each . -2 m In the same experiment, L. wingfieldii point of attack; heavy accumulation of killed vigorous Scots pines when inocu- resin and secondary metabolites occurs in lated at a density of 800 points mover a -2 this zone that gradually turns into a 60-cm wide band, while severely pruned necrotic zone within which aggressors are trees were killed by an inoculation den- confined [2, 6, 21, 27]. In the sapwood, sity of 400 m In addition to the impor- . -2 fungi may promote blue staining and tissue tance of attack density, it has been sug- drying [4, 6]. Nevertheless, and similar to gested that the total number of attacks that observed with beetle attacks, inocu- could be of consequence for the outcome lation with such fungi has detrimental of the infection in Norway spruce [4, 25]. effects in the sapwood only when per- This hypothesis has been documented by formed above a threshold density of inoc- Christiansen and Berryman [5] in Norway ulation points [30]. spruce, but no information was available
high, and diameter at breast height was around for Scots until pine. Other than these now 6 in all Dead lateral branches were cm observations, the existence of a threshold cases. 1.80 m, and the inoculations were pruned up to of inoculation density is still poorly sup- made at breast height (D on May 10 and ), 130 ported by experimental evidence in Scots 11, under eight different conditions: two den- pine. sities of inoculation points distributed over belts of four different widths (table I), with a The present study aimed at testing the staggered disposition in order to avoid coales- response of Scots pine trees to artificial cence of the induced reaction zones in the inoculations with L. wingfieldii as modu- phloem. Spacing between inoculation points lated by two factors: i) density, and ii) was 7 cm at 200 m and 5 cm at 400 m -2 . -2 total number of inoculation points. Results were expected to provide approximate val- ues for the threshold of inoculation density 2.2. Inoculation of the fungus, in the case of young Scots pines, and to and measurement of sapwood allow further research on the effect of and phloem reactions environmental factors such as drought on Scots pine resistance to bark beetle-asso- L. wingfieldii was collected from bark bee- tle galleries in the forest of Orléans, and grown ciated fungi. malt agar medium as a monospore strain. on a At each inoculation point, bark and phloem were removed with a 5-mm diameter cork 2. MATERIALS AND METHODS borer, and a 5-mm diameter disc of a 3-week- old malt agar culture was inserted, with the fungus side close to the sapwood. Thereafter, the bark plug was returned to maintain the 2.1. Experimental layout mycelium, and to avoid contamination [32]. During April 1993, 40 young Scots pine Three months after inoculation, the trees (Pinus sylvestris L.) growing in the forest of felled, and three stem discs were cut in the were Orléans (Loiret, central France) were selected middle and close to both ends of the inocu- in the plot n° 531 which was flat, and on an lated section of each stem. Areas of blue- homogenous soil. They were 8 years old, 3.5 m stained, dried and resin-soaked sapwood were
measured on the discs (figure 1). Data were variance (ANOVA) was achieved using SAS obtained by drawing the areas on a transpar- software (SAS Institute, Cary, NC, USA), and ent paper and by measuring them with a differences between means were tested using planimetre (ΔT area metre, ΔT Devices, Cam- multiple comparison tests of Bonferroni (α = bridge, UK). The outer bark was removed at 0.05, n= 5). around 10 inoculation points in each tree, and the visible length of the induced reaction zones was measured at the external side of the 3. RESULTS phloem. Three months after the inoculation with L. wingfieldii, several trees displayed 2.3. Decline assessment external symptoms of decline, with severe needle yellowing; in particular, trees Before felling the trees, we visually assessed exposed to the highest inoculation density the degree of yellowing in the crown, and (400 m were severely affected, while ) -2 ordered them according to three classes: 1) those inoculated at 200 m remained -2 completely green needles, 2) faint yellowing distributed over the whole crown and 3) severe almost unaffected (figure 2). The induced yellowing of the whole crown. reaction zone surrounding inoculation points in the phloem was approximately 10 cm long; this length remained unaf- 2.4. Statistical methods fected by density or total number of inoc- ulation points (figure 3). Mean values of damage extent were calcu- The total area of damaged sapwood lated in each tree, and resulting values were increased significantly with inoculation compared between modalities. Means are pre- density and number of inoculations sented with their standard error. Analysis of
(figure 3). Resin-soaked and desiccated parameter and the damaged sapwood area sapwood areas were around 5-15 % in found (table II). As expected, the per- was of resin-soaked and dried sapwood response to all treatments, and did not dis- cent were correlated as well as with the per- play any change with density or number of cent total damaged sapwood. inoculations. The fraction of blue-stained sapwood was very low at 200 mfor all -2 belt widths. It increased dramatically at 400 mfor the highest belt widths (fig- -2 4. DISCUSSION ure 3). In fact, the interaction between the two factors (density and belt width) was L. wingfieldii is known to display a significant (P > F 0.0135): the increase high pathogenicity and to have the capa- due to higher densities was only detectable bility to kill Scots pine trees at least 1 year above 70 cm belt width. after inoculation at high densities [21, 29, 30]. The effects of artificial inoculation Despite the lack of effect of treatments of a strain of this species into the trunk of on the length of the induced reaction zone, young trees at two densities and over four a significant correlation between this belt widths were very contradictory. High-
densities and largest belts resulted in of the pigmented fungal hyphae [1, est ence visible yellowing, but did not induce tree 11, 13] that preferentially develop in the mortality after 3 months. A similar obser- ray parenchyma and resin ducts [1, 20]. vation was previously reported by Bois Tracheids are colonized secondarily, and [3], who inoculated young Scots pine at the progress of the hyphae occurs via bor- a high density (400 m and on 1-m belt ) -2 dered pits or through direct penetration of width. In fact, our observations correspond the walls [20]. The rate of fungal colo- to early events of tree decline, and the lack nization is slow in the nutrient-rich ray of tree death after 3 months cannot be used cells and faster in the tracheids. Develop- to affirm that the density of inoculation ment of dried, non-conducting zones in points was too low to induce tree decline. the sapwood of conifers was frequently In fact, Solheim et al. [30] observed tree observed in response to fungal invasion death 1 year after inoculation at a density [7, 8]. They are generally located at the of 400 m . -2 margin of the stained areas and elaborated in response to fungal activity [12, 28]. It is Despite this lack of mortality, impor- assumed that desiccation of the sapwood damage (i.e. occurrence of large areas tant occurs before blue staining, i.e. before of blue-stained, resin-soaked and desic- penetration of the fungus into the medullar cated sapwood) was induced in the sap- rays [15].In addition, desiccation seems to wood and its extent depended heavily on be specifically induced by the fungus; a the inoculation density. The highest den- simple inoculation with sterile agar has sity promoted extended damage. Such no effect [12]. Chemicals such as oxalic results are in agreement with inoculation acid [8] or others [9, 14] are produced by density responses reported for the same several fungi and may play a role in induc- fungus-tree model [30]. Beyond this very ing embolization and subsequent desicca- general observation, a careful analysis of tion of sapwood at a distance from the symptoms revealed differential effects. zone where fungi are present. Blue staining increased dramatically with inoculation density, whereas resin soak- Our observations fit into this general ing or desiccation extension did not vary frame. The fact that desiccated sapwood significantly. Development of blue stain- was relatively stable while blue staining ing is the direct manifestation of the pres- increased significantly only above an inoc-
by Solheim et al. [30] with the shown ulation density of 400 mwith a width -2 species but somewhat older and same of 70 cm suggests that generalized colo- taller trees (20 years, 5.5 m). In the same nization of sapwood probably occurs only way, the critical threshold of inoculation above a severe inoculum constraint. We density probably depends on tree vigour. may hypothesize that the progression of For instance, Långström et al. [18] showed L.wingfieldii in the sapwood is accompa- that pruned Scots pine were more heav- nied by a gradual desiccation of wood tis- ily affected by bark beetle (Tomicus sues that are secondarily invaded by piniperda) attacks than unpruned ones. mycelia. As such, we may state that a Mahoney [24], Raffa and Berryman [27], threshold of inoculation density must be and Waring and Pitman [31]observed that reached before the invasion of the sap- Pinus contorta trees with a low sapwood wood by mycelial strains is possible. The productivity were more sensitive to Den- sequence of events leading to the infec- droctonus ponderosae attacks than trees tion stage observed in our stem sections with a high sapwood productivity. Similar needs, nevertheless, to be better docu- results were obtained with Picea abies mented by sequential anatomical obser- attacked by Ips typographus [25]. Using vations during the course of infection. the technique of mass inoculation with The impact of the total number of inoc- Ceratocystis polonica, Christiansen [4] ulation points at a given density has been showed that suppressed trees appeared suggested by Christiansen [4], Mulock more susceptible than more vigorous trees. and Christiansen [25] and Homtvedt and It is therefore extremely difficult to extrap- Solheim [16]. More recently, Christiansen olate our results to other stands of Scots and Berryman [5] observed that the blue- pine without additional information on the stained sapwood cross-sectional area was relationships between growth conditions, dependent on the inoculated belt width in tree vigour and susceptibility to L. wing- Norway spruce. They evidenced that at an fieldii. inoculation density of 400 mwith Cer- -2 A last but interesting result of our study atocystis polonica, blue staining was lies in the stability of the length of the greatly enhanced as soon as the belt width induced reaction zone in the phloem, overcame 50 cm in a susceptible clone, which was significantly affected neither while in two resistant ones, a width above by the density nor by the total amount of 90 cm was needed to obtain similar dam- inoculation points. This observation is age. Our results clearly confirm the impor- similar to those made by Solheim et al. tance of the total number of inoculation [30] and by Bois [3]. It points to a rela- points, as a large increase of blue stain- tive independence between the responses ing was observed in the sapwood with an observed in the phloem and the spread of increasing amount of inoculation points, at the fungus in the sapwood. The slight cor- a density of 400 m Nevertheless, we . -2 relation found between this parameter and also showed that this response depended the total damage in the sapwood tempers on inoculation density and was not this assertion. detected at a lower density of 200 m . -2 Finally, our experiment suggests that the threshold of inoculation density with L. ACKNOWLEDGEMENTS wingfieldii is close to 400 mon a belt -2 width of 100 cm. Above such conditions, The authors are grateful to the ’Office death of young Scots pines may be National des Forêts’ for providing the Scots expected. This value is close to the thresh- pine stand in the forest of Orléans, and the old of 600 mover a 60-cm belt width -2 technical help of J. Garcia and P. Romary is
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