Anomalies de croissance et dépérissement du tilleul au Québec : à prévenir dès la production des plants en pépinière

Cet article présente des résultats d'analyse d'échantillons de quelque 30 disques de tilleuls (principalement de lignées ou cultivars issus du Tilia cordata Mill.) atteints de dépérissement prononcé, dans la ville de Québec (Québec) Canada, et ayant dû être coupés récemment. Depuis 1993, près de 5 % de la population de tilleuls situés dans cette ville ont été abattus pour la même raison. Parmi les anomalies observées sur ces arbres, la formation de nombreuses excroissances tumescentes sur le tronc et la présence de gros renflements à leur base dans certains cas ont paru prépondérantes vis-à-vis l'apparition de ce mal. Ces anomalies semblent avoir conduit à la venue de plusieurs autres agents, supposément secondaires, soit des insectes et des micro-organismes, et à d'autres conditions dommageables, telles la formation de crevasses et anfractuosités prononcées de l'écorce, le décollement en plaques de celle-ci, etc. Les dissections ont montré que l'origine de ces anomalies pouvait remonter au tout jeune âge de l'arbre; celles-ci s'étaient graduellement amplifiées, devenant ouvertement nuisibles il y a plusieurs années. Les résultats d'inventaires conduits à Québec et ailleurs, comprenant plus de 900 jeunes arbres plantés ces dernières années, ont montré que de telles anomalies commençaient à se développer sur un très grand nombre de ceux-ci, mais avec des différences notables selon les variétés. Ces données montrent l'importance de développer et d'appliquer des méthodes de prévention susceptibles de contrer l'emballement de ces anomalies de croissance dès les stades de production et de plantation de l'arbre.Since 1993, a pronounced decline in the health condition of linden trees (mainly from Tilia cordata Mill. lineage) has been responsible for the removal of 5% of the total planted linden trees in Quebec City (Quebec, Canada). Among the various growth anomalies observed on these trees, numerous gall-like outgrowths developing on the trunks and pronounced swellings at the base have appeared to be preponderant factors leading to decline. These anomalies have favoured the action of secondary agents such as opportunistic insects and microorganisms, and of damaging disturbances, such as a pronounced fissuring and cracking of the bark, bark peeling, and so on. Analysis of disks eut from 30 felled trees has shown that the anomalies had originated at the centre of the tree, and that their growth and effect had started to increase rapidly several years ago. The results of surveys conducted on more than 900 recently planted linden trees in Quebec City and elsewhere indicate that such outgrowths on the stem as well as the swellings at the tree base were present on a large number of the trees and were starting to expand, but with marked differences between varieties. These data point to the necessity of developing preventive measures and applying them early in the tree life to prevent such anomalies

Extracellular sheath formation by Sphaeropsis hypodermia and association with its infection in elm trees

Nous avons observé le mode de développement de Sphaeropsis hypodermia (isolé d'une branche d'orme (Ulmus americana) atteinte d'un chancre) dans les milieux suivants : un milieu gélosé (PDA), un tissu essuie-tout (Kimwipes), des blocs de bois d'orme stérilisés à l'autoclave, et des ormes d'Amérique inoculés en serre. Des échantillons de chaque substrat ont été fixés pour les observations en microscopie photonique et en microscopie électronique avec du glutaraldéhyde et du tétroxyde d'osmium. Observés en microscopie électronique, les hyphes accolés au substrat étaient entourés d'une épaisse couche extracellulaire devenant circonscrite par des bandes rigides et contenant des corps opaques simples ou agrégés, ou des masses de matière opaque plus volumineuses dans le cas des blocs de bois d'orme. Sur celui-ci et sur le tissu Kimwipes, la couche extracellulaire s'étendait sur une distance appréciable, loin des cellules fongiques, et avait également pénétré les parois de cellules de parenchyme et des fibres, selon le cas. Dans les ormes inoculés, le champignon a rapidement produit des dommages notables dans les tissus du cambium et colonisé abondamment les régions de l'écorce et du xylème avoisinantes. La pénétration et la dégradation des parois cellulaires de l'écorce étaient marquées, en relation également avec de la matière opaque entourant les cellules du champignon. Cette matière ressemblait à celle liée aux cellules fongiques sur les milieux stérilisés. Bien que les cellules du xylème étaient généralement colonisées, des altérations pariétales n'étaient apparentes que dans les cellules récemment formées. En outre, le passage du champignon d'une cellule à l'autre à travers les parois n'a été observé que dans le cas des éléments de vaisseaux et des cellules de rayon. Concernant les fibres, seule y était visible une bande de matière filamenteuse dans les parois et liant les cellules fongiques présentes dans la lumière de ces fibres. En réponse à la dégradation de parois cellulaires liée à de la matière opaque, l'hypertrophie et l'hyperplasie des cellules du cambium et de l'écorce interne ont été observées, liées possiblement à la formation d'une barrière de protection. On discute du rôle possible de la couche extracellulaire des cellules fongiques in vivo et in vitro.Sphaeropsis hypodermia, isolated from a cankered American elm branch, was grown on agar medium (PDA), on autoclaved wiping paper (Kimwipes), and American elm (Ulmus americana) wood chips, or inoculated into greenhouse-grown American elm saplings. Samples from each treatment were double-fixed with glutaraldehyde and osmium tetroxide and examined with the light and the transmission electron microscopes. Ultrastructurally, the hyphae on PDA and inert substrates appeared surrounded by large extracellular sheaths which were delimited by rigid opaque bands of various thicknesses. The sheaths extended appreciable distances from the fungal cells, as evidenced by their adherence to rigid substrates. Individual or aggregated opaque bodies, even as large masses on elm wood chips, were the main components of the sheath. This opaque material was often associated with penetration and ruptures of the wood cells. Inoculated into elm trees, the fungus rapidly caused pronounced alterations of cambial tissues and colonized the adjoining bark and xylem cells. The prominent penetration and breakdown of the inner and outer bark cells by the fungus were associated with opaque material, particularly in cortical fibres. The material was structurally similar to the sheath formed on the rigid sterilized substrates. In the xylem, only the walls of the recently deposited cells were visibly altered, and although mature fibres were generally colonized, the passage of the fungus from one fibre to another was rarely observed, contrary to the passage from vessel and ray cells to adjoining cells. In that instance, only bands of opaque material present in the walls of fibres were connected with fungal cells in their lumen. In the inner bark and cambial regions, cell hypertrophy and hyperplasia occurred next to host walls that were altered and contained similar opaque material. The extracellular sheath of S. hypodermia under in vitro conditions and the opaque material associated with host wall alterations in vivo are considered to be analogous

Ultrastructure of in vivo interactions of the antagonist bacteria Bacillus cereus X16 and B. thuringiensis 55T with Fusarium roseum var. sambucinum, the casual agent of potato dry rot

The interaction of Fusarium roseum var. sambucinum, the causal agent of potato dry rot, with two antagonistic bacteria, Bacillus cereus X16 and B. thuringiensis 55T, was studied on wounded potato tubers using light and electron microscopy. Application of B. cereus X16 or B. thuringiensis 55T to potato wounds before challenge with the pathogen suppressed dry rot and restricted fungal growth in plant tissues to the first few cell layers beneath the site of inoculation. Both bacterial antagonists penetrated into potato tissues and established themselves through intercellular and intracellular proliferation. The extent of Fusarium colonization was appreciably reduced in the bacterized tubers, and most fungal cells in these tubers were severely damaged, with appreciable morphological and structural changes. In potato tubers bacterized by B. thuringiensis 55T, Fusarium invasion of the host tissues did not stimulate structural host reactions, and direct parasitism, which operates by degradation of the fungal cell walls and disintegration of the fungal cytoplasm, seemed to play a key role in the antagonism against Fusarium hyphae. In potato tubers inoculated with B. cereus X16 and challenged with the pathogen, on the other hand, a set of defense reactions, were triggered, including modifications of the primary cell walls and the occlusion of some cells and vascular tissues with different types of electron-opaque materials. Fungal hyphae in the vicinity of these barriers, apparently containing higher than usual levels of phenol-like compounds, usually showed advanced stages of disorganization, suggesting the existence of a fungitoxic environment. The results presented here show that the two antagonistic bacilli use different biocontrol strategies to suppress Fusarium dry rot development

Peculiar structures occurring in vessel walls of the susceptible carnation cultivar Early Sam infected with Fusarium oxysporum f.sp. dianthi

Ultrastructural and cytochemical studies of infection by Fusarium oxysporum f.sp. dianthi (Prillieux and Delacroix) Snyder and Hansen in susceptible carnation (Dianthus caryophyllus L.) plants have disclosed conspicuous amounts of an unusual dense material that extends long distances in vessel walls. This material, which at times was circumscribed by bands of material that labelled for chitin and was closely linked to large fungal or microhyphal cells, contained membranous and filamentous structures and opaque particles of uniform appearance. At times, the latter aggregated into crystalline bodies.</p

Reactions of paratracheal cells of resistant and susceptible carnation (Dianthus caryophyllus) cultivars to vascular invasion by Fusarium oxysporum f. sp. dianthi

Here, cell and wall modifications in resistant (cv. I) and susceptible (cv. II) carnation inoculated with Fusarium oxysporum are characterized. Samples, of various days after inoculation (dai), were double-fixed and gold-complexed probes used for cellulose, pectin, chitin. In cv. I, mature paratracheal cells contained appositions mostly bound by a cellulose layer over the densely opaque native walls. Middle lamellae of younger cells, many hyperplastic and much distorted, were obliterated, and appositions displayed a mingling of cellulose-labelled and -unlabelled components; adjoining cells contained inner suberin layers. Cell wall modifications in cv. II were limited, but early pathogen invasion was restricted to middle lamellae despite pronounced host cell content alterations. In both cvs, microfilaments linked to often small, thin-walled fungal cells in vessels and in turn to coating on their walls, extended into adjoining cells, seemingly associated in cv. I with cell reactions, and in cv. II excreted as bodies into vacuoles. The present results highlight that extensive cell reactions and wall modifications, including suberin deposition in hyperplastic tissue, are the main initial factors of defence conducive to final pathogen confinement