Une comparaison entre plants sains et plants spontanément atteints de crown gall a été effectuée dans un dispositif expérimental de comparaison de clones de peupliers grisards (obtenus par croisements contrôlés entre Populus tremula et P alba). Les arbres présentant des tumeurs au collet sont plus petits que les arbres sains (- 18%) alors que la présence de tumeurs limitées aux racines n’a pas d’effet significatif. La fréquence des plants atteints de crown gall a été notée chez 31 clones de grisards lors de plusieurs campagnes de production dans la même pépinière. En dépit d’importantes variations annuelles et clonales, certaines familles d’hybrides sont toujours moins atteintes que d’autres, suggérant l’existence d’une base génétique de résistance au crown gall exploitable en sélection. En outre, une étude épidémiologique a montré la présence de la même population caractéristique d’Agrobacterium tumefaciens pathogène dans plusieurs pépinières de grisards. Ceci indique que les agrobactéries à l’origine de la contagion du crown gall chez les peupliers Leuce sont probablement transportées de pépinière en pépinière par les plantes elles-mêmes. La diffusion de grisards porteurs de bactéries pathogènes latentes fait courir un grave risque épidémiologique aux pépinières forestières car les plasmides Ti en cause peuvent potentiellement infecter de nombreuses essences forestières. La sélection de clones résistant au crown gall doit, de ce fait, être accompagnée de mesures prophylactiques sévères visant à limiter la diffusion des plants porteurs sains.

Crowngalled and crown-gall free plants of Leuce poplar (Populus tremula x P alba) were observed to be spontaneously contaminated by Agrobacterium tumefaciens in experimental plots primarly designed to compare agronomic properties of poplar clones. This offered a rare possibility for the survey of clonal responses to spontaneous attacks of crown gall in the field, with the aim of serving as basic reference for breeding purposes. This study took into account the vigour and aspect of the plant, the crown gall frequency of the clone and an estimation of the epidemiological risks in nurseries. Regarding the effect of crown gall on shoot size, 2-yr-old poplars harbouring crown galls were statistically shorter than apparently healthy plants growing in the same plot (table I). Regarding the effect of size and location of galls on shoot growth inhibition, even small galls (< 2 cm in diameter) reduced relative shoot lengths (- 8%), but the biggest growth inhibitions were recorded with the largest galls (- 19% with galls > 5 cm). The location of the galls at the crown instead of on the roots had a significant effect : trees with crown-located galls were 18% shorter than healthy trees, while root-located galls had no significant effect (table II). Regarding clonal and family effect of spontaneous crown gall frequencies, the frequencies of crown-galled plants of 31 clones belonging to 7 breeding families were recorded over a period of several years in a single nursery (table III). In spite of significant annual and clonal variations, some hybrid families were constantly less crown-galled than others, indicating a possible genetic basis for selection of resistant poplars. Regarding the epidemiological survey, the pathogenic population of A tumefaciens, already described in a single poplar tree nursery (Nesme et al, 1987) was looked for in galls originating from several other nurseries. This showed the constant occurrence of the same pathogenic population of agrobacteria in every Leuce poplar nursery. This indicates that agrobacteria, which mediates crown-gall contagion in Leuce poplars were mainly carried from nursery to nursery by poplar plants themselves. Therefore, selection of crown gall-resistant poplar clones must be combined with severe prophylactic measures to limit the diffusion of contaminated plants carrying latent pathogenic agrobacteria.