Pseudomonas syringae, bactérie épiphyte, glaçogène et pathogène

Abstract

Pseudomonas syringae, one of the 50 phytopathogenic bacterial species, is the most frequently occurring bacterium under temperate climatic conditions. P syringae has been isolated from ≈ 400 different plant hosts (177 hosts for pv syringae) and is characterized by high genetic, physiological and biological variability expressed through 52 pathovars. Three important properties are associated with this species: pathogenicity, ice nucleation activity and ability for epiphytic survival. The general characteristics of P syringae, apart from the genetic aspects, which have not been examined here, are presented in the first part of this review. In the second part, the 3 remarkable properties of the species have been analyzed. Epiphytic survival, expressed by a capacity to colonize aerial parts of plants or by a significant epiphytic multiplication characterizes most pathovars. High levels of epiphytic populations can be recovered in the spring and also in autumn for perennial plants. These epiphytic populations constitute an inoculum source and are essential to the development of infection. The generally abundant distribution of epiphytic populations is not homogeneous, either between or within plants; their distribution on the surface of the leaf which constitutes their main support is also very heterogeneous. The nature of the interaction between epiphytic bacteria and their host is not yet known, but a relation between host specificity for pathogenicity and host specificity for epiphytic capability within P syringae has been hypothesized. The ice nucleation, activity of P syringae, expressed as the ability to induce an early nucleation of water, concerns only some pathovars, mainly the pathovar syringae. It has been shown to be involved in the ice nucleation of plants and frost damage in spring (eg grapevine, tomato, potato) and to be one of the favorable factors in the first stages of some bacterial infections. The pathogenicity of Pseudomonas syringae is dependent on the pathovar except for pv syringae which exhibits a rather wide host range. Some authors have contested this host range for the latter pathovar. The development of infection caused by Pseudomonas syringae is conditioned by the existence of natural (these are limited) or artificial (wounds) means of penetration into the plant tissues and by the inoculum level. Climatic conditions during the first stages of infection or during the incubation period appear to be involved in the modulation of disease intensity. These biological data are useful in understanding the life cycle of P syringae and in elaborating an efficient strategy for disease control; different examples are then presented for illustration. Finally, further developments of research on this phytopathogenic bacterium have been discussed. © 1993.