PSEUDOMONAS-SYRINGAE, AN EPIPHYTIC ICE NU CLEATION ACTIVE AND PHYTOPATHOGENIC BACTERIUM

Pseudomonas syringae, one of the 50 phytopathogenic bacterial species, is the most frequently occurring bacterium under temperate climatic c onditions. P syringae has been isolated from almost-equal-to 400 diffe rent plant hosts (177 hosts for pv syringae) and is characterized by h igh genetic, physiological and biological variability expressed throug h 52 pathovars. Three important properties are associated with this sp ecies: pathogenicity, ice nucleation activity and ability for epiphyti c survival. The general characteristics of P syringae, apart from the genetic aspects, which have not been examined here, are presented in t he first part of this review. In the second part, the 3 remarkable pro perties of the species have been analyzed. Epiphytic survival, express ed by a capacity to colonize aerial parts of plants or by a significan t epiphytic multiplication characterizes most pathovars. High levels o f epiphytic populations can be recovered in the spring and also in aut umn for perennial plants. These epiphytic populations constitute an in oculum source and are essential to the development of infection. The g enerally abundant distribution of epiphytic populations is not homogen eous, either between or within plants; their distribution on the surfa ce of the leaf which constitutes their main support is also very heter ogeneous. 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 sy ringae, expressed as the ability to induce an early nucleation of wate r, concerns only some pathovars, mainly the pathovar syringae. It has been shown to be involved in the ice nucleation of plants and frost da mage in spring (eg grapevine, tomato, potato) and to be one of the fav orable factors in the first stages of some bacterial infections. The p athogenicity of Pseudomonas syringae is dependent on the pathovar exce pt for pv syringae which exhibits a rather wide host range. Some autho rs have contested this host range for the latter pathovar. The develop ment 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. Clim atic conditions during the first stages of infection or during the inc ubation period appear to be involved in the modulation of disease inte nsity. These biological data are useful in understanding the life cycl e of P syringae and in elaborating an efficient strategy for disease c ontrol, different examples are then presented for illustration. Finall y, further developments of research on this phytopathogenic bacterium have been discussed.