Phytopathogenic Pseudomonads and Related Plant-Associated Pseudomonads
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Phytopathogenic Pseudomonads and Related Plant-Associated Pseudomonads MILTON N. SCHROTH, DONALD C. HILDEBRAND AND NICKOLAS PANOPOULOS
Phytopathogenic pseudomonads are a very diverse group of bacteria with respect to their genetics, ecology, and the kinds of diseases they cause. They currently are grouped into approximately 25 species that could easily be placed into at least three distinct genera. Moreover, there are approximately 50 pathovars in the species P. syringae, most of which attack different hosts. There also are pathovars in other species. These pseudomonads are worldwide in distribution and cause diseases on most major groups of higher plants. Some of the world’s most serious bacterial diseases are caused by pseudomonads, such as Pseudomonas solanacearum. Because of the genetic diversity of the phytopathogenic pseudomonads, disease symptomatology in plants ranges from necrotic lesions, spots, cankers, and twig dieback (blights) to hyperplasias (galls, scabs), tissue maceration (rots), and vascular infections (wilts). Some pseudomonads, for example, P. aeruginosa and P. cepacia, appear to infect both plants and animals, although more work needs to be done on the comparison of strains from both sources. Research on the phytopathogenic pseudomonads during the 1980s has been narrowly based with greatest emphasis given to such areas as molecular biology, where support funds have been more plentiful. Consequently, considerable progress has been made on understanding the genetic and physiological bases for such characteristics as virulence and host specificity. The development of DNA probes and immunological techniques to detect strains in nature also has progressed well. On the other hand, pseudomonad taxonomy and nomenclature remain perplexing since systematics has received little attention. This is unfortunate, since the defining of taxonomic relationships provides valuable information for those who are searching for clues concerning bacterial properties that relate to pathogenesis and ecological fitness. Many of the studies on the genetics of phytopathogenic
This chapter was taken unchanged from the second edition.
pseudomonads in the 1980s were based on findings from previous investigations comparing the physiological and pathological differences among taxa. Upon reviewing our previous chapter on the phytopathogenic pseudomonads in the first edition of The Prokaryotes (Schroth et al. 1981), it is evident that there has not been a good transfer of basic information to problem solving during this decade. The diseases in general are not better controlled today than they were ten years ago and not much more is known about the ecology of the causal agents with the exception of a few taxa, such as P. syringae pv. syringae. There also have not been any particularly new innovations on management of diseases. However, it is expected that with time, the application of new information gained in the 1980s from studying such fact
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