Читать книгу Plant Pathology and Plant Pathogens - John A. Lucas - Страница 32

The classification of pathogenic microorganisms is based initially on the same morphological, physiological, and molecular criteria as other groups. However, conventional taxonomy does not accommodate all the characteristics of importance in pathology. Thus, different isolates of a pathogen which may appear identical in morphology and cultural characters can differ in pathogenicity and in the range of host species attacked. The same problem also occurs in medical microbiology. For instance, the common gut bacterium Escherichia coli is normally a harmless species resident in the human intestine, but certain isolates of this species can infect the gut, causing gastroenteritis and severe illness. The differences between the pathogenic isolates and normal E. coli are relatively minor and are coded for by a few genes often carried on extrachromosomal plasmids. Similar subtleties are common with plant pathogens. In some cases, differences in pathogenic behavior may be due to only a single gene. Differences in host range may be sufficient to define particular groups, or pathotypes, adapted to particular host species. In fungi, where such host specialization is clear, it may be possible to recognize form species. For instance, the black stem rust fungus Puccinia graminis occurs on various grasses including wheat (P. graminis f.sp. tritici) and barley (P. graminis f.sp. hordei). With plant pathogenic bacteria, particular pathovars adapted to different host plants may also be distinguished.

The classification of plant pathogenic viruses presents particular problems as many have very wide host ranges, infecting different plant species, genera, and even families. Nevertheless, different strains occur which vary in important properties such as the relative severity of disease they cause or frequency of transmission by different insect vectors. Such variation needs to be accommodated in any scheme for classifying viruses responsible for disease in plants.