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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.

Koch's Postulates

      To determine with certainty that a particular microorganism is the cause of a disease rather than some incidental contaminant, it is necessary to critically examine its relationship with the host. This dilemma was first recognized in studies of pathogens of humans and other animals. In 1876, Robert Koch provided the first experimental proof of disease causation by applying a set of rules which have since come to be known as Koch's postulates. Koch considered that these rules must be satisfied before any microorganism can be regarded as a pathogen. The rules involve five steps outlined below.

      1 The suspected pathogen must be consistently associated with the same symptoms.

      2 The organism should be isolated into culture, away from the host. This precludes the possibility that the disease may be due to malignant tissues or other disorders of the host itself.

      3 The organism should then be reinoculated into a healthy host.

      4 Symptoms should then develop which are identical to those observed in the original outbreak of disease.

      5 The causal agent should be reisolated from the test host into pure culture and be shown to be identical to the microorganism initially isolated.

An actual example of the use of Koch's rules is shown in Figure 2.5. An apparently new disease of orange trees, with symptoms of chlorosis, stunting, and dieback of branches, was reported in South America. Leaves from affected trees were surface sterilized and plated onto a nutrient medium. Colonies of a small, gram‐negative bacterium were obtained. Suspensions of the bacterium were then injected into healthy citrus saplings, and after a period of incubation, some of these artificially inoculated trees developed symptoms very similar to those seen in the original infected tree. The same small bacterium was reisolated from these trees.

      This procedure completed Koch's postulates and showed that the new disease, named citrus variegated chlorosis, was due to a bacterium. In reality, a lot more work, including light and electron microscopy and the use of specific antisera, was required to actually identify the agent as a new strain of the xylem‐inhabiting pathogen Xylella fastidiosa. A few years later, in 2000, X. fastidiosa became the first cellular plant pathogen to have its complete genome sequenced.

      Procedures for the detection and diagnosis of specific pathogens are described in more detail in Chapter 4.

This example shows that even today, Koch's rules are still relevant, although they cannot be rigidly applied in their original form to all pathogens. The most important exceptions in plant pathology are when the pathogen cannot be grown in artificial culture, for example the viruses and some biotrophic fungi. The problem of isolating viruses from their host plants is generally overcome by using indicator plants. These are alternative hosts which develop symptoms which are specific for a particular virus. Healthy specimens of the original host may then be reinoculated. In addition, electron microscopy of plant sap or of purified crystalline samples of the virus, coupled with serological techniques, may be employed to investigate the type(s) of virus present at each step of the procedure. There are also a number of new and powerful methods for detecting nucleic acid sequences specific to particular pathogens.

Figure 2.5 Use of Koch’s postulates to establish the etiology of a new disease of citrus caused by the bacterium Xylella fastidiosa.

      Source: Based on Hartung et al. (1994).

The application of Koch's rules to nonculturable fungal pathogens presents fewer problems because these agents produce spores. Such propagules can be removed from the host and then used in reinoculation experiments. In many instances, spore morphology is also a valuable aid to identification of the inoculated and reisolated pathogens.

      Further difficulties in satisfying these postulates may be experienced in cases where symptoms result from mixed infections or when dealing with previously undescribed disease agents. For instance, few pathologists would have predicted the existence of the viroids, which scarcely conform to our preconceptions of a successful parasite (see Chapter 3).

Host Resistance and Pathogen Virulence

      All crops are exposed to a wide variety of potentially pathogenic microorganisms present in soil, water, and the surrounding atmosphere. Yet most plants remain healthy most of the time. Consequently, the majority of pathogens are unable to infect the plants with which they come into contact. Even where a specific pathogen can attack a particular host species, there are marked variations in the extent to which individual plants succumb to disease. These differences are paralleled by variation in the pathogen population, reflecting differences in the genetic constitution of both the host and the pathogen. The ability of the pathogen to cause disease, and the host to respond to invasion, has been shown to be determined by specific genes. This discovery has important implications both in the analysis of disease and in its control.

      Resistance and Susceptibility

      When a microorganism makes contact with a plant, it may be able to penetrate the potential host or it may be completely excluded. Following penetration, development of the pathogen may be halted by a host response or, alternatively, growth continues within the host tissues.

Describing the interaction between a microbial pathogen and its host presents problems as the outcome needs to be defined in terms of both partners (Figure 2.6). At one extreme is the situation where a microorganism is incapable of causing disease in the host under any conditions, and so is described as a nonpathogen of that host. Likewise, plants able to completely prevent penetration by a microbial agent are nonhosts, and are considered to be immune to that organism. The majority of interactions between microbes and green plants are likely to be of this type. It may, however, be difficult to establish whether a plant is immune to a particular pathogen, as the absence of visible symptoms does not automatically imply a failure to penetrate. Nowadays, the term immunity is mainly used in the context of innate plant defense, or should only be applied in cases where precise descriptions of the microbe–plant interactions are available.

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