Plant Pathology and Plant Pathogens. John A. Lucas

Plant Pathology and Plant Pathogens - John A. Lucas


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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.

      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.

      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.

Flow diagram of the use of Koch’s postulates to establish the etiology of a new disease of citrus caused by the bacterium Xylella fastidiosa with arrows from “Citrus tree” leading to “bacterial cultures compared.”

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

      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).

      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.

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