apparent damage to host cells, but water‐soaked lesions which become necrotic then appear. The apple scab fungus Venturia inaequalis grows beneath the cuticle of host leaves for several days without causing obvious necrosis, but as the lesions age, the host tissues are eventually killed and the typical scabs develop (Figure 2.3). Some species of the anthracnose fungus, Colletrotrichum, penetrate directly into host cells which remain alive for several days (see Chapter 6, Figure 6.9); subsequently, necrotic, spreading lesions are formed. The term hemibiotroph has been used to describe such behavior. The factors responsible for this switch from a balanced mode of parasitism to rapid killing of host cells have in many cases not yet been identified.
In nature, necrotrophs may grow on both living and dead host tissues. Pathogens such as Pythium and Rhizoctonia may be found growing actively in soil or on subterranean or aerial plant surfaces in competition with the natural microflora. In the absence of a suitable host, they may successfully complete their life cycle by utilizing dead organic resources. The ability of biotrophs to compete for dead organic matter is very limited or even nonexistent. These differences in patterns of natural occurrence of pathogens are reflected in their growth on laboratory culture media. Most necrotrophs are nutritionally undemanding; they grow well on a wide range of simple media. Biotrophs, on the other hand, have traditionally been regarded as fastidious organisms and in extreme cases cannot be grown on any known culture media.
Figure 2.3 Apple scab disease caused by Venturia inaequalis. (Left) Scab lesions on fruit. (Right) Scanning electron micrograph of apple leaf fractured through a scab lesion, showing sporulation of the fungus on the surface, and intact, uncolonized host tissues beneath. Bar = 10 μm.
Source: Courtesy of Alison Daniels.
Figure 2.4 Nutritional modes in heterotrophic microorganisms.
Distinctions based on the criterion of culturability are used to divide pathogens into two nutritional types: facultative and obligate parasites. A further refinement of this scheme distinguishes pathogens which are able to grow relatively well in pure culture, but which in nature are unable to compete with nonparasitic microbes. Such parasites are termed ecologically obligate in contrast to biochemically obligate organisms which are unable to grow apart from the living host either in vivo or in vitro. The basis of obligate parasitism remains largely unresolved; such microorganisms may be unable to synthesize essential metabolites and therefore have to obtain them from the host, lack particular nutrient uptake mechanisms, or may require developmental cues that are only provided in the presence of the host plant.
Figure 2.4 summarizes these different relationships and modes of nutrition in heterotrophic microorganisms.
Pathogen Classification
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.
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
