Plant Pathology and Plant Pathogens. John A. Lucas

wilt of tomatoes, bacterial wilt of cucurbits Hypertrophy Growth regulation Club root of brassicas, maize smut, peach leaf curl Hyperplasia Growth regulation Crown gall, cocoa witches broom, peach leaf curl Leaf abscission Growth regulation Leaf blight of rubber, coffee rust, black spot of roses Etiolation Growth regulation Bakanae disease of rice Inhibition of flowering and fruiting Reproduction Choke of grasses Ergot of grasses, cereal smut diseases Abnormal coloration Pigment synthesis Grapevine leaf roll virus, citrus greening, tulip breaking virus

The symptoms listed in Table 1.1 will also interact in numerous ways. In club root of cabbage, the basic symptoms are hypertrophy (abnormal enlargement of cells) and hyperplasia (uncontrolled cell proliferation) in root tissue (Figure 1.3), but the first visible symptom is often wilting of the aerial parts of the plant. Any disruption of normal root development inevitably affects other functions such as water and nutrient transport. In view of the highly integrated nature of life processes, it is hardly surprising that attempts to define symptoms often lack precision.

The relative importance of any symptom will vary, depending not only upon its duration and severity but also on the habit or life form of the plant affected. Hence, necrosis in the stem of an herbaceous seedling will probably lead to the death of the whole plant, while necrotic lesions (known as cankers) in the stem of a woody perennial may only result in the loss of a twig or branch. If, however, such a lesion girdles the trunk of a tree, then translocation will be disrupted to the extent that the plant will die.

Figure 1.3 Club root disease of brassicas. (a) Primary infection causes distortion of root hairs, which contain plasmodia of the pathogen Plasmodiophora brassicae. Bar = 50 µm (b). Secondary infection of the main root leads to division and enlargement of cortical cells to produce the typical “clubbed” root symptom.

Figure 1.4 Rust of willows caused by Melampsora species. (a) Aerial view of experimental trial of willow clones in summer. Plots which appear empty have lost their leaves due to severe rust infection.

      Source: Courtesy of D.J. Royle.

      (b) Scanning electron micrograph of rust pustule on willow leaf showing spiny urediospores of the fungus.

      Source: Spiers and Hopcroft (1996).

Pathogens which actually kill plants are the exception. More commonly, disease symptoms indicate an impairment of the efficiency of plant physiological and metabolic processes (Table 1.1). Some symptoms, such as local changes in pigmentation, may be trivial in terms of overall plant performance. Often, the most important consideration is the stage in the life cycle at which symptoms first appear. Severe chlorosis or even necrosis of the first‐formed leaves of a cereal may have little effect upon the final yield, as these leaves senesce naturally during crop growth, and most of the photosynthetic products required for grain filling are provided by the top three leaves and ear tissues. Accelerated abscission of leaves is unlikely to be a problem in annual herbaceous plants but in perennials, it may exert a severe drain on the food reserves of the plant. For instance, Figure 1.4 shows defoliation of some willow clones due to infection by the rust fungus Melampsora. Loss of photosynthetic tissue reduces the biomass produced by the crop. A similar symptom can be seen in coffee bushes affected by another rust, Hemileia, or in rubber trees affected by the leaf blight fungus Microcyclus ulei. In both these evergreen crops, early leaf fall is often followed by the production of a second flush of leaves. If these are also prematurely lost due to further infection then the plant loses vigor and may eventually die.

      While visual symptoms are still routinely used to diagnose diseases and disorders in crops growing in the field, in recent years a range of molecular assay techniques have become available to directly detect the agents causing the symptoms. Such molecular diagnostics are discussed in more detail in Chapter 4.

Causes of Disease

Any agent capable of adversely affecting green plants may be regarded as lying within the scope of plant pathology. The principal agents involved in plant disease are shown in Figure 1.5. Partial or total crop failure may be due to one or more agents. Where more than one agent is responsible, each may act independently, or they may interact. In the latter instance, there may be synergism, that is, two or more agents acting in combination to cause symptoms that are more severe than those produced by either agent alone. Synergism has been demonstrated to occur with several combinations of viruses. For example, tobacco mosaic virus and potato virus X each cause relatively mild mottling symptoms in tomato. But if by chance they both occur together in the same host, then severe necrosis develops and this may even result in the death of the plant.

      A useful distinction can be drawn between animate (biotic) and inanimate (abiotic) causes of disease (Figure 1.5). Many of the animate agents, including the microbial pathogens, the parasitic angiosperms, and some of the animal pests, are infectious. Due to their capacity for growth, reproduction, and dispersal, these agents spread from one host plant to another. Under particularly favorable conditions, they may be dispersed rapidly over wide areas and even entire continents.

      Pests

Among the animals exploiting plants are many pests which cause damage to roots, leaves, shoots, flowers, and fruits. Usually these pests, which include insects such as aphids and leafhoppers, and some nematodes, spend relatively brief periods on individual plants before moving on to explore new food supplies. Other pests, such as leaf miners, gall‐forming sawflies, and endoparasitic nematodes, spend their entire life cycle, or a major part of it, on one plant. Pest attack may result simply in a drain on host nutrients or, alternatively, in extensive destruction of tissues. Aphids and whiteflies on leaves and stems extract sap from the phloem with almost clinical efficiency. Many caterpillars are simply small herbivores; nevertheless, they can consume large areas of the leaf lamina.