Plant Pathology and Plant Pathogens. John A. Lucas

Figure 1.1 A plant life cycle and some effects of disease.

      Disease may disrupt the activities of the plant at one or more of these levels. Some disorders involve subtle alterations in metabolism which do not affect the successful completion of the life cycle. Certain diseases caused by viruses have only slight effects on the growth of the plant; in such cases, it may be difficult even to recognize the existence of a disease problem. For instance, potato virus X was known as potato healthy virus until virus‐free seed potatoes became widely available. Comparisons with infected plants then showed the virus to be capable of causing a 5–10% loss in yield. Other more destructive diseases may interfere with numerous molecular, cellular, and physiological processes and lead to premature death of the plant.

While everyone is familiar with the idea of disease, in practice there may be difficulties in drawing a precise distinction between healthy and diseased plants. No single definition of disease has found universal acceptance; the most widely used involves some reference to the “normal” plant, for instance “a condition where the normal functions are disturbed and harmed” (Holliday 1989). However, there is no consensus as to the exact extent of deviations from this norm which may constitute the diseased state. The problem of defining normality, in terms of the processes outlined above, is further complicated by the variation inherent in all plant populations. Such variation is particularly common in natural populations, especially where hybrids occur, but even within apparently uniform populations of crop plants, there may be differences between individuals. Such differences either have a genetic basis or are due to environmental factors operating during the growth of the crop. If, for instance, one sows seed of an old cereal variety alongside that of a modern, improved cultivar, one will observe major differences between the two crops. In particular, the modern cultivar will be shorter, form much larger seed heads and heavier grains, and the final yield will be greater. The difference in this case is due to intensive selection and genetic improvement rather than to any disease in the old variety, but this example highlights the importance of understanding the initial potential of the plant before accurate estimates of disease can be obtained.

      Damage or Disease?

      It can be argued that short‐term harmful effects on plants, such as injury due to grazing, do not constitute disease. Indeed, some plants, such as the grasses, are well adapted to regular grazing and respond with increased growth if so affected. In cases where damage is sustained over a longer period of time, such as progressive destruction of roots by migratory nematodes or distortion of aerial shoots by exposure to persistent herbicides, the outcome is clearly within the scope of pathology. However, these fine distinctions are of limited use in arriving at a working definition of disease. Such a definition will depend in part on the situation in which it is intended to be used. For example, the biochemist may well be concerned with a malfunction involving a single enzyme and hence view disease as a specific metabolic lesion, whereas the farmer is normally only interested in changes which affect the overall performance of the crop and reduce its value.

      Although at present, definitions of disease lack precision, it may ultimately be possible to describe all malfunctions in terms of biochemical changes. To date, this has been achieved in only a few exceptional cases, notably in diseases caused by fungi which produce host‐specific toxins, where all the symptoms are due to a single toxic compound acting at a specific target site (see Chapter 8).

Symptoms of Disease

      A doctor diagnoses illness in a patient by looking for visible or measurable signs that the body is not functioning normally. Such signs are known as symptoms and they may occur singly or in characteristic combinations and sequences. For example, someone suffering from influenza may have a sore throat, fever, and muscular aches and pains. Such a group of symptoms occurring together and in a regular sequence is termed a disease syndrome. For many diseases, the occurrence of a particular combination of symptoms is sufficient to arrive at an accurate diagnosis. Alternatively, symptoms may be common to a wide variety of diseases (for instance, fever is a generalized response to both infection and certain types of injury). In such cases, detailed microbiological and biochemical analyses will be necessary to detect other diagnostic symptoms.

Similar considerations apply to the diagnosis of disease in plants. Just as with doctors and human disease, plant pathologists must be aware of the range of visual disease symptoms, the organs affected (Figure 1.2) and what these suggest as the cause of the problem.

The major symptoms of disease in plants are listed in Table 1.1 on the basis of the functions affected. This approach is used because it directs attention to the underlying nature of the disorder. For instance, the presence of galls or other cancerous growths immediately suggests some malfunction in the control of cell division; this in turn implicates a hormonal imbalance and/or genetic change in host cells. It should be realized that this classification of symptoms is to some extent arbitrary and nonspecific. Permanent wilting provides a useful example. Although this symptom suggests that something is interfering with the uptake and transport of water, the symptom itself tells us little about the actual site or cause of the interference. The problem could be due either to a blockage in the vascular system, as in vascular wilt diseases, or to a general destruction of root tissues. It is also possible that the problem has little to do with water uptake or transport; in some diseases, such as infections of leaves by rust fungi, wilting is a sign of excessive water loss due to increased transpiration.