1 The Diseased Plant
Since it is not known whether plants feel pain or discomfort, and since, in any case, plants do not speak or otherwise communicate to us, it is difficult to pinpoint exactly when a plant is diseased.
(George N. Agrios, 1936–2010)
The significance of disease in plants varies depending upon biological, agricultural, and socioeconomic factors. At one extreme, disease may be so severe that the farmer is faced with total crop failure, and the need for control measures is immediately obvious. In other cases, it may be difficult to define disease symptoms, the cause of the problem is not initially clear, and any benefits obtained from control measures are not easy to predict. This chapter discusses the nature of disease and surveys the range of pathogens, pests, and other agents which adversely affect plants. The impact of disease, both in natural plant communities and in agriculture, forestry and horticulture, is then considered.
Concepts of Disease
To fully understand the nature of disease, one must first identify the processes occurring during the growth and development of the healthy plant. Such an analysis may be done at three levels:
the sequence of events comprising the normal plant life cycle
the physiological processes involved in plant growth and development
the metabolic pathways and molecular reactions underlying these processes.
Seed germination, maturation of vegetative structures, the initiation of reproduction, and the formation and dispersal of fruits and seeds are all critical phases of the life cycle at which disease may occur (Figure 1.1). At each stage in this developmental sequence, the integration of several physiological processes is essential for the continued development of the plant. Cell division and differentiation, the fixation and utilization of energy (photosynthesis and biosynthesis), transport of water and nutrients (transpiration and translocation), and storage of reserve compounds are all necessities for growth. Each of these functions involves a complex series of molecular events which comprise the overall metabolism of the plant. The nature and regulation of metabolism are themselves determined by the genetic make‐up of the plant, interacting with the environment in which it is growing.
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
