Plant Pathology and Plant Pathogens. John A. Lucas

Part I Plant Disease

       We see our cattle fall and our plants wither without being able to render them assistance, lacking as we do understanding of their condition.

      (J.C. Fabricius, 1745–1808)

      The health of green plants is of vital importance to everyone, although few people may realize it. As the primary producers in the ecosystem, green plants provide the energy and carbon skeletons upon which almost all other organisms depend. The growth and productivity of plants determine the food supply of animal populations, including the human population. Factors affecting plant productivity, including disease, therefore affect the quantity, quality, and availability of staple foods throughout the world. Nowadays crop failure, due to adverse climate, pests, weeds, or diseases, is rare in developed agriculture, and instead there are surpluses of some foods. Nevertheless, disease still takes a toll, and much time, effort, and money are spent on protecting crops from harmful agents. In developing countries, the consequences of plant disease may be more serious, and crop failure can damage local or national economies, and lead directly to famine and hardship. Improvements in the diagnosis and management of plant disease are a priority in such instances. Furthermore, the pressures on plant productivity are increasing. The area of cultivated land available per person on the planet declined from around 0.4 ha in the 1960s to less than 0.3 by the year 2000 (FAO data), and as the human population continues to multiply the area will further decrease.

      As well as supplying staple foods, plants provide many other vital commodities such as timber, fibers, oils, spices, and drugs. The use of plants as alternative renewable sources of energy and chemical feedstocks is becoming more and more important, as other resources such as fossil fuels are depleted and the need to mitigate climate change becomes a priority. Finally, the quality of the natural environment, from wilderness areas to urban parks, sports fields, and gardens, also depends to a large extent on the health of plants.

Healthy plants provide a series of benefits for the farmer, food chain, and the environment (Table 1). The yield and quality of crop products are ensured, and healthy plants are more efficient at using precious resources such as water and nutrients. In doing so, they also prevent losses of nitrogen and other nutrients to the wider environment, and reduce pollution of rivers and ground water, which in many areas is a potential problem for drinking supplies. Vigorous, healthy plants are more competitive with weeds, and are easier to harvest than crops that are stunted or collapsed. Plant root systems play an important role in reducing soil erosion, and thereby help to conserve another precious resource. Finally, given the mounting concerns about climate change, it should be noted that healthy crops have a lower carbon footprint than diseased crops, due to their greater productivity and more efficient use of inputs per area of cultivated land.

Table 1 Benefits of healthy plants

      The science of plant pathology is the study of all aspects of disease in plants, including causal agents, their diagnosis, physiological effects, population dynamics and control. It is a science of synthesis, using data and techniques from fields as diverse as agriculture, microbiology, meteorology, engineering, genetics, genomics, and biochemistry. But first and foremost, plant pathology is an applied science, concerned with practical solutions to the problem of plant disease. Part of the appeal of the subject is to be found in this mixture of pure and applied aspects of biology.

      The scope of plant pathology is difficult to define. On a practical level, any shortcoming in the performance of a crop is a problem for the plant pathologist. In the field, he or she may well be regarded in the same way as the family doctor – expected to provide advice on all aspects of plant health! A distinction is often drawn between disease caused by infectious agents and disorders due to noninfectious agents such as mineral deficiency, chemical pollutants, or adverse climatic factors. The main emphasis of this book is on disease caused by plant pathogenic microorganisms such as fungi, oomycetes, bacteria, and viruses. Under favorable conditions, these pathogens can multiply and spread rapidly through plant populations to cause destructive disease epidemics. Many of the principles discussed apply equally well, however, to other damaging agents such as insect pests and nematodes.

A fundamental concept in plant pathology is the disease triangle (Figure 1) which shows that disease results from an interaction between the host plant, the pathogen, and the environment. This can be enlarged to include a further component, the host–pathogen complex (Figure 1), which is not simply the sum of the two partners, as the properties of each are changed by the presence of the other.

A comprehensive analysis of plant disease must take all four components into account. Obviously, one needs to be familiar with the characteristics of the host and the pathogen in isolation. The successful establishment of a pathogen in its host gives rise to the host–pathogen complex. Unraveling the dynamic sequence of events during infection, the molecular “cross‐talk” taking place between the partners, is one of the most challenging problems in experimental biology. In addition, the effects of the environment on each of other components must be understood. This includes not only physical and chemical factors but also macro‐ and microbiological agents. The two‐way arrow between the host and the environment in Figure 1b should not be overlooked, as populations of crop plants often have important effects on their surrounding microclimate. For example, the relative humidity within a crop canopy is higher than that outside and this will favor the development of some microorganisms. Plants attacked by pests and pathogens often produce volatile compounds that can act as signals sensed by neighboring plants. Effects of pathogens on the environment are more subtle but may be significant; some fungi, for instance, produce the volatile hormone ethylene, which can in turn affect the development of adjacent host plants.