Plant Pathology and Plant Pathogens. John A. Lucas

or all of the plant. Some plantation crops grown in subtropical regions, such as citrus and coffee, are especially vulnerable. For instance, frosts in southern Brazil in 1975 affected most of the coffee‐growing area with production losses estimated at more than 60%. Many of these physical effects are relatively unsubtle and the symptoms associated with them are nonspecific. Drought is also an important cause of loss, due to reduced yield or even complete crop failure. Recently, prolonged dry spells in the USA, Australia, and Europe have highlighted this threat to crop productivity, and increased concerns about the impact of global climate disruption.

      While the current models of climate change predict varying future scenarios, they all agree that greater fluctuations in temperature and rainfall are likely to occur, with consequent annual variations in crop yield. Rising temperatures may pose a threat not only due to drought. Episodes of higher than usual temperatures can disrupt important developmental processes, such as fertilization in cereals and fruit set in tomatoes. In the longer term, such effects may lead to changes in the areas where certain crops can be reliably grown. The frequency of extreme weather events is also predicted to increase. High winds can have catastrophic effects on plants; perennial plantation crops such as bananas have been destroyed by hurricanes in the West Indies, Cuba, and Central America. Hailstorms are particularly damaging to soft fruit crops and grapevines. There are other, less common environmental hazards. The massive eruption of Mount St Helens in Washington State in 1980 deposited volcanic ash over a wide area with a variety of effects on agriculture. Ash on plant leaves reduced photosynthesis by as much as 90%, some crops such as alfalfa actually lodged (collapsed) under the weight of ash, but eventual crop losses were less than expected, at around 7% of the total.

      Chemical deficiencies or imbalances often result in distinctive symptoms, which may be diagnostic in the case of deficiencies of essential cations. For example, magnesium deficiency in swedes is associated with an abnormal purplish pigmentation in interveinal leaf areas, whereas boron deficiency in the same crop causes brown‐heart symptoms in the storage root. Such deficiency diseases are commonplace, especially in the intensive cropping systems of present‐day agriculture. In the UK, recent reductions in atmospheric sulfur, and subsequent deposition by rainfall, are now leading to deficiencies occurring in sulfur‐demanding crops such as oilseed rape. Sulfur deficiency can affect crop quality as well as quantity, for instance by reducing the bread‐making quality of wheat flour.

Excess amounts of certain mineral ions may be equally harmful, due to their effects on the availability or uptake of other essential ions. When insufficient iron is taken up, plants become chlorotic. Such a shortage may be due to inhibition of iron uptake by high levels of calcium or manganese in the soil, rather than any absolute shortage. Imbalances of soil nitrogen, phosphorus, and potassium result in the development of plant tissues which are particularly prone to infection by microorganisms or damage by other agents. Aluminum is the most abundant metallic element in soil, and at acid pH can become soluble and toxic to plant growth. This is an important constraint to crop production in some tropical soils. The problem can be rectified by raising the pH of soil by the addition of lime. Liming has been used successfully to open up new areas for crop production, notably the Brazilian cerrado (a type of savannah), which is now a major production region for soybeans.

      A common difficulty in diagnosing disorders caused by chemical agents is the similarity between the symptoms they produce and those due to infection by microorganisms. Foliar symptoms in barley resulting from a deficiency of manganese resemble those caused by the leaf blotch fungus Rhynchosporium. Symptoms of other deficiency diseases bear a striking resemblance to those caused by viruses. Recently, molecular work on the cellular systems responsible for the uptake of specific nutrient ions has identified some of the transporter proteins involved. The genes encoding these transporters are in many cases regulated by levels of the appropriate nutrients. In the long term, it may therefore be possible to engineer components of such transport pathways to recognize a deficiency or excess of ions by producing a reporter chemical which is visible in the plant. These so‐called “smart plants” would be sown at intervals within a crop and hence provide an early warning of nutrient imbalance.

      Pollutants are substances which are either unnatural components of the environment, such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT), or naturally occurring substances present in abnormal concentrations, such as ozone, sodium chloride and the acid mist which forms when oxides of sulfur and nitrogen dissolve in atmospheric moisture. Photochemical smog is increasingly common in urban areas and results from the interaction of waste gases, especially from automobiles, particulates, and sunlight. High concentrations of certain chemicals may be a normal feature of some habitats, but problems arise when human activities redistribute these substances. High salt concentrations, which are nontoxic to salt marsh plants, can severely injure inland species, as in the case of roadside communities which are damaged by splash following the application of sodium chloride to roads to prevent ice formation. Irrigation of desert soils can also lead to an accumulation of salts, a process described as salination.

      The influence of many of these compounds on higher plants is now well known and the symptoms induced include abnormal growth due to meristem damage, chlorosis, and necrosis. Some species of plants are especially sensitive to particular pollutants. However, within a species the response of different cultivars may vary considerably. Plant species or genotypes which are tolerant of high levels of pollutants, and which can accumulate them from soil, are of value in reclaiming contaminated sites, a process known as bioremediation.

This discussion has only considered the direct effects of biological, physical, and chemical agents acting independently. In reality, all these agents interact with each other in a more or less complex manner. For instance, infection of the stem base of many crop plants predisposes them to collapse in wind or heavy rain; such lodging then results in problems at harvest. It is difficult to distinguish between the damaging effects of each of these factors. Other interactions are even more complex. The widespread defoliation and death of trees observed in some industrialized countries, a condition known as forest decline, is believed to be due to aerial pollution. However, there is dispute over the relative importance of different atmospheric pollutants and acid rain as contributory factors. It has even been claimed that the premature death of some trees is a normal part of the forest cycle. Most likely, several factors interact to affect tree health, including direct toxic effects, soil acidity, release of toxic ions such as aluminum, and indirect effects on root function, including inhibition of beneficial mycorrhizal fungi and enhanced activity of minor root pathogens. Forest decline provides a good example of a complex disease syndrome, and also illustrates the difficulty of reaching a conclusive diagnosis when several interacting factors are involved.

Significance of Disease

      Disease in Natural Plant Communities

It is often assumed that disease outbreaks are less frequent and less severe in wild plant populations than in crops. This is because there are several important differences between natural and agricultural plant communities (Table 1.3). Wild species are more diverse, both genetically and in the age structure of the population. Hence individual plants will differ in their relative susceptibility to infection. Natural populations also tend to be spatially dispersed as part of a mixed plant community, thereby reducing opportunities for the spread of disease. A recent survey of grassland plots differing in the number of plant species present found that the number of groups of fungal pathogens increased with diversity of the plant community, but that the severity of infection on individual plants decreased. This supports the idea that disease is commonplace in natural plant populations but that mixed communities are less prone to severe disease outbreaks. Often, nutrients are added to crops as fertilizers, and in some cases this can lead to increased susceptibility to infection. Finally, it is likely that plants in natural communities have co‐evolved with their pathogens over long periods of time, leading to some kind of host–pathogen equilibrium. Analysis of wild populations of the genetic model plant Arabidopsis thaliana found very high levels of polymorphism in genes determining recognition of pathogens, supporting the idea that reciprocal selection between host and pathogen over