Disease is one of the most complex and challenging research topics in biology. At every level of organization, from molecular interactions through the physiology of plant–pathogen interactions to population genetics and ecology, the relationship between hosts and parasites is a potent driving force for evolution by natural selection and the generation of biological diversity. Parasites exploit a huge range of features of their hosts while, in turn, plants defend themselves by recognizing the presence of parasites, disabling functions essential for pathogenicity. Disease and responses to antagonists are thus closely integrated with many other aspects of an organism's biology. It is not surprising, therefore, that as research on pathogenicity and disease resistance has accelerated in recent years, it has become apparent that these processes involve a wide range of trade-offs with other traits.
The study of fitness in plant disease is also a subject of increasing practical relevance. As early as 1963, Vanderplank raised the concern that selection for higher yield in the absence of disease may lead to evolution of lower levels of what he called horizontal resistance, which is nowadays more often called partial or quantitative resistance. Over the last 40 years or so, production of arable and horticultural crops has benefitted from protection by effective, systemic fungicides. Disease resistance in crop varieties is now increasingly seen as essential rather than desirable, as many pathogens have evolved insensitivity to important pesticides, fewer new modes of action are being developed and public opinion favours greater restrictions on applications of pesticides to crops. The agricultural and horticultural industries now face the challenge of producing varieties which combine resistance with many other essential characteristics such as quality, yield and a wide range of agronomic properties. Plant breeders in particular must maintain and improve resistance to ever-changing populations of parasites without incurring significant costs in terms of other crucial traits.
This supplement on ‘Fitness Costs and Trade-offs in Plant Disease’ brings together fundamental biology with applications to disease control. Some of the costs and trade-offs described here have a long history in the plant pathology literature while others are newly discovered ways in which disease and defence are integrated with other biological processes. The papers span the range from molecular evolution of hosts and parasites to the large-scale evolution of populations. In between, they report the diverse and often surprising ways in which costs of pathogenicity and defence affect parasites, their hosts and the interactions between them. They cover fungal, bacterial and viral diseases; crops and model plant species; fundamental research and applications to the plant breeding and agrochemical industries; and a wide range of disciplines including molecular biology, physiology, genetics and ecology. It will be evident that research on the costs and trade-offs involved in disease touches on very many other areas of plant and pathogen biology. It is fitting, although it was unintentional, that this supplement appears on the fiftieth anniversary of Vanderplank's (1963) seminal work, which has done so much to stimulate both research on fitness costs and consideration of the relevance to crop production. Whilst there are inevitably some notable omissions, this supplement has brought together, for the first time, papers covering a wide range of topics in this area which, it is hoped, will stimulate advances in research by plant pathologists and others.