Plant Diseases and Resistance
The relationship between a plant and a pest1 is very complex. The ability of a pest to cause disease in a plant depends on environmental conditions, the properties of the organism itself and the capacity of the plant to defend itself. Varieties within a plant species can differ in their ability to defend themselves. Under different climatic conditions the interaction between the same plant and pest may have different outcomes.
Pathogens are known to develop and form new biotypes, pathotypes, races or strains that can cause damage to plants, which remain unaffected by the original form of the pathogen. Resistance is the ability of a plant variety to restrict the growth and development of a specified pathogen or the damage they cause when compared to susceptible plant varieties under similar environmental conditions and pathogen pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pathogen pressure.
Consistency in Terminology and Nomenclature
To promote consistency in the terms used to describe the reaction of a plant to a pest, the ISF Vegetable and Ornamental Crops Section has adopted definitions for terms describing reactions of plants to pests and recommends their use to vegetable seed companies around the world. The paper defines two levels of resistance. Company claims on the level of resistance are based on tests carried out with well-characterized isolates of a pest in controlled environmental conditions. An isolate is a population of (micro)organisms that has been obtained in pure culture from a field, from another location or in a laboratory, and that has been characterised using differential hosts. A characterised isolate is representative of a biotype, pathotype, race or strain of a pest and will not have any characteristic differentiating it from another isolate of the same biotype, pathotype, race or strain.
Seed companies are also encouraged to use uniform codes for pests affecting vegetable and cereal crops in catalogues and other communication with customers (see Pathogen Codes).
Disease resistance is a major goal in breeding new varieties and plays a key role in vegetable crop production and integrated pest management practices. It is also carefully described to differentiate new varieties from older ones on the market. Resistance genes may be effective against all or some biotypes, pathotypes, races or strains of a pest, and the emergence of new biotypes, pathotypes, races or strains is not uncommon.
To identify and distinguish different isolates within a species plant pathologists use different methods including 'differential hosts' (see Differential Hosts). Isolates that after characterization do not match an already described pathotype, biotype, race or strain of the pest are designated as a new biotype, pathotype, race or strain following proof that it is established in nature in a scientific publication.
A consistent naming of pathogen races/strains builds stakeholder confidence in product performance under disease pressure. However, the system of naming pathogen races and strains varies from region to region. In some instances there is a single system in use everywhere, as in the case of leaf mould in tomato (Ff) and near wilt in pea (Fop). Different naming systems cause no problem when there is no overlap in strain/race names, as in anthracnose in bean (Cl) in Europe and USA, or where through the use of joint host differential sets the systems are 'linked’, as is the case with downy mildew in lettuce (Bl). The most problematic case is when the use of differing systems causes an overlap in race/strain names, e.g. in Fusarium wilt in tomato (Fol 0, 1 and 2 in Europe, Fol 1, 2 and 3 in USA) and downy mildew in spinach (Pfs).
1 The FAO defines a pest as: Any species, strain or biotype of plant, animal or pathogenic agent injurious to plants or plant products. Pathogens (microorganisms such as bacteria, viruses and fungi that cause a disease) are, therefore, included in the term ‘pest’.