Plant Diseases and Resistance
The relationship between a plant and plant pathogen is very complex. The ability of a plant pest or pathogen 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 plant pathogen may have different outcomes. Plant pathogens are known to develop and form new races or strains that can cause damage to plants that 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 pest or pathogen and/or the damage they cause when compared to susceptible plant varieties under similar environmental conditions and pest or pathogen pressure. Resistant varieties may exhibit some disease symptoms or damage under heavy pest or pathogen pressure.
Consistency in Terminology and Nomenclature
To promote consistency in the terms used to describe the reaction of a plant to a plant pest or pathogen, the ISF Vegetable and Ornamental Crops Section has adopted definitions for terms describing reaction of plants to pests or pathogens and to abiotic stresses, and recommends their use to vegetable seed companies around the world.
Seed companies are also encouraged to use uniform codes for pest organisms 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 strains of a pathogen, and the emergence of new pathogenic strains is not uncommon. To identify and distinguish different species of a pathogen and/or races within a species plant pathologists use 'differential hosts' (see Differential Hosts).
A consistent naming of pathogen strains builds stakeholder confidence in product performance under disease pressure. Strain names, however, vary from region to region. In some instances the same system is in use everywhere, as in the case of leaf mould in tomato (Ff) and near wilt in pea (Fop). In other cases, the different systems in use cause no problem either because there is no overlap in strain 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 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).
Together with representatives from public research institutions and regulatory organizations, the vegetable seed industry is engaged in developing a system to standardize identification of pathogen strains and races (see Strain Identification).