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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 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.
Resistance is the ability of a plant variety to restrict the growth and development of a specified pest 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 pest pressure. Pests are known to adapt genotypically and form sub-species, formae speciales, biotypes, pathotypes or races that can cause disease in plants previously unaffected by, or resistant to the original form of the pathogen.
Resistance genes may be effective against all or some sub-species, formae speciales, biotypes, pathotypes or races of a pest, and the emergence of new sub-species, formae speciales, biotypes, pathotypes or races is not uncommon. To identify and distinguish different isolates within a species, sub-species, forma specialis, biotype, pathotype or race, plant pathologists use different methods including ‘differential hosts’ – see Differential Hosts. After characterization, isolates that do not match an already described sub-species, forma specialis, biotype, pathotype or race of the pest, are designated as a new sub-species, forma specialis, biotype, pathotype or race after verifying it has substantially established in nature. Verification of establishment occurs through documentation of repeated occurrence in multiple locations over several seasons, adopted by the ISF Disease Resistance Terminology Working Group and are reported in a peer reviewed scientific publication.
Consistent terminology
To promote consistency in the terminology used to describe the reaction of a plant to a pest, the ISF Coordination Group (CG) Phytosanitary has adopted sets of agreed definitions.
The paper Definitions of the Terminology Describing the Reactions of Plants to Pests for the Vegetable and Ornamental Seed Industry defines two levels of resistance: Intermediate Resistance and High Resistance. Claims on the level of resistance are based primarily on tests carried out with well-characterized isolates of a pest under controlled environmental conditions. However, in specific cases claims can be based on field tests carried out under uncontrolled but carefully monitored natural conditions.
The paper Definitions of Terminology Describing the Validation of Disease Testing defines the different types of reference material (Pests and Controls) that are used by the Vegetable and Ornamental Seed Industry to validate results obtained in different disease resistance tests.