Over the years various terms and definitions have been used to describe the interaction between plants and the pests or pathogens which attack them causing disease. The seed industry has now agreed to define a number of standard terms aimed at characterisation of this reaction. It is important that all people in our industry recognise and understand what these terms mean when describing plant and pest reactions. These terms are set out below:-

Immunity: Indicates plants would not be subject to attack or disease infection by a specified pest or pathogen.

Resistance: Due to the complex nature of plant pest reactions, resistance has been defined into two levels. These are highly resistant and intermediate resistant. Resistance is described as the ability of a plant variety to restrict the growth and development of a specified pest or pathogen. Resistant plant varieties restrict the damage that pests or pathogens cause when compared to susceptible plant varieties under similar growing conditions and disease pressure. It should be noted that resistant varieties may exhibit some disease symptoms or damage under heavy pest pressure.

Highly resistant plant varieties would highly restrict the growth and development of the specified pest or pathogen under normal growing conditions and pest pressure when compared to susceptible varieties. These plant varieties may, however, exhibit some symptoms or damage under heavy pest pressure or stressful growing conditions.

Intermediately resistant plant varieties would restrict the growth and development of the specified pest or pathogen.

The plan variety may however exhibit a greater range or degree of symptoms when compared to a highly resistant variety. Intermediate resistant plant varieties will still show less severe symptoms or disease damage than susceptible plant varieties when the crop is grown under similar environmental conditions and is subjected to the same disease or pest pressure.

Susceptibility: Indicates the inability of a plant variety to restrict the growth and development of a specified pest or pathogen. Tolerance will no longer be used to describe disease and pest reaction.

The international seed industry is also currently working towards standardising the way in which diseases are abbreviated in published material. The aim is to ensure abbreviations for diseases are the same all over the world, so that anyone involved in the industry is able to recognise which disease resistance is being claimed. An example of the change would be Black rot in brassica where BR has been changed to Xc. Another example is the replacement of DM for downy mildew in lettuces to Bl. Once this list has been finalised it will be released to the industry.