Introduction

Confirmation that a plant problem is due to attack by a pathogen, and the identification of the causal agent, are important prerequisites to the selection of control measures. The extent to which disease has progressed also needs to be considered. Diagnosis of disease in the early stages of infection is important with pathogens which have a long latent period prior to symptom expression. Examples of such diseases include septoria on wheat and viruses in seed potatoes, whose early detection may enable control measures to be implemented before they reach damaging levels.

Diagnosis is based on one or more unique features of the causal agent or disease. These may include the appearance of the disease in the field, the morphology of an isolated microorganism, the unique immunological characteristics of the causal agent or the structure of its genetic material.

IDENTIFICATION BASED ON SYMPTOMS

Symptom assessment is the most common method of disease identification. In many cases, as seen in Chapter 2, characteristic symptoms are associated with particular diseases and these are often reflected in the common name of the disease. Accurate diagnosis of disease from symptoms often enables instant decisions to be made regarding the application of control measures – especially the selection and deployment of fungicides. Indeed, the importance of accurate diagnosis of disease is reflected by the fact that, in the UK, personnel undertaking the British Agrochemical Standards Inspection Scheme (BASIS) examinations must demonstrate a very high degree of proficiency in the identification of diseases on the major crops of the UK.