It is important to be aware that some symptoms in crops which may look like a disease can have non pathogenic causes, for example, nutritional deficiencies or toxicities, poor growing conditions and damage from chemicals (crop burn).
When plants are grown under optimal conditions, they tend only to be at risk from pathogens that can only develop in living tissues (obligate parasites) such as powdery mildew. However, when plants are stressed, they become more susceptible to a wider range of pathogens including ones that have the ability to survive under different conditions and which can survive on non-living organic matter (facultative parasites) such as Botrytis and Pythium.
Effective management of diseases depends on being able to quickly recognise disease symptoms and correctly identify the cause. Be careful when attempting to diagnose diseases from pictures or descriptions – you may have an uncommon or new disease that superficially looks like another or the symptoms may be a result of growing conditions or factors.
At the first sign of an unfamiliar symptom, sufficient samples should be sent for diagnosis by an expert. Not only will this ensure that you apply the correct and lawful method of control, but new diseases can be identified.
Correct diagnosis requires a plant sample that shows the full range of symptoms present on the crop. When sending plant specimens, include several affected plants that range from early symptoms on younger plants to some with severe or 'full-blown' symptoms. It is useful to also include an apparently healthy plant.
Samples should, ideally, consist of whole plants including roots and some attached growing substrate. Keep samples as fresh as possible by wrapping in moist newspaper. Refrigerate them if they are to be kept overnight. Hand deliver, courier or send the sample by Express Post. Clearly mark your name and address on the package. Material dispatched by post or courier should be sent early in the week to avoid being held up during the weekend.
Further details and contact information: Plant Health Diagnostic Service