How Are Plant Diseases Detected and Identified?

Plant diseases are detected by observing symptoms and then identified using a gardening guide or website, laboratory testing, or a test kit containing monoclonal antibodies.

Detecting plant disease

A plant may be diseased if it shows symptoms such as:

• stunted growth
• spots on leaves
• areas of decay or rot
• abnormal growths
• malformed stems or leaves
• discolouration
• the presence of pests

These symptoms show that something is wrong, but they do not always reveal the exact cause. Plants can be infected by viral, bacterial or fungal pathogens, and insects can also damage plants.

For example, tobacco mosaic virus causes a mosaic pattern of discolouration on leaves. Rose black spot causes purple or black spots, after which leaves may turn yellow and fall early.

Identifying the cause

Once symptoms have been detected, the cause can be identified by:

1. Comparing symptoms with a gardening manual or website. This may provide a quick identification.
2. Sending a sample to a laboratory. Scientists can test the infected plant material to identify the pathogen.
3. Using a test kit containing monoclonal antibodies. These antibodies bind to a specific antigen associated with a particular pathogen, so they can give a specific test result.

An antigen is a molecule with a particular shape that an antibody can bind to.

Common exam mistake

Do not assume that every unhealthy plant has an infectious disease. Similar symptoms can result from mineral ion deficiencies. Nitrate deficiency causes stunted growth, while magnesium deficiency causes chlorosis, meaning yellowing of leaves.

Also, remember the difference between the two stages: detecting means noticing symptoms, while identifying means finding the particular cause.

Scope: This answer assumes Higher Tier AQA GCSE Biology. Detection and identification of plant diseases is biology-only content, so it is not required for AQA Combined Science students.