Plants can be infected by viral, bacterial and fungal pathogens, and by insects. You need three examples. Tobacco mosaic virus is a viral disease that leaves a mosaic of discoloured patches on the leaves, so less photosynthesis happens. Black spot is a fungal disease that causes dark spots on the leaves of plants such as roses. Aphids are small insects that feed on the plant and are pests.
Plants can also be damaged by a lack of ions in the soil. Nitrate ions are needed to make proteins, so a lack of nitrate causes stunted growth. Magnesium ions are needed to make chlorophyll, so a lack of magnesium causes chlorosis, where the leaves turn yellow.
Plant diseases can be detected from signs such as stunted growth, spots on leaves, areas of decay (rot), growths, malformed stems or leaves, discolouration and the presence of pests.
The cause can then be identified in three ways. You can look it up in a gardening manual or on a website. You can take the infected plant to a laboratory to identify the pathogen. You can also use a testing kit that contains monoclonal antibodies, which bind to one particular pathogen.