Plants can be infected by a range of viral, bacterial and fungal pathogens…

The idea

Plants are not attacked by a single type of organism; viruses, bacteria, fungi and insects can all cause disease or damage. Viruses, such as tobacco mosaic virus, are tiny pathogens that reproduce inside plant cells and often cause mosaic patterns or distorted growth. Bacteria are single-celled pathogens that can multiply in plant tissue and cause wilting, spots or soft rot. Fungi, such as black spot, grow on or inside plants and often produce visible spots, moulds or decay. Insects, such as aphids, are not pathogens in the same way but damage plants directly by feeding and can also transmit pathogens. Recognising the broad groups helps a grower predict how a disease spreads and what control method may work.

What earns the marks

• Plants can be infected by viral pathogens, which reproduce inside plant cells and cause symptoms such as mosaic patterns.
• Plants can be infected by bacterial pathogens, which are single-celled and can cause wilting, spots or rotting tissue.
• Plants can be infected by fungal pathogens, which grow on or in plant tissue and often produce visible spots or decay.
• Insects can damage plants directly by feeding and can also act as vectors that transmit pathogens.
• Different pathogen groups have different structures and reproduction methods, which affects how disease spreads and is controlled.
• A single plant can be affected by more than one type of pathogen or pest at the same time.

Common mistakes and how to put them right

• Treating insects as a type of pathogen.

  Put it right: Insects are animals that damage plants and can carry pathogens, but they are not themselves viral, bacterial or fungal pathogens.

• Assuming all plant diseases are caused by fungi.

  Put it right: Viruses, bacteria and insects also cause disease or damage.

• Believing bacteria and viruses are the same size and treated the same way.

  Put it right: Viruses are much smaller and reproduce inside cells, while bacteria are cells that can often be treated with antibiotics in other contexts, though plant control differs.

Exam tips

• Name the group and give one example or symptom for each type of pathogen or pest.
• Use the term vector correctly when describing insects that transmit pathogens.
• If asked to compare groups, focus on one clear difference such as cell structure or method of reproduction.