2.5 - Plant Diseases

Plant diseases and their signs

• Plants can be infected by pathogens, just as animals can: viruses, bacteria, and fungi.
• They can also be infested and damaged by larger organisms such as insects.
• Several pathogens are specific to plants.

Examples of plant diseases

• Tobacco mosaic virus (viral) - Gives a mottled, discoloured pattern on the leaves of many plants.
• Ash dieback (fungal) - A disease of ash trees, carried from tree to tree through the air by the wind. It produces leaf loss and lesions on the bark.
• Crown gall disease (bacterial) - Caused by Agrobacterium tumefaciens, which lives freely in some soils and on plant roots:
  1. When the bacteria get into a plant, they make growths (tumours) called galls form on the roots, stems, and branches.
  2. These galls damage the plant's tissues and restrict the flow of water through it, so the plant becomes weaker and may eventually die.

Signs of disease

• Diseased plants show characteristic signs, so it's usually fairly clear when a plant is diseased. Common signs include:
  • Stunted growth
  • Spots on the leaves
  • Patches of decay or rot
  • Abnormal growths, such as lumps
  • Malformed stems or leaves
  • Discolouration
• Different diseases produce different combinations of these signs - this is what makes it possible to tell one disease from another.