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.

Detecting and identifying plant disease

• A plant pathologist is an expert in plant disease.
• Detection usually starts with observation in the field, and moves to the laboratory when field observation isn't enough.

Detecting disease in the field

• The pathologist recognises the signs and matches them to a disease - for example by looking them up in a gardening manual or website:
  • Growths on the stems might point to crown gall disease.
  • Mottled leaves might point to tobacco mosaic virus.
• The distribution of diseased plants is a clue to the pathogen, because different pathogens spread in different ways:
  • Patches of diseased plants suggest a pathogen spread through the soil.
  • Plants diseased at random suggest an airborne pathogen.
• A sign such as yellow leaves may not be caused by a pathogen at all, but by an environmental problem such as a nutrient shortage. This can be ruled out by changing the conditions - e.g. adding nutrients to the soil - and seeing whether the symptoms improve.

Identifying the pathogen in the laboratory

• When field observation isn't enough, an infected sample is taken to a laboratory to identify the pathogen accurately.
• A microscope can be used to look at the sample in more detail, and to tell apart pathogens that look alike (such as similar-looking fungi) and can't be distinguished by eye.

Two laboratory methods identify a pathogen from molecules unique to it:

1. Detecting the pathogen's antigens - Antigens are the unique molecules on a pathogen's surface. Antibodies that bind to a particular pathogen's antigen show it's present. The ELISA test uses such antibodies linked to an enzyme that gives a colour change when the antigen is found.
2. Detecting the pathogen's DNA - A technique such as the polymerase chain reaction (PCR) copies pathogen DNA from the sample many times over, until even a small amount can be detected - identifying exactly which pathogen is present.

Physical plant defences

• Plants don't have an immune system of specialised cells and antibodies like animals do.
• Instead, they use physical defences - barriers that stop pathogens getting into the plant in the first place.

The two physical barriers

1. Waxy cuticle - A tough layer covering most leaves and stems. It blocks pathogens from entering, and stops water collecting on the leaf - which lowers the risk of infection by pathogens carried in water.
2. Cellulose cell wall - Every plant cell is surrounded by a cell wall made of cellulose, sitting beneath the cuticle. It forms a further barrier against any pathogen that gets past the cuticle.

Chemical plant defences

As well as physical barriers, many plants make chemical defences - substances that either kill pathogens or deter the animals that would eat the plant.

Chemicals that kill or limit pathogens

• Antimicrobial chemicals (also called antibacterials or antiseptics) - Kill or stop the growth of bacteria and fungi. Examples include mint and witch hazel.
• Phytoalexins - Antimicrobial substances produced in response to infection that help limit the spread of the pathogen.

Poisons that deter herbivores

• Some plants produce poisons to deter animals that would eat them.
• Examples include tobacco plants, foxgloves, and deadly nightshade.
• The poison digitalis comes from the foxglove.