5.5 - Plant Diseases

Plant diseases and their signs

• Plants can be infected by the same types of pathogen as animals: viruses, bacteria, and fungi.
• They can also be infested and damaged by larger organisms, such as insects.
• It's usually fairly clear when a plant has a disease, because diseased plants show characteristic signs.
• Different diseases produce different combinations of these signs - that's what makes it possible to tell one disease from another.

Ash dieback - A fungal disease

• Ash dieback is a disease of ash trees caused by a fungus.
• It's carried from tree to tree through the air by the wind.
• It produces leaf loss and lesions on the bark, and eventually kills the tree.

Common signs of plant disease

• Stunted growth
• Spots on the leaves
• Patches of decay or rot
• Abnormal growths, such as lumps
• Malformed stems or leaves
• Discolouration

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 kill pathogens or deter the animals that would eat the plant.

Antimicrobial chemicals

• Some plants produce antimicrobial chemicals (also called antibacterials or antiseptics), which kill or stop the growth of bacteria and fungi.
• Examples: mint and witch hazel.

Poisons to deter herbivores

• Other plants produce poisons that deter herbivores.
• Tobacco plants, foxgloves, and deadly nightshade all make poisonous chemicals.
• The poison digitalis comes from the foxglove.

Plant chemicals as human medicines

• Quinine - Comes from cinchona bark and has long been used to treat malaria.
• Aspirin - Comes from willow.