Pathogens as causes of plant diseases

A pathogen is an organism that causes a disease. There are four main types of pathogen: viruses, bacteria, fungi, and protists. All types of pathogen have a simple life cycle. They infect a host, reproduce (or replicate themselves if a virus), spread from their host and then infect other organisms.

Diseases caused by pathogens are called communicable diseases. This means they can be caught. The spread of communicable diseases is called transmission.

Plant pathogens kill or reduce the growth of many plants, which in turn can reduce biodiversity. Plant pathogens of food crops are especially serious, such as the Irish potato famine around 1850. A fungus-like species infected the potato crops of Ireland with a disease called potato blight. Potatoes were the main source of food for many people at this time. This disease caused the deaths of approximately one million people.

Chalara ash dieback

A plant affected by ash dieback disease
A plant affected by ash dieback disease

A species of fungus called Hymenoscyphus fraxineus infects ash trees. Infected plants suffer the following symptoms:

  • leaf loss
  • die back of new shoots
  • bark lesions

This species of pathogenic fungus is relatively new to the UK. It was first found in 2012. The fungus spreads through the air, so we call it an airborne infection. Since 2012 it has spread to about one third of all large woodlands in the UK.

Scientists are not yet sure how quickly this fungus will spread or what its full impact will be on our ash population. They are trying to identify the genetic factors that mean that some ash trees are naturally resistant to the fungus.