Pathogens as causes of plant diseases

A is an organism that causes a disease. There are four main types of pathogen:

All types of pathogen infect a , reproduce (or replicate themselves if a virus), spread from their host and then infect other organisms.

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

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, which caused the deaths of large numbers of people through starvation. The pathogen caused 'blight' - an infestation that discolours the leaves but also rotted the tubers – making the potatoes inedible.

Viruses

A tobacco leaf infected with tobacco mosaic virus — Figure caption,
Tobacco mosaic virus (TMV) chlorosis seen in a backlit tobacco leaf (Nicotiana tabacum)

The tobacco mosaic virus infects tobacco and lots of other closely related plant species like tomatoes and peppers. It is transmitted by contact between plants, either naturally or on the hands of farmers. It infects the of plant leaves and changes their colour from green to yellow or white in a mosaic pattern. It can also make leaves crinkled or curled up. This reduces the plant's ability to and grow properly, which can reduce farmers' crop .

There is no cure, so farmers' only option is to stop their crops from being infected or reduce the spread if crops are infected.

Barley powdery mildew

Figure caption,
A barley plant covered with powdery mildew

The fungus Erysiphe graminis causes a disease commonly called barley powdery mildew. It infects grass plants, including crops like barley and causes a fluffy white growth on the leaves. The disease is common in cooler, damper climates, such as in the UK. Eventually the plant stops being able to make and so cannot photosynthesise to make its own food. This leads to the death of the plant. The fungus produces in order to reproduce. The spores can be spread by the wind. Powdery mildew is treatable using and by removing and carefully destroying infected leaves.

Bacteria

Agrobacterium tumefaciens is a bacterium that causes crown gall disease in plants. This is like cancer and a tumour develops where the bacterium has infected the plant. Often the tumours appear in the stems or roots of the plants. Many plant species can be infected by the bacterium and so it is a huge concern for farmers.

Agrobacterium tumefaciens transfers some of its own DNA to the infected plant cell's DNA. This means that scientists can test the DNA of the infected plant to see if it has been infected by Agrobacterium tumefaciens and confirm that the disease is crown gall disease.

Insects

Green Aphid sucking sap from stem of nettle — Figure caption,
Green aphids suck on the sap of plants, which eventually kills the plant

As well as microbial pathogens, insects can infest a plant. are a common example of these seen in UK gardens and greenhouses. In particular, they infest roses and tomatoes. They are found in large numbers on new shoots, from which they suck sap. This reduces the growth rate of plants and can eventually kill them. Aphids are eaten by ladybirds, so gardeners often try to attract these to their gardens as a natural way of reducing aphids.

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Plant disease - OCR Gateway Pathogens as causes of plant diseases