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Plant diseases - viruses and fungi

A is an organism that causes a disease. There are four types of pathogen that cause diseases in plants:

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

Plant pathogens infect a , reproduce inside the host - or replicate themselves in the case of a virus - then spread from their host to infect other individuals.

Plant pathogens kill or reduce the growth of many plants. Plant pathogens of food crops usually reduce and can be especially serious.

Viruses

A tobacco leaf infected with tobacco mosaic virus
Figure caption,
The pattern produced by tobacco mosaic virus (TMV) in a tobacco leaf (Nicotiana tabacum)

The tobacco mosaic virus (TMV) infects tobacco plants and other closely related species, such as tomatoes and peppers.

TMV is transmitted by contact between plants, either naturally or on the hands of farmers or contaminated clothes or tools.

TMV infects the of plant leaves and reduces the plant's ability to . The leaves change colour from green to yellow or white and develop a mosaic pattern. It can also make leaves crinkled or curled up. This and the reduction in photosynthesis means that the plant will not grow properly, and will reduce farmers' crop yields.

Fungi

A leaf infected with rose black spot fungus
Figure caption,
Rose black spot fungus causes black and purple spots on leaves

Rose black spot is caused by a fungus which infects roses. It infects leaves and causes black or purple spots on the leaves. The rest of the leaves often turn yellow and drop off the plant. This reduces the plant's ability to photosynthesise and therefore reduces growth.

Rose black spot is spread by the spores of the fungus that are produced in the black spots.

The spores can be transmitted in air or water - blown by the wind or in raindrops - as well as being transferred from plant to plant by gardeners.