Pathogens as causes of plant disease

• Pathogens are disease-causing viruses, bacteria, fungi or protists which attack plants as well as animals.
• Plants have physical and chemical defences against pathogens.
• Pathogens can cause diseases in plants, which can reduce biodiversity and affect food security.
• Pathogens of food crops, such as the Irish potato famine around 1850, can cause deaths of large numbers of people through starvation.
• The pathogen caused 'blight' - an infestation that discolours the leaves but also rotted the tubers making them inedible.
• Aphids infest roses and tomatoes, found in large numbers on new shoots, from which they suck sap, reducing the growth rate of plants and can eventually kill them.
• The tobacco mosaic virus is transmitted by contact between plants, either naturally or on the hands of farmers.
• Rose black spot fungus causes black and purple spots on leaves.
• Aphids are a common example of insects that infest plants, seen in UK gardens and greenhouses.
• The tobacco mosaic virus can also make leaves crinkled or curled up, reducing the plant's ability to photosynthesise.
• Rose black spot is caused by a fungus which infects roses.
• The rest of the leaves often turn yellow and can drop off the plant, reducing the plant's ability to photosynthesise and reducing growth.
• Rose black spot infects leaves and causes black or purple spots on the leaves.
• Black rose spot is treatable using fungicides and by removing and destroying infected leaves.
• Aphids (greenfly) are tiny insects that feed on phloem from plants.
• The tobacco mosaic virus changes the colour of plant leaves from green to yellow or white in a mosaic pattern.
• The tobacco mosaic virus infects the chloroplasts close to the chloroplast.
• The tobacco mosaic virus contains the green pigment chlorophyll; the site of photosynthesis.
• Black rose spot can be transmitted in air or water as well as direct contact by gardeners.
• The tobacco mosaic virus infects tobacco and lots of other closely related species like tomatoes and peppers.
• Aphids are eaten by ladybirds, so gardeners often try to cultivate these in their gardens as a natural way of reducing aphids.
• Tobacco mosaic virus (TMV) causes chlorosis seen in a backlit tobacco leaf (Nicotiana tabacum).
• There is no cure for the tobacco mosaic virus, thus farmers' only option is to stop their crops from being infected or reduce the spread if they have.