A pathogen is an organism that causes a disease. There are four main types of pathogen:
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.
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 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.
The tobacco mosaic virus infects tobacco and lots of other closely related species like tomatoes and peppers. It is transmitted by contact between plants, either naturally or on the hands of farmers. It infects the chloroplasts 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 photosynthesise and grow properly, which can reduce farmers' crop yields.
There is no cure, thus farmers' only option is to stop their crops from being infected or reduce the spread if they have.
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 can drop off the plant. This reduces the plant's ability to photosynthesise and reduces growth. Black rose spot can be transmitted in air or water as well as direct contact by gardeners. It is treatable using fungicides and by removing and destroying infected leaves.
As well as microbial pathogens, insects can infest a plant. Aphids 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 that growth rate of plants and can eventually kill them. Aphids are eaten by ladybirds, so gardeners often try to cultivate these in their gardens as a natural way of reducing aphids.