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The blight we know as vegetable growers is a fungus, not a virus, which is carried in the air and deposited onto wet leaves where it quickly spreads through the whole plant.
It survives on infected plant material and is persistent because it can overwinter successfully in most of the UK (Scotland and parts of Northern England escape it) on heaps of infected plants in field margins or elsewhere. It starts its spread when a certain temperature and humidity are achieved for a period of time - these can be predicted and farmers can spray preventative chemicals.
There are different types of blight but the one that most commonly gets our potatoes and tomatoes in the UK is "late blight" - phytophthora infestans.
Unless there is poor hygiene in a garden on allotment site (i.e. infected material not properly disposed of), the blight is blown in new every year - there's probably a source nearby such as a farm. If you save potatoes which are infected and use them the next year you will continue the lifecycle of the disease.
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