What colour is this "mould"?
Have a look at my photos page, on the bottom row of page 3 is two photos of blight. One is on a potato and the other is of a tomato. If your plant looks like this then I'm afraid there is nothing you can do. Destroy the infected plant and either burn or bin it. Don't put it in your compost heap as it will infect the surrounding soil and can live there for several years.

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