You need to check your potato and tomato crops regularly and take action if you spot the problem. You can't really mistake it. There are pictures in the Shop and shown below.
If you see any signs on tomato plants you can remove the offending parts and spray all of the clean remainder with a fungicide such as Dithane (available in the Shop). If you have a good clean tomato crop then you may wish to spray anyway as a preventative. Once the problem has reached the main stem then it’s too late to do anything other than remove the plant.
Blight cannot survive in soil or fully composted plant material. It over-winters in living plant material and is spread on the wind the following year.
If you see any signs on your potatoes then you can remove odd leaves but beyond that all you can do is cut the foliage to the ground to save your crop.
Don't put affected foliage on your compost heap or leave it lying around - bin it or burn it.
Please note - this problem spreads rapidly and it's not fair on other plot-holders if affected crops are left in the ground. If a plot is seen to be infected and rapid action isn't taken by the plot-holder then our rules allow us to intervene.