Blight, a fungus that causes brown spots then kills off the leaves, is incurable. It also affects tomatoes.
Once present, in the right conditions, it only takes one or two days to decimate the crop.
If left it spreads to the potatoes themselves, creating a stinking squidgy mess. It also spreads like wildfire across the neighbourhood if left - so, for the sake of your neighbours, keep a check!
However, warnings of Blight conditions (warm nights and humid air) are advertised on the noticeboard, here on the website, or directly if you can access the 'Blightwatch' website.
Spraying Bordeaux mixture gives some preventative protection but is not a cure.
Alternative sprays suggested in various places are:-
Aspirin @ 1 tablet to 1 gallon/4.5 litres
Compost liquid ?Comfrey/Nettle tea @1:20 dilution
Milk (semi-skim) @ 1:8 dilution
Milk spray is recommended weekly, the others at least fortnightly, or you might wait for the Blightwatch warning. Some recommend a drop of detergent to help the spray stick to the leaves. Spray until dripping wet - and don't forget the lower leaves!
If Blight strikes the odd leaf you can try removing affected leaves and destroying them (burn or take to the tip). With a general attack some advise you to cut off the top growth, wait a fortnight for the fungus spores to die off and then harvest whatever you have got. (You can dig up some if you don't want to store them
Google 'Blightwatch' to get the technical info and, if you wish, register for Blight warnings.
An American site publishes a 'scientific' paper that gives lots of detail - if you want to know the up to date knowledge on Blight.