There are various fungicidal products available to spray potato plants to prevent blight. However, as we grow all our vegetables organically in Stephanie’s Kitchen Garden, I am regrettably unable to comment on their performance or recommend a particular one.
We prefer to try and avoid the issue by growing varieties that are more resistant to blight as well as making sure our plants are strong and healthy in the hope that they will be less susceptible to an attack. We try, wherever possible, to ensure our crop has consistent levels of water by irrigating them in dry spells. If you are watering your potatoes, it is advisable to use soaker hoses or irrigation pipes in order to prevent water splashing the foliage of your plants.
I would recommend signing up to Blightwatch.co.uk. This is a really useful website that will warn you of cases of blight in your area, as well as notifying you when weather/humidity conditions are liable to lead to blight occurring. This will also give you advance notice of when to apply fungicide, should you wish to do so. You can also register your own instances of blight to help alert other growers. There is also useful information regarding blight on the website.
I am sorry to not have any definitive solutions for you, but wish you a good blight-free potato growing season.
If I can be of any further assistance, please do not hesitate to contact me.