Paintedlady - If you want to be organic you will have to cut the tops off when blight strikes.
I don't use it & never have, nor ever used dithane, I merely answered someone's question to what "used to be" the spray for preventing blight
Touch wood, I've been fortunate not to have had potato blight or it has appeared when the crop was naturally dying back anyway so the haulms could be cut right back with no harm to the crop. Tomato blight on the other hand .....
BTW, I've never claimed to be organic though I do try to use methods that avoid unnecessary use of chemicals unless more drastic action needs to be taken and even then, as a last resort especially for long term projects and with great care and attention to minimise harm to beneficial organisms. As for it being banned, a quick google and it still seems to be available
http://www.hardware-ironmongers.com/details.aspx?code=7740262 (I'm not advertising here and have nothing personal to gain from posting this address!) From a little research, it would seem making your own has been banned not the commercial product.
The harmful component of Bordeaux Mix is the copper sulfate - copper (metal) has always been an effective fungicide but sadly the sulfate compound can leach out and into water courses - in large quantities this is toxic to the environment particularly fish and earthworms, so I would agree that caution should be taken when using it if you have to or want to. The most sensible alternate to using chemicals is to look out for potato varieties that have a higher resistance to diseases in particular blight
This goes for tomatoes and fruit varieties at risk from fungal diseases eg mildew resistant gooseberries etc
PS I've only just noticed that someone had felt the need to point something out to me personally hence my apologies for hauling this thread out of the archives ... hopefully I've cleared up the confusion on this matter now.