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how do you know you have blight?  ty

The main problem seems to be distinguishing blight from normal die back of the haulms.

Here's a picture of blight on a leaf - it's a lesion with a pale surround rather than just spots of brown.

http://www.potato.org.uk/department/knowledge_transfer/pests_and_diseases/ref.html?item=12

Other pictures

http://www.blackthornarable.co.uk/photography/potdis1.htm
(click on photos to enlarge)

It moves to the stem very quickly, turning it brown and from there to the tubers. Some people say it has a distinctive smell.

With blight, you very soon see patches of dead dissue on the leaves. In dry weather, it gets no worse, but in wet, the whole soon begins to rot. I'm not so clear about the picture with toms, but some potato varieties are very much more susceptible than others.

thank you both for the info
liza x