If you have a relatively small area devoted to potatoes it is possible to minimise the effect of blight. First of all, you have to be able to recognise the symptoms at an early stage. This means walking around the plot each day, looking closely at the foliage. Usually, during May and June, it will be a healthy green colour. But at some point towards the end of June or the beginning of July you will notice on some of the leaves (which still look healthy) some tiny brown freckles. This is the first indication of blight, but if you treat it whilst the plants are still young and vigorous you can minimise the effect. If you ignore the freckles they will soon grow into brown blotches - the classic symptom of blight.
As soon as you notice any affected leaves, remove them. This will not stunt the plant, because the younger leaves beneath will be more exposed to the light and will replace them. As soon as you have removed the leaves (and there should only be a few, if you have been vigilant) you must make up some Bordeaux mixture and give the whole plot a thorough spraying, especially the infected area.
You now have to go around the potato plot, once a day; Bordeaux mixture in one hand and a bucket for blighted leaves in the other. Remove all diseased leaves as you see them, and any yellow ones from lower down. If you do it every day, blight will not get a hold, its spores will not grow on leaves which are dosed with copper. Don't leave the blight until symptoms are obvious. If you do, the plants won't be vigorous enough to replace the leaves you remove, the Bordeaux won't help, and your plants will have a shorter life than they should, with a corresponding effect on yield. The affected leaves should be put in the dustbin, not the compost heap.
Incidentally, if you also grow tomatoes - they also suffer from blight; any dubious leaves should be removed and the plants sprayed with Bordeaux.
Nigel Deacon / Diversity website
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