Hi all. Potato blight (phytophorus Infestans) is always going to be a problem on allotments. The issue is little control when it does appear because there's little we can do organically, poor hygiene and rotations too close together. Last year was a particularly bad year due to heavy rain and warm days causing long periods of high humidity. This in turn causes the blight to multiply rapidly. Before your potatoes emerge this year, make sure there are no old tubers or potato tops on your plots or your neighbour's plots or in compost heaps nearby. If there are, burn them. If you have access to a flame gun, run over the ridges before the shoots come through. This will help to keep the spores down. Have a look upwind from your plot and check for discarded potatoes from last year and ask if they can be destroyed too.
This is about all that you can do organically - be watchful if we get similar weather this year as we did last and at the first signs, cut off and burn the affected foliage. A good mulch over the ridges will help too.
Hope this helps.
