Also be wary of accepting onion and cabbage plants from a friendly neighbour, they may well carry the dreadful diseases of onion white rot and clubroot. You will never be able to get rid of these diseases so err on the side of caution and 'just say no'. If you have an allotment with either of these diseases then don't even use the same tools or boots in your own garden because you will spread them. So to all the beginners who thought they were just going to do a straight swap from conventional to organic gardening by simply changing their sprays it is not quite so simple. Planning, forethought, observation and vigilance make for success in the organic garden.
Prevention is better than the cure.