Answer: Based on your description it is very difficult to make a positive identification of the problem. There is a possibility that it is a bacterial blight which spreads during wet weather such as we have had this spring. However, since you did not mention any discoloration or other symptoms on the leaves, I suspect the culprit is slugs.
If this is the case, there are several methods of eliminating the slugs. You can hand-pick them (just after dark or very early in the morning work best) and drop them into soapy water, trap them with saucers of beer, or use a commercial slug bait.
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