Answer: Over-watering which results in soggy soil and extended periods of wet foliage which can promote decay in a number of garden plants. I should also point out that the head lettuces are difficult to grow successfully in your area. However the leaf lettuces do stupendously!
For best results with lettuce, mix lots of compost into your clay soil prior to planting. Plant on slightly raised beds to divert excess water away from the plants. Select a leaf type lettuce, and water well every few days rather than daily.
Thanks for the question and good luck with your garden!
Q&A Library Searching Tips