We have a 12 x 16' backyard greenhouse, unheated, where we attempt to grow lettuce through the winter months. Each year we are having increasing problems with a disease. The plants turn to mush right at the base where the leaves meet the roots. In 24 hours they go from a healthy plant to a very sick one. Is there anything to help? I usually start the lettuce in flats and then transplant them so that the plants are spaced far enough apart. Help!