Yes, definitely possible. We've also had a very wet spring. I've just never encountered this with lettuce before... All the plants that died, except the Romaine, grew from self-seed volunteers from last year's plants. The Romaine was from starts I bought at the feed store about a month ago. It has seemed very healthy and ready for harvest. I was surprised to see that one plant just collapse overnight.Is there any possibility it could be just root rot? If they all start dying then I would think that it is fusarium with the description.