Cause A bacterium, Pseudomonas cichorii, which can survive in soil for some time after the lettuce crop is removed.
Symptoms Dark-brown, firm, necrotic spots a few millimeters in diameter occur on leaves and petioles of lettuce several layers within the lettuce head. Because outer layers of leaves are rarely affected, usually it is impossible to detect the disease without destroying the head.
- Several years of crop rotation to non-hosts.
- Minimizing overhead irrigation the 3 weeks before harvest may help.