Bay trees are susceptible to Dutch elm disease, a common disorder of shade and ornamental trees. The disease is caused by Ophiostoma ulmi fungus that is transmitted to trees by bark beetles. Though more frequent in elm trees, the fungus has a wide host range, including bay.
The disease spreads within the water-conducting tree tissues. Initial symptoms include curling, wilting and yellowing foliage that eventually turns brown. The damage starts in the upper area of the tree and progresses downward. Branches start to wilt, and there are streaks on bark and sapwood. Brown foliage remains attached to the tree.
Irrigate trees well during periods of high heat. Plant resistant tree species. In case of light damage, prune and remove all infected tree areas to about 10 feet of non-infected wood. Bury or burn infected pruned tree areas.