Bay trees, Laurus nobilis, also called bay laurel, sweet bay or laurel tree, are probably most popularly known for their leaves that are used in cooking, primarily in soups and stews to give them flavour. Bay laurel trees also are famous for being honoured in ancient Greece and Rome. Emperors, poets and other important people would wear wreaths around their heads made of laurel branches. Bay trees are tall and originate from the Mediterranean area. The leaves are somewhat ovoid shaped with serrated edges. Although bay trees are hardy, they can still be afflicted with diseases.
Bay Sucker Disease
The main disease that affects the bay tree is the bay sucker. This is a small jumping louse that sucks the sap from the tree. It causes the leaves to curl, die and then fall off. Most of the time, the bay tree will not die from an infestation, but it does weaken the tree. The best way to cure a bay tree of the bay sucker lice is to pick off the leaves as soon as you notice them curling. Make sure to dispose of them properly so any lice eggs do not fall off into the soil or onto other parts of the tree. A spray solution made of dishwashing detergent and water will work very well to keep the lice off the leaves. Spray the solution back side of the leaves.
A fungus is the cause of shot hole disease or coryneum blight. The fungus creates spots on the leaves that look black, dark purple or red. The spots will eventually eat all the way through the leaf, leaving a hole in the middle of the leaf. The fungus will thrive in warm and wet summers. Unfortunately, once a bay tree is infected, there is not too much that can be done to cure it. The fungal spores will live safely in the winter time inside the buds of the tree or inside little cuts in the bark or branches. When the weather warms up and rain falls, the water splashes onto the cuts or buds and spreads the spores to other parts of the tree or to nearby trees. The good thing about shot hole disease is that the fungus will only live on the tree's new growth, not the older areas. To avoid contamination of neighbouring trees, make sure sprinkler systems do not hit the upper leaves and pluck off any infected leaves or branches.
Powdery mildew is caused by a fungus that makes white powdery areas on the leaves of the bay tree. The fungal spores are spread primarily by wind, which can spread them to the rest of the tree or to nearby trees. At first, the fungus will be in small patches, but as it spreads, the powdery mildew will grow into large patches. If a bay tree is completely infected with powdery mildew, its growth can be affected or the leaves can die and fall off much too early. This is because the fungi are sucking the nutrients out of the leaves. If the bay tree leaves are used for cooking, then this condition can make it unusable. Powdery mildew can be controlled with a fungicide, but not if you plan to use the leaves. In this case, hand picking of the infected leaves is the only viable solution.