White alder trees are native to North America and grow quickly, often reaching 75 to 100 feet in height. They are often used for landscaping and as shade trees because of their large canopy, according to the USDA. While the white alder tree does not have very many natural enemies, those that are a problem can kill your tree if the infection is not controlled. Understanding the diseases of the white alder tree can help you keep your white alder happy and healthy for as long as possible.
Cercospora infections are characterized by brown or gray uneven discolorations on the surface of the leaves. Left untreated, these lesions will enlarge and spread until they have destroyed the leaf, resulting in leaf fall and an overall weakening of the tree. This infection can generally be controlled without fungicides, using sterile pruning and careful, thorough disposal of the plant material in a sealed bag or by burning. Carefully monitor your tree to insure that you find any new disease spots on the leaves for quick removal.
Because the canopy of the white alder tree is so thick, these trees can develop powdery mildew on lower leaves fairly easily if the leaves are frequently wet. Powdery mildew looks like a heavy dusting of black, white, gray or pink chalk dust on the leaves, twigs and branches of the tree. You can control powdery mildew by removing all affected foliage from the tree. In between each cut, wipe down pruning shears with rubbing alcohol. Dispose of the plant material in a sealed garbage bag and remove additional fallen debris from the base of the tree to prevent reinfection.
In California, the flat-headed borer is a serious problem for white alder trees. In the event of a flat-headed borer infestation, the tree will start to develop discolored leaves and will defoliate. You will notice holes in twigs and branches where the beetle has entered the tree. In late summer and fall, it will be easy to spot infested branches because the tree will start to lose its leaves. During this time, the beetle cannot fly, so you can prune away infested branches and dispose of them by burning or in sealed bags to prevent reinfection once the insects develop flight ability.