White alder, known botanically as Alnus rhombiflia, is a deciduous tree related to birch that is native to North America. It is a fast-growing tree that reaches up to 100 feet in height at maturity under ideal growing conditions with a canopy spread of up to 40 feet.
While not known as a tree widely susceptible to life-threatening diseases, white alder can suffer from several common maladies affecting the appearance and performance of broad leaf trees.
White alder is most susceptible to the Nectria bacterial canker strain, which causes wounds and die-back on woody tissues that can weep or callus over. It can show itself in discolored or wilting leaves and off-season or premature leaf drop. Fungi may bloom at the infection sites on both leaves and branches. Large or deep cankers can kill branches and trunk tissue and the entire tree if advanced. Bacterial canker tend to invade stressed or compromised tree tissues.
Wood rot can occur in the root zone as well in the trunk and limbs of white alder. Often this is an invisible disease until it becomes advanced enough to disrupt the tree's systemic flow of water and nutrients through the wood, choking off life support to the tissues. Branches and limbs can drop their foliage and wither.
Mushrooms, shelf fungi and conks, or white mold-like spots of rot, can also appear at the base of the tree or along the root path and is a sign of active infection and decay. Also a fungal disease, wood rot seizes upon damaged, wounded or stressed trees and enters the tissues at a weak point.
Rust & Leaf Spot
Rust and leaf spot are fungal diseases that can distort the size, shape and color of white alder catkins and foliage. Thin, dry red to yellow crusts will form splotches on the lower leaf, and the top of the leaf can discolor entirely and dot the catkin tissues. Leaves from infected branches and trees often drop prematurely, causing a creeping defoliation pattern. In heavy infestations, cankers can form on the woody tissues, causing die-back.
When catkins are infected, they can become stunted and gnarled instead of hanging straight. The tree is most significantly affected by woody tissue infections, as those are the versions of the fungus that can lead to tree death.