Alder root and collar rot (Phytophthora alni) is a fungal pathogen of alder trees. It is a native of Europe.
Infected trees have small, yellow leaves which fall prematurely. Dieback in twigs and branches of the crown may be the first symptoms of the disease. Tree death may occur quickly, but in many cases tree health deteriorates over many years. Alder dieback has spread steadily throughout Europe since 1994.