Background
Phytophthora diseases are quite common on a number of broadleaf tree species and several species of Phytophthora invade the stems and roots of trees. For example in the UK:
- P. cinnamomi, the cause of ink disease in sweet chestnut
- P. cambivora, the cause of root disease and lower trunk lesions various broadleaf tree species including sweet chestnut, beech, maple and various fruit trees
- P. plurivora (previously known as P. citricola) causes aerial cankers on sycamore
- P. ilicis causes a leaf and twig blight on holly.
However, Phytophthora infection of alder was considered to be uncommon until the discovery of a new Phytopthora causing root collar and stem lesions on common alder.
The alder Phytophthora hybrid
Based initially on studies of the morphological and cultural characteristics and then molecular analysis, it became clear that the alder Phytophthora was a hybrid between P. cambivora and another Phytophthora probably closely related to P. fragariae – a pathogen of strawberry. This new Phytophthora is now spreading across Europe as a ‘hybrid swarm’ which consists of multiple forms or variants of the pathogen. Some of the variants are very damaging, and pose a serious threat to alder and the stability of riparian ecosystems.
The most common and pathogenic form of the pathogen has recently been named as Phytophthora alni subspecies alni, while the other different hybrid variants are collectively known as P. alni subspecies uniformis and P. alni subspecies multiformis.
