Both Common and Italian Alders are susceptible to Phytophthora disease of alder, caused by the oomycete (water mould) Phytophthora alni. P. alni was first discovered in Britain in 1993. The symptoms are described on the Forest Research website and include small, yellow and sparse leaves, thinning of the tree crown and a tarry bleeding of the trunk.

There are now several Phytophthora diseases of trees in the UK, with species affected including Japanese Larch (P. ramorum), Beech (P. kernoviae), Lawson’s Cypress and Yew (P. lateralis), Juniper (P. austrocedrae), Sweet Chestnut (P. cambivora and P. cinnamomi), Holly (P. ilicis), Beech and Sycamore (P. plurivora). The website Forest Phytophthoras of the World lists several more and there is an online global Phytophthora database as well. Phytophthora ramorum in particular is proving to be very destructive and in three years more than three million larch trees have been felled in the United Kingdom, in an attempt to prevent its spread. In September 2012 the disease was discovered in Sussex and Surrey for the first time (see press release).

A mild outbreak of Potato Blight, Phytophthora infestans

The transport of wood and wood products around the world has helped these diseases to spread and new hybrids have formed as new species have been brought into contact with one another. (For example,  Phytophthora alni is a hybrid between P. cambivora and another Phytophthora probably closely related to P. fragariae, a pathogen of strawberry (reference).)

The most infamous Phytophthora species is Potato Late Blight, Phytophthora infestans, pictured left. The infamous outbreak in Ireland in 1845–57 caused mass starvation and the disease continues to affect potato crops today.

Over 100 species of Phytophthora have been descibed so far and several hundred more probably exist. The genus derives its name from the Greek phytón (plant) and phthorá (destruction), so literally means “the plant-destroyer” – an apt name indeed.

When I was at university, oomycetes (water moulds) were thought to be fungi. Nowadays they are recognised as a distinct group of eukaryotic organisms,with a morphology like fungi, but with genetic similarities to brown algae and diatoms.