Sunday, August 1, 2010

Sudden Oak Death: An impending threat?

Sudden Oak Death is caused by a fungal-like pathogen called Phytophthora ramorum and has hit the headlines again recently when the disease spread across the Bristol Channel to South Wales.

First found in Britain in 2002 (where it was infecting Viburnum tinus in a garden centre), it has been causing problems in the US since the mid-nineties.

In 2000, researchers at the University of California identified the cause of dying oak trees in many parts of the state to be a previously unknown species of Phytophthora. Another Phytophthora species, P. infestans, causes late blight in potatoes- a topic we have covered previously on this blog.

Alternative hosts for P. ramorum include rhododendron, viburnum, bay laurel, douglas fir, redwoods, yew, horse chestnut, beech.

The conclusive link between the pathogen and sidden oak death was not made until work published in 2002 (Phytophthora ramorum as the cause of extensive mortality of Quercus spp. and Lithocarpus densiflora in California. D.M. Rizzo, M. Garbelotto, J.M. Davidson, G.W. Slaughter, and S.T. Koike.Plant Disease 86: 205-214. 2002).

By 2005, the pathogen had been found in Northern Ireland.

However, despite the problem in Europe and North America arising around the same time, research has shown that the mating-type from the American isolates (called A2 type) are different to those found in Europe (A1 type). This indicates that the pathogen did not come from North America to Europe or vice-versa. It is thought to have originated in an as-yet unidentified third country.

The pathogen can cause cankers of the bark or can cuase damage to the leaves. Depending on the host plant, differing symptoms are visible.

The pathogen may reproduce by sexual or asexual means. To reproduce sexually, both the A1 and A2 mating types (i.e. the European and American types) need to be present together and, apart from a single exception) this has not been the case up until now. Hence, the spread of the pathogen is generally via the production of asexual spores (called zoospores) which are spread by rain and wind.

In order to eradicate the pathogen in California, authorites cut and burn infected trees along with all other host plants within 100 feet. Controlling the spread of infected (or potentially-infected plant material) is crucial.

Control of the disease in Ireland is governed by European phytosanitary legislation and despite it being found on rhododendron and viburnum species in parts of the country, it has yet to be found on trees. With trees now infected Wales, how long until it spreads across the Irish Sea?

Spread of P. ramorum in Europe (2006)


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