SUDDEN OAK DEATH

Sudden Oak Death (SOD), caused by a new species of Phytophthora pathogen, is active and spreading on certain oak species in coastal California. It is now also known to occur in Germany and the Netherlands on rhododendron and viburnum.

Whether it occurs on European oaks, and the relative susceptibility of European oaks such as common pedunculate oak (Quercus robur) or sessile oak (Q. petraea), is unknown at present. Other possible hosts in the UK are Sweet Chestnut, Beech and other fagaceae species such as southern beech.

In the UK, there is already oak mortality and dieback of complex cause known as ‘oak decline’. This is fairly widespread, although individual pockets can be localised and intense. In some cases, the decline is associated with infection by other phytophthoras, but these are mainly root infecting species whereas SOD Phytophthora causes stem cankers. Oak decline also involves recurrent episodes of drought, other root infecting fungi and recurrent insect defoliation and scale insect attack.

This combination of decline factors will complicate surveying to detect whether SOD is present in the UK. However, DEFRA and the Forestry Commission is carrying out complementary investigations over the summer to determine whether the disease is present. These studies will concentrate on areas of potentially high susceptibility, such as sites close to major rhododendron nurseries, sites with understorey rhododendron, and sites where ‘oak decline’ is prevalent. Sample material will be isolated and examined at the Forest Research laboratories at Farnham, Surrey. The movement of rhododendron nursery stock is probably the most likely pathway for introduction of the disease.
Phytophthora ramorum is a member of the same fungal genus as the causative agent of Potato Blight, P. infestans. They are primitive fungi, dependent to a large degree on moisture for the transmission of their asexual, dispersal, spores of which there is a variety.
Symptoms on oak are dark red to black sap oozing from the trunk and rapid colour change of foliage and death throughout the crown.

Forestry professionals are saying “no panic, but it’s worth keeping an eye out for bleeding stem cankers on oaks, beech and sweet chestnut”.

Further information can be found on the Forestry Commission or DEFRA websites, at
www.forestry.gov.uk or www.defra.gov.uk

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