Distribution and etiology of aerial stem infections of P. ramorum and P. taxon C at three woodland sites in the UK

Anna Brown, Clive Brasier, Sandra Denman, Joan Rose, Susan Kirk and Joan Webber, Forest Research, Farnham, Surrey GI10 4LH UK; anna.brown@forestry.gsi.gov.uk

Phytophthora ramorum and Phytophthora taxon C are recently invasive pathogens in woodlands in southern Britain. P. taxon C is a newly discovered taxon, shortly to be named P. kernovii sp. nov. Both species can aggressively infect foliage and shoots of Rhododendron ponticum and then spread aerially to attack the inner bark of tree stems, especially Fagus sylvatica and some Quercus species, causing bleeding lesions. Three woodlands in south-west England with extensive naturalised understory R. ponticum infected with P. ramorum and/or P. taxon C were investigated for spread of these pathogens onto trees.

Following a preliminary survey, 127 trees with 'suspicious' external stem symptoms were investigated for Phytophthora by isolation. Overall, 18/49 (37%) of trees investigated at Site 1 yielded a Phytophthora, 24/63 (38%) at site two and 10/15 (67%) at site 3. P. ramorum was isolated from aerial stem lesions on F. sylvatica, Q. cerris, Aesculus hippocastanum and Nothofagus obliqua. P. taxon C was isolated frequently from beech and a Liriodendron tulipifera. P.citricola, an established endemic, was reguarly isolated from Acer pseudoplatanus, and may also have spread from the adjacent diseased rhododendron. P. ilicis, another recently invasive aerial Phytophthora, was obtained from stems of two Chinese Ilex species. Surprisingly, P.gonapodyides, a weak parasite and common soil inhabitant, and P. cambivora¸ an established endemic collar pathogen, were also obtained from aerial bark lesions on several beech trees.

26 mature (50-140 cm diam) F. sylvatica were investigated for processes of local spread and infection. These included proximity to rhododendron, pattern and mode of spread within a tree, and tree response. Three trees were infected with P. ramorum, 22 with P. taxon C and one with both Phytophthoras. Aerial bark lesions, ranged in size from small ca 3 cm diam.fresh infections to lesions of >1m diam. These occurred from near ground level to 11m. One tree had completely girdling lesions for over 6 m of its length.

The study indicated: (i) intact beech bark can be penetrated and infected by P. ramorum zoospores and probably by P. taxon C zoospores; (ii) direct contact or close proximity (< 3m) to infected rhododendron is a major but not universal factor in infection; (iii) rainwater accumulation and run-off at branch forks may account for a minority of infections; (iv) simultaneous multiple infection of stems is occurring; (vii) vertical spread may occur within a tree; (vi) P. ramorum and P. taxon C can penetrate and be recovered from the underlying wood; (vii) the tree may respond to a bark lesion by attempting to wall it off with callus; (viii) P. taxon C (at least) may pump water out of the xylem into the inner bark; (ix) stem lesions sometimes extend 10-20 cm below ground level; (x) some trees are multiply infected by P. taxon C, P. ramorum and P. gonapodyides; (xi) developing lesions are soon invaded by unknown basidiomycetes and ascomycetes.

Wider risk issues arising from these observations will be discussed.

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