Colonization of tree xylem by Phytophthora ramorum, P. kernoviae and other Phytophthora species

Abstract

The aetiology and frequency of Phytophthora spp. in discoloured xylem tissue beneath phloem lesions was investigated in a range of broadleaved trees infected with P. ramorum, P. kernoviae, P. cambivora, P. citricola and other species. Isolation was attempted from the inner surface of 81 sterilized discoloured wood panels (6 × 4 cm) from 53 trees. Discolouration mostly extended 1-5 mm into the xylem (75%), but incursions of 6 -10 mm (10%) and 10 -25 mm (15%) were frequent. Of the wood panels, 81% yielded Phytophthora spp. In 66 cases, both a wood panel and an overlying phloem panel were sampled. In 56% of these, a Phytophthora sp. was isolated from both the wood and the phloem panel. In 23% the Phytophthora sp. was isolated from the wood panel only and in 8% from the phloem panel only. Small 'island' phloem lesions, often in linear arrays adjacent to main lesions, were a common feature of Fagus sylvatica and Quercus spp. trees infected with P. ramorum or P. kernoviae . Island lesions were often connected by underlying strips or intermittent pits of discoloured xylem in line with the wood grain. Phytophthora ramorum, P. kernoviae and other Phytophthora spp. were successfully isolated from these connecting xylem features with P. ramorum and P. kernoviae also recovered from discoloured tissue 5-25 mm below exposed xylem surfaces 24-27 months after the overlying phloem was removed. These results show that these pathogens commonly occupy xylem beneath phloem lesions; that they can perennate in xylem tissue; that they can spread in xylem tissue ahead of phloem lesions; and indicate that they may initiate new phloem lesions in this way. Such colonization must lead to at least local xylem dysfunction. It is recommended that, if xylem discoloration is present, isolation of the Phytophthora sp. should be attempted from the xylem as well as the bark; also, that removal of infected outer sapwood should be undertaken during excision of bleeding lesions for disease control and in protocols aimed at preventing national or international spread of these tree stem pathogens.

Fishing for Phytophthora from Western Australia’s waterways: a distribution and diversity survey

“…Under favourable conditions some of these Clade 6 species, such as P. inundata, P. taxon PgChlamydo, P. gonapodyides and P. lacustris (e.g. Hansen and Delatour 1999;Greslebin et al 2005;Smith et al 2009;Randall 2011;Reeser et al 2011;Nechwatal et al 2012), can be aggressive tree pathogens (Brasier et al 2003b;Brown and Brasier 2007;Jung 2009). P. inundata has been isolated from dying native vegetation in Western Australia (WA) (Stukely et al 2007).…”

Sudden oak death and Phytophthora ramorum: a summary of the literature

“…Brown and Brasier (2007) reported survival of the pathogen in exposed wood of trees for up to 2 years.…”

“…Discoloration was generally more extensive in the cambium and secondary phloem tissues than in the xylem; typically black discoloration extended less than 1 mm into xylem tissues, but was occasionally observed up to 3 cm into the xylem. Brown and Brasier (2007) found that P. ramorum commonly occupies xylem beneath phloem lesions, that the pathogen can penetrate xylem tissue, and that it spreads in xylem tissue ahead of phloem lesions. Parke and others (2007) demonstrated that the pathogen colonized tanoak sapwood, and then spread through xylem vessels.…”

“…Following entry through the bark of woody hosts, hyphae of P. ramorum grow within the phloem and cambial tissues, and then into xylem tissues (Brown and Brasier 2007, Parke and others 2007, Rizzo and others 2002a. Xylem colonization is the norm.…”

Potential ofPhytophthora pinifoliato spread via sawn green lumber: a preliminary investigation

“…Some species of Phytophthora, like P. austrocedrae, P. cambivora, P. cinnamomi and P. ramorum among others can be found in the phloem, normally 4 associated with stem cankers (Hamm and Hansen 1987, Rizzo et al 2002, Brown and Brasier 2007, Parke et al 2007, Moralejo et al 2009, Greslebin and Hansen 2010. There are also a few Phytophthora species that have been detected in the xylem tissue.…”