Invasion of xylem of mature tree stems by Phytophthora ramorum and P. kernoviae

The aetiology and frequency of Phytophthoras in discoloured xylem tissue beneath phloem lesions was investigated in a range of broadleaved trees infected with P. ramorum, P. kernoviae and several other Phytophthoras. Isolation was attempted from the inner surface of 81, 6 x 4 cm sterilised discoloured wood panels from 53 trees. Discolouration mostly extended 1 to 5 mm into the xylem (75 percent) but incursions of 6 to 10 mm (10 percent) and 10 to 25 mm (15 percent) were frequent. Phytophthora was isolated from 81 percent of the wood panels. In 66 cases, both a wood panel and an overlying phloem panel were sampled. In 56 percent of these Phytophthora was isolated from both the wood and the phloem panel. In 23 percent it was isolated from the wood panel only and in 8 percent from the phloem panel only.