First report of collar and root rot caused by Phytophthora hedraiandra on Viburnum in the UK

Viburnum is a popular shrub grown mainly for its flowers, fragrance, berries, autumn colour, and it is also used as a hedge. In 2011, samples of Viburnum showing symptoms of collar and root rot collected in North Yorkshire (RHS166247) were received by the advisory service at the Royal Horticultural Society. The plant was part of a hedge showing signs of severe dieback with many plants dying. A Phytophthora species was isolated from root samples using rhododendron leaves for baiting. Freshly picked leaves of Rhododendron catawbiense ‘Cunningham’s White’ were washed under running tap water and blotted dry on filter paper. The roots were placed in a plastic tray and flooded with filtered pond water and the intact rhododendron leaves were floated on top of the water.  The trays were left at room temperature (18-25°C) on the bench. After two to eight days, 5 mm² sections were aseptically removed from the margins of developing lesions, plated onto P5ARP (Erwin & Ribeiro, 1996) and incubated at 20°C in the dark. Single hyphal tip cultures were obtained by transferring individual hyphal tips from the P5ARP plates onto carrot agar (CA) (Erwin & Ribeiro, 1996). Sporangia were produced and zoospores released in pond water following floating of CA plugs.

Phytophthora hedraiandra RHS166247 was identified using morphological characteristics and confirmed by DNA sequencing. Sporangia were conspicuously papillate (av. 2.3 µm) and were ovoid to obpyriform, caducous with average dimensions of 37.7 ± 1.1 x 26.1 ± 0.8 µm (average l/b ratio 1.45) and short pedicels (average 1.2 µm). The gametangia were readily produced on CA within 7 days. The fungus was homothallic and the antheridia were paragynous occasionally amphigynous. Oogonia had smooth walls and were usually globose to slightly subglobose. They had a mean diameter of 36.0 ± 0.5 µm. Oospores were slightly aplerotic and measured 29.2 ± 0.5 µm. The oospore wall index was 0.22. Antheridia measured 10.5 ± 0.3 x 9.9 ± 0.6 µm. The morphology of sporangia and gametangia agreed with the original description of Phytophthora hedraiandra by De Cock & Lévesque (2004). The identification of the isolate RHS166247 was confirmed by direct sequencing of the ITS region using a semi-nested PCR reaction. The primers ITS4 (White et al., 1990) and DC6 (Bonants et al., 1997) were used in the first round and ITS4 and ITS6 (Cooke et al., 2000) in the second round. The PCR conditions for both rounds were the same as those described by Cooke et al. (2000) except for the annealing temperature which was 62°C in the first round and 63°C in the second round. Sequencing of the ITS region revealed that the nucleotide sequence of the isolate RHS16624 7 (GenBank Accession No. KF793827) is 99% identical to the ex-type culture of Phytophthora hedraiandra CBS111725 (AY707987). The ITS sequence also has three polymorphisms at positions 73, 99 and 100 of the ITS1 which differentiates it from its close relative P. cactorum.  

Six plants of Viburnum tinus were stem inoculated with 3 mm plugs from a seven-day-old Phytophthora culture (RHS166247) grown on CA. A bark incision was made using a sterile 3 mm cork borer to expose the cambium and the plug placed in the incision. The wounds were wrapped in damp sterile cotton wool, Parafilm and foil. The plants were kept in a polycarbonate grow dome where the temperature was maintained at 20°C during the day and at 15°C during the night. P. hedraiandra was pathogenic to Viburnum tinus causing necrotic phloem lesions 15 days after inoculation, with average lengths of 35.9 ± 6.0 mm whereas the agar control produced lesions of 11.5 ± 2.3 mm. P. hedraiandra was successfully re-isolated from the margins of the lesions of the inoculated plants but not from the controls. P. hedraiandra occurs on Rhododendron and Viburnum species in Italy, Spain, Slovenia, the Netherlands and the USA (Farr et al., 2013).  It causes wilting, stem cankers and root and collar rots. To our knowledge, this is the first finding of this species in the UK. Following its detection another case in a garden in London was confirmed by direct sequencing of plant material taken at the collar of Viburnum tinus (GenBank Accession No. KF793828). No statutory action is taken against this species.