Phytophthora kernoviae

Overview

Phytophthora kernoviae Brasier, Beales & S.A. Kirk (2005) was first observed in Cornwall, southwest England in 2003. The new species was described in 2005. It causes leaf lesions on rhododendron and stem lesions on European beech in gardens and woodlands in the UK. Other trees and shrubs are also affected. The pathogen appears to match isolates recovered in New Zealand from a diseased cherimoya orchard in 2006 and from soil isolates recovered previously from a native kauri forest and a radiata pine plantation. Etymology: ‘Kernow’, the old name for Cornwall.

Morphology

Sporangia (34-52 x 19-31 µm, mean range ca 38.5-45.5 x 22.3 x 27 µm) papillate and caducous, formed occasionally on carrot agar (CA) in the light, produced abundantly on CA plugs in nonsterile pond water or soil leachate, ovoid, limoniform to asymmetric or ‘mouse-shaped’, most with a conspicuous vacuole, pedicel length 5-19 µm , borne on sympodially branched sporangiophores. Hyphae sometimes denticulate or tuberculate. Chlamydospores not observed. Colonies in dark on CA largely submerged with small central area of aerial mycelium, with alternating rings of mycelium in diurnal light. Homothallic, gametangia abundant on CA after 10 d. Oogonia diameter 21-28 µm (mean 23.5-25.5 µm), often with tapered stalks. Oospores 19-25 µm (mean 21.1-22.5 µm), plerotic, wall thickness 3.5-5 µm (mean ca. 3.5 µm). Antheridia amphigynous, 10-14 x 9 x 12 µm, commonly 10-14 x 9-12 µm. Compared to UK isolates described above (Brasier et al., 2005), New Zealand isolates are reported to grow somewhat slower at 20° C and have a few small differences in the size of oogonia, sporangia, and pedicel length (Ramsfield et al. 2009).

Genetics

P. kernoviae is placed in Clade 9, with P. boehmeriae as its closest relative (Blair et al., 2008). ITS sequences of New Zealand isolates matched 812/813 base pairs with the UK reference isolate AY040661. The UK isolate AY040661 has adenine in position 679, whereas New Zealand isolates have either guanine in that position or are polymorphic for adenine and guanine. This polymorphism in the ITS sequence is interpreted as evidence for some genetic diversity in the New Zealand population (Ramsfield et al. 2009). A possible origin in the southern hemisphere is hypothesized for P. kernoviae, and although present in New Zealand since at least 1953, it is not known if it is an endemic species (Ramsfield et al. 2009).

Growth

Temperature optimum ca. 18° C, max. ca. 26° C. Growth rate in dark at 20° C on carrot agar 3.8-4.6 mm/d (mean 4.2 mm/day).

Distinguishing characteristics for identification

P. kernoviae may be distinguished from other homothallic species with caducous, papillate sporangia with medium-length pedicels by its lower optimal temperature (cfr. P. botryosa and P. hevea); higher optimum temperature (cfr. P. nemerosa), often tapered oogonial stalks (cfr. P. meadii, P. botryosa, P. nemerosa), often asymmetric sporangia (cfr. P. meadii, P. megakarya, P. nemerosa), and longer pedicels (cfr. P. boehmeriae). Compared to P. ramorum, which often occurs in similar habitats in the UK, P. kernoviae is homothallic instead of heterothallic, is papillate instead of semi-papillate, does not produce chlamydospores, and has a longer pedicel length.

The searchable web-based database phytophthora-id.org is useful for rapid identification of Phytophthora species based on sequencing of the ITS or Cox spacer regions, followed by BLAST searching the database. The database includes only sequences that are associated with published Phytophthora species descriptions or classic Phytophthora phylogenetics references.

Disease History

A new Phytophthora species was isolated from diseased rhododendrons and European beech in Cornwall, UK in 2003. Initially called ‘Phytophthora taxon C’ or ‘P. kernovii’, it was later named P. kernoviae (etymology: ‘Kernow’, the old name for Cornwall). It causes stem lesions on members of Fagaceae, and leaf lesions and dieback on plants in several families. Hosts include Rhododendron ponticum, an understory species on which it sporulates. It is believed to be an introduced pathogen in the UK. In 2006, P. kernoviae was reported from an abandoned orchard in the Northland region of New Zealand on rotting fruit of custard apple Annona cherimola (Braithwaite et al, 2007). It appears to match earlier soil isolates recovered in New Zealand from a native kauri forest and a radiata pine plantation

Impacts in the Forest

In the UK, P. kernoviae has been found mainly on rhododendron in woodlands of southwest England with a few sites in northern England, Wales and Scotland. Between 2002 and 2008, 66 outbreaks occurred in managed and unmanaged land, and 5 outbreaks occurred in nurseries and garden centers in England and Wales (Walters et al. 2010). The pathogen produces abundant sporangia on leaves of Rhododendron ponticum that serve as primary inoculum for tree infections, so management efforts have focused on removal of this invasive species. Recent findings indicate that P. kernoviae can persist as oospores in rhododendron roots, complicating this management strategy (Fichtner et al. 2011). In 2009, P. kernoviae was reported on Vaccinium myrtilllus in three heathland locations in Cornwall (Beales et al. 2009) and later on the Isle of Arran (Scotland). The pathogen sporulates profusely on leaves. It is postulated that long distance disease spread may occur when coastal winds carry infected, abscised leaves. Local spread may occur via infected roots and rhizomes (Fichtner et al. 2010). Other heathland species including Vaccinium vitis-idaea and Arctostaphylos uva-ursi appear to be highly susceptible in artificial inoculation tests, suggesting that this valuable botanical resource could be endangered (Beales et al. 2009).

Forest and Wildland Hosts and Symptoms

In the UK, P. kernoviae causes a serious disease on European beech (Fagus sylvatica). Most of the infestations are associated with the invasive understory plant, Rhododendron ponticum, widely distributed in woodlands and gardens. The pathogen’s host range includes Chilean hazelnut, tulip tree, Gevuina avellana, winters bark, Magnolia spp., Pieris spp., Michelia dolsopa, Holm oak, English oak, cherry laurel, ivy, and variegated holly. Bilberry (Vaccinium myrtillus) is an important heathland species has become infected under natural conditions (Beales et al. 2009). Nursery infestations have occurred rarely and were eradicated. Like P. ramorum, P. kernoviae causes bleeding stem cankers on members of the Fagaceae and foliar blight and shoot dieback on other hosts.

In New Zealand, the pathogen was associated with diseased leaves, shoots, and fruits of cherimoya trees that were near native shrublands and forest in Northland (Braithwaite et al. 2007). Previous isolates from soil in kauri forests 50 km away were not associated with disease, nor were isolates from near Tokoroa, 400 km to the south, from Pinus radiata plantations (Ramsfield et al. 2009). P. kernoviae seems to have been first observed by Newhook in New Zealand in 1953 (Ramsfield et al. 2009).

Educational and Management Materials

• P. kernoviae disease fact sheet (English) - FERA, UK
• P. kernoviae disease fact sheet (Welsh) - FERA, UK
• P. kernoviae slide show – Biosecurity New Zealand
• Recovery plan for P. kernoviae - USDA–ARS
• P. kernoviae: Growing threat to native heathlands – UK Forest Research
• Future management of risk from P. kernoviae and P. ramorum – DEFRA, UK