Phytophthora kernoviae

 Species profile prepared by M. Dick and J. L. Parke

To cite, see Forest Phytophthoras (2012) doi: 10.5399/osu/fp.2.1.3051

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

External lesion caused by P. kernoviae on a beech tree (top), necrosis of Rhododendron leaves (bottom).

View more images...
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).

Host Latin Name Host Common Name Symptoms Habitat Region
Annona cherimola Custard apple Dieback, Fruit rot Agricultural setting New Zealand - Northland
Drimys winteri Winter's bark Dieback, Leaf necrosis Gardens England
Fagus sylvatica European Beech Canker Forest, Gardens, Parklands England, England - Cornwall, Wales
Gevuina avellana Chilean hazel Dieback, Leaf necrosis Gardens England
Hedera helix Common ivy Dieback, Leaf necrosis Gardens England
Host unknown None, found in streams or soil None, found in streams or soil Forest, Plantations New Zealand - Northland, New Zealand - Tokoroa
Ilex aquilfolium English holly Decline, Leaf necrosis Gardens England
Liriodendron tulipifera Tulip tree Dieback, Leaf necrosis Gardens England
Magnolia spp. Magnolia Dieback, Leaf necrosis Gardens England
Michelia doltsopa Sweet michelia Dieback, Leaf necrosis Gardens England
Pieris formosa Chinese pieris Dieback, Leaf necrosis Gardens England
Prunus laurocerasus Cherry laurel Dieback, Leaf necrosis Gardens England
Quercus ilex Holm oak, Oak Dieback, Leaf necrosis Gardens England
Quercus robur English oak, Oak Dieback, Leaf necrosis Gardens England
Rhododendron ponticum Pontic rhododendron Blight, Decline, Wilting Forest, Gardens, Parklands England, England - Cornwall, Scotland, Wales
Vaccinium myrtillus Bilberry Dieback, Leaf necrosis Wildland England - Cornwall, Scotland - Isle of Arran