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

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.

Papillate and caducous sporangia, photos from Q-bank, used with permission.

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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).  

Oogonia with amphigynous antheridia, photos from Q-bank, used with permission.

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Genetics: 

P. kernoviae is placed in Clade 10, 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).

Phylogenetic tree from http://www.phytophthoradb.org/kernoviae (Blair et al 2008).

Growth: 

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

Colony morphology at 7 days at 18°C on V8 (left).   Growth on cornmeal agar with pimaricin, ampicillin, rifampicin, hymexazol, and Terrachlor, Plant dis. 70, 1038-1043 (right)

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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. nemorosa), often tapered oogonial stalks (cfr. P. meadii, P. botryosa, P. nemorosa), often asymmetric sporangia (cfr. P. meadii, P. megakaryaP. nemorosa),  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 http://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.

For more information about Phytophthora kernoviae, visit our Disease, Education and Management materials, and Reference sections.