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Figure 1. Culture of Phytophthora kernoviae grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

Figure 2. Representative Phytophthora kernoviae sporangia. 18–22, Regular, ovoid limoniform. 23–26, Asymmetrical or mouse-shaped sporangia. Bar = 10 µm. (Reproduced, by permission of The British Mycological Society, from Brasier et al., 2005) Click image to see larger view.

Figure 3. Representative oogonia, antheridia, and thick-walled plerotic oospores of Phytophthora kernoviae. Compare oogonia with tapered bases (10–14 and 16) with those without this feature (9, 15, and 17). Bar = 10 µm. (Reproduced, by permission of The British Mycological Society, from Brasier et al., 2005) Click image to see larger view.

Figure 4. Thick-walled plerotic oospore of Phytophthora kernoviae (×1,000). (Courtesy Jean B. Ristaino)

Figure 5. Necrosis of rhododendron leaves caused by Phytophthora kernoviae. (Courtesy Paul Beales; © Crown copyright; Central Science Laboratory Archive, www.bugwood.org)

Figure 6. Necrotic lesion on stem of rhododendron caused by Phytophthora kernoviae. (Courtesy Paul Beales; © Crown copyright; Central Science Laboratory Archive, www.bugwood.org)

Figure 7. Bleeding lesion on trunk of European beech caused by Phytophthora kernoviae. (Courtesy Paul Beales; © Crown copyright; Central Science Laboratory Archive, www.bugwood.org)

Introduction

Phytophthora kernoviae  Brasier, Beales & S. A. Kirk (2005)

 

Phytophthora kernoviae was first described as Phytophthora taxon C and found during a survey for P. ramorum in the Cornwall region of the United Kingdom (Brasier et al., 2004). It causes bleeding cankers in European beech in southern England and in rhododendron in woodland sites. Molecular evidence based on internal transcribed space (ITS) sequence show it is closely related to P. boehmeriae (Cooke et al., 2000). P. kernoviae and P. boehmeriae are more closely related to each other and to members of ITS clades 9 and 10, including P. macrochlamydospora, P. insolita, and P. richardiae, than to members of the main cluster of Phytophthora spp. in ITS clades 1–8 (Cooke et al., 2000). 

Cultural Characteristics

On carrot agar, the optimum temperature for growth is 18°C, and the maximum temperature for growth is 26°C (Fig. 1). 

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores are sympodial.

 

Sporangia:

Sporangia are papillate, caducous, and ovoid or limoniform to distinctly asymmetrical or mouse shaped, with one rounded and one flatter side. Sporangia pedicels are 8.6–14.1 µm long. Sporangia are an average of 19–31 × 34–52 µm (Brasier et al., 2005) (Fig. 2).

 

Chlamydospores:

No chlamydospores have been observed.

 

Hyphae:

Hyphae are sometimes denticulate or tuberculate.

  

Sexual Structures

 

P. kernoviae is homothallic and gametangia form abundantly on carrot agar.

 

Antheridia:

Antheridia are amphigynous and 10–10.5 × 11.5–12.5 µm.

 

Oogonia:

Oogonia are relatively small, averaging 23.5–25.5 µm in diameter, often with tapered stalks.

 

Oospores:

Oospores are plerotic and 21.1–22.5 µm in diameter, with a wall thickness of 3.5 µm (Figs. 3 and 4).

Host Range and Distribution

Host	Common Name	Disease	Geographical Distribution
Drimys winterii	Winter's bark	Leaf necrosis	United Kingdom
Fagus sylvatica	European beech	Dieback; bark cankers; foliar necrosis	United Kingdom
Gevuina avellana	Chilean hazelnut	Leaf necrosis	United Kingdom
Hedera helix	Ivy	Leaf necrosis	United Kingdom
Ilex aquifolium ‘Variegata’	Holly	Stem necrosis	United Kingdom
Liriodendron tulipifera, Magnolia spp., Michelia doltsopa	Tuliptree, tulip magnolia, Michelia tulip poplar, yellow poplar	Dieback; bark cankers; foliar necrosis; bud blast; blossom blight	United Kingdom
Podocarpus salignus	Podocarpus	Leaf necrosis	United Kingdom
Prunus laurocerasus	Cherry laurel	Leaf necrosis; stem dieback	United Kingdom
Quercus robur, Q. ilex	English oak, Holm oak	Dieback; bark cankers; foliar necrosis	United Kingdom
Rhododendron spp., Pieris spp.	Rhododendron, Pieris	Dieback; bark cankers; foliar necrosis	United Kingdom
Vaccinium myrtillus	Bilberry	Leaf necrosis	United Kingdom

Symptoms

It causes disease on bark, leaves, and shoots of a number of woody trees and shrubs. The pathogen causes foliar necrosis and shoot dieback of the dense understory rhododendron. Dieback, wilting, and foliar necroses are observed in lower and upper stems, and leaves may abscise, leading to defoliation (Figs. 5 and 6). Lesions develop that are sunken or erumpent bark cankers. On beech, oak, and magnolia, it is associated with bark necrosis, bark canker, and bleeding stem lesions, especially on aerial stems (Fig. 7). Pathogenicity has been confirmed by inoculation studies.

References

Brasier, C. M., Denman, S., Brown, A., and Webber, J. F. 2004. Sudden oak death (Phytophthora ramorum) discovered on trees in Europe. Mycol. Res. 108:1107-1110.

 

Brasier, C. M., Beales, P. A., Kirk, S. A., Denman, S., and Rose, J. 2005. Phytophthora kernoviae sp. nov., an invasive pathogen causing bleeding stem lesions on forest trees and foliar necrosis of ornamentals in the UK. Mycol. Res. 109:853-859.

 

Cooke, D. E. L., Drenth, A., Duncan, J. M., Wagels, G., and Brasier, C. M. 2000. A molecular phylogeny of Phytophthora and related oomycetes. Fungal Genet. Biol. 30:17-32.