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

Figure 2. Colony, sporangial, and gametangial types of Phytophthora inundata (Phytophthora sp. O-group) isolates. 1–4, Colonies on carrot agar of isolates P210, P246B, P895, and P1052, respectively. 5–8, Representative sporangia (stained with cotton blue), including proliferating sporangia (8). 9–12, Representative oogonia and antheridia. Bar = 20 µm. (Reproduced, by permission of The British Mycological Society, from Brasier et al., 2003) Click image to see larger view.

Figure 3. Oogonia and antheridia of Phytophthora inundata (×1,000). (Courtesy Jean B. Ristaino)

Introduction

Phytophthora inundata  Brasier, Sánchez-Hernandez & S. A. Kirk (2003)

 

Phytophthora inundata was first isolated by Brasier from roots of a dying horse chestnut (Aesculus hippocastanum) and a river-flooded willow (Salix matsudana). It was described by Brasier et al. in 2003 from a variety of riparian habitats (Brasier et al., 2003). The pathogen superficially resembles P. gonapodyides. It is a member of clade 6 according to Cooke et al. (2000) and falls in subclade 1. 

Cultural Characteristics

P. inundata grows well on carrot agar (Figs. 1 and 2.1–2.4). The minimum temperature for growth is 25°C, the optimum temperature for growth is 28–30°C, and the maximum temperature for growth is 38°C. Colonies are irregular, with stellate to broad-lobed patterns.

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores are single or sympodial.

 

Sporangia:

Sporangia are terminal with rounded bases, noncaducous, nonpapillate, and ovoid or ovoid-obpyriform. They proliferate internally and are 37–58 × 49–72 µm (average 47.7 × 64.5 µm) (Fig. 2.5–2.8).

 

Chlamydospores:

Chlamydospores are not produced.

 

Hyphae:

Small hyphal swellings occur in water.

 

Sexual Structures

P. inundata is heterothallic, but some isolates are self fertile.

 

Antheridia:

Antheridia are amphigynous and 15.5–16.5 × 15.1–18.4 µm (average 15.9 × 16.5 µm).

 

Oogonia:

Oogonia are smooth, subspherical, and 34.7–43.4 µm in diameter (average 40.1 µm).

 

Oospores:

Oospores are thick walled, often yellowish, smooth, subspherical, and aplerotic. Oospores are 31.4–38.2 µm in diameter (average 35.7 µm) (Figs. 2.9–2.12 and 3).

Host Range and Distribution

Host	Common Name	Disease	Geographical Distribution
Alnus spp.	Alder	Root rot	United Kingdom, Denmark
Aesculus hippocastanum	Horse chestnut	Root and collar rot	United Kingdom
Olea spp.	Olive , black ironwood	Root and collar rot	Spain
Salix matsudana	Willow	Root and collar rot	United Kingdom
Vitis spp.	Grape	Root and collar rot	South America

Symptoms

It is associated with root and collar rots of hardwood trees and shrubs, especially after flooding or on very wet soils. It is also found on debris in rivers and ponds.

References

Brasier, C. M., Sanchez-Hernandez, E., and Kirk, S. A. 2003. Phytophthora inundata sp. nov., a part heterothallic pathogen of trees and shrubs in wet or flooded soils. Mycol. Res. 107:477-484.

 

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.