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

Figure 2. Typical nonpapillate sporangia of Phytophthora alni subsp. alni. (Courtesy Clive Brasier; Reproduced, by permission of The British Mycological Society, from Brasier et al., 2004). Click image to see larger view.

Figure 3. Oogonium of Phytophthora alni with a tapered stalk and two-celled antheridium (×1,000). (Courtesy Jean B. Ristaino)

Figure 4. Oogonia and antheridia of Phytophthora alni subsp. alni. Images 13–15, Large ornamented oogonia and two-celled antheridia; 16 and 17, oogonia with scarce, single-celled antheridia; 18 and 19, comma-shaped oogonia; 20–23, partially developed small oogonia; and 24, oogonium with aborted oosphere. (Courtesy Clive Brasier; Reproduced, by permission of The British Mycological Society, from Brasier et al., 2004). Click image to see larger view.

Figure 5. Bark of Phytophthora alni-infected alder stem cut away to show basal lesion. (© Crown copyright; Forestry Commission Picture Library/John Gibbs)

Figure 6. Italian alder in the remnant of a woodland planting subject to occasional flooding. One tree has recently died from Phytophthora alni infection. (© Crown copyright; Forestry Commission Picture Library/John Gibbs)

Figure 7. Alder killed by Phytophthora alni. (© Crown copyright; Forestry Commission Picture Library/John Gibbs)

Introduction

Phytophthora alni  Brasier & S. A. Kirk (2004)

 

Phytophthora alni was first discovered in 1993 (Brasier et al., 1995) as a Phytophthora species responsible for alder disease in Alnus trees and was formally described in 2004 as P. alni by Brasier et al. (2004). The species consists of a range of heteroploid organisms. P. alni subsp. alni is a tetraploid, while P. alni subsp. uniformis and P. alni subsp. multiformis have chromosome numbers between a diploid and a tetraploid. P. alni subsp. alni was formerly called the standard variant and P. alni subsp. uniformis was called the Swedish variant. P. alni subsp. multiformis consists of three variants from Germany, Denmark, and the United Kingdom (Brasier et al., 2004). P. alni subsp. alni is more aggressive than the other types and is referred to as the standard variant (Brasier and Kirk, 2001). The pathogen is believed to be of hybrid origin between P. cambivora and a P. fragariae-like species (Brasier et al., 1999). P. alni subsp. alni may have been generated on numerous occasions by hybridizations with P. alni subsp. uniformis and P. alni subsp. multiformis (Ioos et al., 2006). P. alni subsp. uniformis probably has P. cambivora as an ancestor, but the origin of P. alni subsp. multiformis is less clear (Ioos et al., 2006). These Phytophthora species superficially resembled P. cambivora in the morphology of its gametangia but differ from P. cambivora in being self-fertile rather than outcrossing, having a submerged rather than an aerial colony type, and having different optimum and upper temperature limits for growth (Brasier et al., 2004).

Cultural Characteristics

Cultures have a submerged colony type with an irregular colony outline (Fig. 1). P. alni subsp. alni can be grown on carrot agar and has an optimum temperature for growth of 23–25°C (Brasier et al., 2004). The maximum temperature for growth is 29°C.

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangia are borne singly on long sporangiophores (Brasier et al., 2004).

 

Sporangia:

Sporangia are ellipsoid, nonpapillate, and noncaducous, with a broad exit pore (Fig. 2). Sporangia are not constricted. The mean sporangial lengths of 10 isolates range from 48 to 59.8 μm, with an overall range of 35 to 70 μm. The mean widths range from 31.3 to 42.8 μm, with an overall range of 27.5 to 50 μm. Mean length–width ratios are 1.32–1.62 (Brasier et al., 2004).

 

Chlamydospores:

No chlamydospores have been observed.

 

Sexual Structures

 

P. alni subsp. alni is homothallic.

 

Antheridia:

Antheridia predominately have two cells and are amphigynous (Fig. 3). The mean lengths of five isolates range from 23.5 to 27 μm, with an overall range of 20 to 30 μm. The mean widths of antheridia of five isolates range from 18.5 to 19.5 μm, with an overall range of 15 to 20 μm (Brasier et al., 2004).

 

Oogonia:

Some variants of P. alni subsp. alni have oogonia that are larger and more mature and have tapered stalks (Fig. 4). The mean diameters of five isolates range from 42.8 to 50 μm, with an overall range of 37 to 55 μm. Other variants of P. alni subsp. alni have smaller oogonia with a diameter of 25–35 μm. Some oogonia are comma shaped, while others are distorted with beaklike or tubelike protuberances (Brasier et al., 2004). P. alni subsp. uniformis forms smooth-walled oogonia, while P. alni subsp. multiformis and P. alni subsp. alni have ornamented oogonial walls.

 

Oospores:

In large oogonia, the mean oospore diameters of five isolates range from 33.3 to 43.5 μm, with an overall range of 27.5 to 50 μm (Brasier et al., 2004).

Host Range and Distribution

P. alni subsp. alni causes aggressive root and collar rot of Alnus glutinosa and other Alnus species (Brasier et al., 2004). Alder dieback is now in 11 European countries: Austria, Belgium, France, Germany, Hungary, Ireland, Italy, Lithuania, Netherlands, Sweden, and the United Kingdom (Webber et al., 2004). In the United Kingdom, the disease has spread steadily since 1994; by 2003, more than 15% of surveyed trees were affected or killed (Webber et al., 2004). Alder dieback is also widespread throughout Austria (Cech, 2004) and has affected more than one-fourth of the alders along streams in the Walloon area of Belgium (Abras, 2005). The pathogen has been found recently in Alaska (Adams et al., 2008; Trummer et al., 2007).

Symptoms

P. alni subsp. alni forms a collar rot on alder and dieback. In 1993, scientists in the United Kingdom determined that the alder dieback they had observed in recent years was a disease caused by a hybrid pathogen, P. alni (Webber et al., 2004) (Figs. 5–7).

References