Figure 1. Morphology of Phytophthora lateralis. Upper row, Ovoid sporangia with various shapes; sporangia forming sympodially; chlamydospores forming laterally from hyphal swellings. A photograph of sporangia by Trione (1974) indicated that shapes were more uniform than those shown by Tucker and Milbrath (1942). Most resembled the sporangia drawn above on the sympodial sporangiophores. Lower row, Globose oogonium with a paragynous antheridium; sporangia after evacuation of zoospores; chlamydospores forming laterally on mycelium. Mycelium is usually smooth but can be gnarled. (Drawing by A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.

 

Figure 2. Culture of Phytophthora lateralis grown on V-8 juice agar. (Courtesy Jean B. Ristaino)

 

Figure 3. Sporangia of Phytophthora lateralis. (Courtesy Everett Hansen, Oregon State University)

 

Figure 4. Port-Orford cedar root disease, caused by Phytophthora lateralis. (Courtesy Alan Kanaskie, Forest Health Management Unit, Oregon Department of Forestry)

 

Figure 5. Chamaecyparis lawsoniana killed by Phytophthora lateralis. (Courtesy Faith T. Campbell, The Nature Conservancy)

 

Figure 6. Diseased hedgerow in Trinidad, CA, infected by Phytophthora lateralis. (Courtesy David Adams, California Department of Forestry, Sacramento, CA)

 

Introduction

Phytophthora lateralis  Tucker & Milbrath (1942)

 

Phytophthora lateralis was first described by Tucker and Milbrath (1942) from diseased Lawson cypress in the northwestern United States, including California. The disease was first described in nursery stock imported from France (Roth et al., 1972), but the causal organism was not described until 1942. The pathogen, with few exceptions, is limited to hosts in the family Cupressaceae and was found to be a serious threat to Port-Orford cedar forests in the Northwest and in British Columbia, Canada (Atkinson, 1965). No synonyms are known for P. lateralis. P. lateralis is a group V Phytophthora species (Stamps et al., 1990; Waterhouse, 1963) (Fig. 1). 

Cultural Characteristics

P. lateralis is slow growing. The minimum temperature for growth is 3°C, and the maximum temperature for growth is less than 26°C (Fig. 2). The optimum temperature for growth is 20°C. 

Reproductive Structures

Asexual Structures

 

Sporangiophores:

A description of morphology can be found in Hall (1991). Sporangiophores are compound and can grow below the base of a sporangium or through the base of an empty sporangium.

 

Sporangia:

Sporangia are nonpapillate, noncaducous, and borne sympodially on simple sporangiophores. They are ovate, obovate, obpyriform, and often elongated or distorted in shape. Sporangia are 12–20 × 20–60 µm (average 15 × 26 µm) (Fig. 3). The length–breadth ratio is 1.6:1–1.9:1. Sporangia form most readily in weak agar media (V-8 juice, 10 ml/liter; whole milk, 20 ml/liter; and cornmeal, 25 g/liter) and in continuous incubation under a combination of blacklight blue and cool white fluorescent lamps.

 

Chlamydospores:

Chlamydospores are abundant, characteristically lateral, sessile on the hyphae, and cinnamon brown . This is a characteristic morphological trait from which the pathogen received its name (Tucker and Milbrath, 1942). Chlamydospores are most often found in agar media at temperatures of 15–25°C. They are 20–77 µm in diameter (average 40 µm). The production of chlamydospores is inhibited by light (Fig. 1).

 

Hyphae:

Hyphae grow best at 15°C and are up to 8 µm wide. Hyphae branch irregularly and can become septate as cultures age. P. lateralis does not produce hyphal swellings (Erwin and Ribeiro, 1996).

 

Sexual Structures

 

P. lateralis is homothallic.

 

Antheridia:

Antheridia are paragynous. They occur singly and are (sub)globulose, colorless, and delimited from hyphae by septa (Hall, 1991).

 

Oogonia:

Oogonia are produced in single culture. They are smooth, spherical, thin walled, and terminal and are 33–50 µm in diameter but absent in most isolates.

 

Oospores:

Oospores are plerotic and made up of 6-µm-thick walls. They are deeply pigmented (color depending on the agar medium used) and grow best on agar of Port-Orford cedar foliage (Trione, 1974). Oospores are 28–46 µm in diameter (average 40 µm).

Host Range and Distribution

Host

Common Name

Disease

Geographical Distribution

Chamaecyparis lawsoniana, C. lawsoniana var. elwoodi, var. cerula, var. erecta-aurea, var. erecta-glauca, var. erecta-viridis, var. glauca, var. lutea, var. minima, var. nestoides, var. stewarti, var. wisseli

Port-Orford cedar, cypress

Root and crown rot

North America

Chamaecyparis obtusa var. crippsii

Golden hinoki cypress

Root and crown rot

North America

Taxus brevifolia

Pacific yew

Root and crown rot

North America

Rhododendron spp.

Rhododendron

Root and crown rot

North America

Thuja orientalis

White cedar

Root and crown rot

North America

 

Hosts are found in North America and include the species listed above (DeNitto and Kliejunas, 1991; Erwin and Ribeiro, 1996; Roth et al., 1972). P. lateralis causes root and crown rot in host species.

Symptoms

Root Rot of Chamaecyparis lawsoniana:

The pathogen is an important host of the timber tree Port-Orford cedar. The first symptom of disease is a light foliage color. Foliage wilts on warm days and gradually changes color from yellow to bronze to light brown or tan. In the end, the foliage becomes crisp and dry. The infection of roots and the spreading of the infection up the tree lead to the color changes in foliage (Figs. 4 and 5). Infected roots appear water-soaked and have a deep cinnamon brown color. When the bark is removed, a clear zone of differentiation is visible between infected and healthy tissues. A black resinous line can be seen in the cambium. Young trees can be killed within weeks and older ones decline over the years (Fig. 6). The disease is more severe under cool wet conditions and rainfall (Zobel et al., 1985).

References

Atkinson, R. G. 1965. Phytophthora species inciting root rot of Chamaecyparis lawsoniana and other ornamentals in coastal British Columbia. Can. J. Bot. 43:1471-1475.

 

DeNitto, G. A., and Kliejunas, J. T. 1991. First report of Phytophthora lateralis on Pacific yew. Plant Dis. 75:968.

 

Erwin, D. C., and Ribeiro, O. K. 1996. Phytophthora Diseases Worldwide. American Phytopathological Society, St. Paul, MN.

 

Hall, G. F. 1991. Phytophthora lateralis. Mycopathologia 115:227-229.

 

Roth, L. F., Bynum, H. H., and Nelson, E. E. 1972. Phytophthora root rot of Port-Orford cedar. U.S. Dep. Agric. For. Pest Leafl. 131.

 

Stamps, D. J., Newhook, F. J., Waterhouse, G. M., and Hall, G. S. 1990. Revised tabular key to the species of Phytophthora de Bary. Mycol. Pap. 162. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

 

Trione, E. J. 1974. Sporulation and germination of Phytophthora lateralis. Phytopathology 64:1531-1533.

 

Tucker, C. M., and Milbrath, J. A. 1942. Root rot of Chamaecyparis caused by a species of Phytophthora. Mycologia 34:94-103.

 

Waterhouse, G. M. 1963. Key to the species of Phytophthora de Bary. Mycol. Pap. 92. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

 

Zobel, D. B., Roth, L. F., and Hawk, G. M. 1985. Ecology, pathology, and management of Port-Orford-cedar (Chamaecyparis lawsoniana). Gen. Tech. Rep. PNW-184. U.S. Dep. Agric. For. Ser. Pac. Northwest For. Range Exp. Stn., Portland, OR.