Figure 1. Morphology of Phytophthora cactorum. Upper row, Papillate sporangia, empty sporangium, and germinated oospore producing sporangia. Lower row, Gnarled mycelium and an oospore in an oogonium with a paragynous antheridium. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.
 

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

Figure 3. Phytophthora cactorum. A, Colony. B–G, Sporangia. H and I, Chlamydospores. J–O, Oogonia and antheridia. Scale bar = 20 µm . All the same magnification except A. (Courtesy Hon H. Ho; Reproduced, by permission of the Institute of Plant and Microbial Biology, from Ho et al., 1995)Click image to see larger view. >
 

Figure 4. Papillate sporangium of Phytophthora cactorum. Bar = 10 µm. (Courtesy Elizabeth A. Bush; Reproduced from Bush et al., 2006)
 

Figure 5. Oogonium, antheridium, and oospore of Phytophthora cactorum (composite drawings of median longitudinal sections) (×700). a, Oogonium and antheridium, a few hours old, grown to full size; the oogonium has about 24 nuclei and its antheridium has about nine nuclei. b, Ooplasm separated from the periplasm; nuclei of the periplasm are in division prior to degeneration. c, Antheridial nucleus has entered the oosphere; oospore wall is forming (i.e., exospore and endospore, the epispore is negligible). d, Immature oospore; the two nuclei have not yet fused. e, Oospore after a period of dormancy; nucleus is preparing for division, oil globule is still undigested, and endospore is becoming eroded in digestion. f, Oospore on the point of germination; nuclei are about 32 in number, neither oil globule nor endospore is completely absorbed; germ tube is through exospore but not yet through oogonium wall. g and h, Later stages in germination; the antheridium occasionally becomes loosened from the oogonial wall. (Reproduced, by permission, from Blackwell, 1949) Click image to see larger view.
 

Figure 6. Mature strawberry with leather rot, caused by Phytophthora cactorum. Infection may result in little color change or in discoloration ranging from whitish gray to purple. (Courtesy M. A. Ellis; Reproduced from Rebollar-Alviter et al., 2005)
 

Figure 7. Strawberry infected by Phytophthora cactorum. (Courtesy Dan Legard, University of California)
 

Figure 8. Infection of rhododendron caused by Phytophthora cactorum. (Courtesy R. K. Jones; Used, by permission, from North Carolina State University, Department of Plant Pathology Slide Collection)
 

Figure 9. Crown rot of apple with typical belowground symptoms of Phytophthora cactorum infection. Note brick red discoloration and line of demarcation between diseased and healthy tissues. (Courtesy Turner Sutton; North Carolina State University)
 

Figure 10. Black discoloration of walnut bark infected by Phytophthora cactorum. (Courtesy B. Teviotdale; Reproduced from APS Digital Image Collections, Diseases of Orchard Fruit and Nut Crops, American Phytopathological Society, St. Paul, MN)
 

Introduction

Phytophthora cactorum  (Lebert & Cohn) J. Schröeter (1886)

 

Phytophthora cactorum was first isolated from cactus and described by Lebert and Cohn in 1871 and named Peronospora cactorum (Lebert and Cohn, 1871). de Bary (1881) renamed the species P. cactorum and grouped it into the omnibus species P. omnivora. Schröeter recognized the original species designation (Schröeter, 1889). Waterhouse said P. omnivora was an illegitimate name for the species and kept P. cactorum. Other synonyms listed by Cline et al. (2008) are Peronospora fagi R. Hartig (1876); P. fagi (R. Hartig) R. Hartig (1876) (note: sometimes erroneously cited as P. fagi Rosenbaum, but Rosenbaum listed P. fagi (R. Hartig) R. Hartig); P. paeoniae D. C. Cooper & Ch. Porter (1928); and Peronospora sempervivi Schenk (1875). P. cactorum is a group I species (Stamps et al., 1990; Waterhouse, 1963) (Fig. 1).

Cultural Characteristics

Colonies are slightly radiate, compact without a definite border, and fluffy but not dense with uniform slightly aerial mycelium (Figs. 2 and 3A). The minimum temperature for growth is 2°C, the optimum temperature for growth is 25°C, and the maximum temperature for growth is 31°C.

Reproductive Structures

Asexual Structures

 

Sporangiophores:

Sporangiophores form a regular simple sympodium with very short stalks in moist air and elongated under water and is slender (0.5–1 µm wide), with a slight swelling at the base of each branch but not elsewhere on the sporangiophore (Blackwell, 1942).

 

Sporangia:

Sporangia are abundant on solid media. They are broadly and regularly ellipsoid, spherical, or ovoid to obpyriform and are 28–35(40) × 36–50(55) µm (Bush et al., 2006) (Figs. 3B–G and 4). They have an apex with a conspicuous hemispherical papilla with apical thickening up to 5 µm deep and are caducous with a short pedicel up to 4 µm long. Sporangia are occluded by a conspicuous septal plug. The average sporangia length–breadth ratio is less than 1.6.

 

Chlamydospores:

Chlamydospores are produced by some but not all isolates. Average diameters are 25–39.7 µm (Fig. 3H and I).

 

Hyphae:

Hyphae are normally about 6 µm wide, but they may be irregularly swollen, although without characteristic hyphal swellings.

  

Sexual Structures

 

P. cactorum is homothallic.

 

Antheridia:

Antheridia are nearly spherical to irregularly clavate, 13 × 15(–21) µm, nearly always applied close to the oogonial stalk, often obscured in a knot of hyphae, and nearly always paragynous and monoclinous.

 

Oogonia:

Oogonia are 25–32 µm in diameter and are spherical or tapering toward the base. The wall is thin and colorless or slightly yellow.

 

Oospores:

Oospores are aplerotic and usually 20–26 µm in diameter. The wall is colorless and 2 µm thick (Figs. 3J–O and 5).

Host Range and Distribution

P. cactorum has a wide host range and can infect at least 154 genera of vascular plants in 54 families (Waterhouse and Waterston, 1966). The pathogen causes damping-off of numerous seedlings, including ash, beech, cheery, and conifer; fruit rots of apple, pear, apricot, strawberry, cucurbits, and eggplant (Figs. 6 and 7); leaf and stem rot of cacti, gooseberry, rhododendron, lilac, ginseng, and rhubarb (Fig. 8); collar rot of apple and other fruits trees; stem canker of avocado, pear, birch, maple, and oak; and root rots in general (Waterhouse and Waterston, 1966). See the table in Erwin and Ribeiro (1996) for a complete list of reported hosts and distribution.

Symptoms

The pathogen causes disease on many woody plants (Erwin and Ribeiro, 1996). It causes a root, collar, and crown rot on many species, including apple (Malus pumila) (Fig. 9) and pear (Pyrus communis). Reddish brown lesions are common. Trees can slowly decline or die quickly depending upon age and location of the infections (Fig. 10). Scion wood can be infected if the graft union is planted close to the soil. Root rot can be seen on some plants, and toppling during windstorms is common. Nursery plants can also be infected and a source of inoculum spread to the fields.

References

Blackwell, E. 1942. The life history of Phytophthora cactorum (Leb. & Cohn) Schroet. Trans. Br. Mycol. Soc. 26:71-89.

 

Blackwell, E. M. 1949. Terminology in Phytophthora. Mycol. Pap. 30. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

 

Bush, E. A., Stromberg, E. L., Hong, C., Richardson, P. A., and Kong, P. 2006. Illustration of key morphological characteristics of Phytophthora species identified in Virginia nursery irrigation water. Plant Health Progress doi:10.1094/PHP-2006-0621-01-RS.

 

Cline, E. T., Farr, D. F., and Rossman, A. Y. 2008. A synopsis of Phytophthora with accurate scientific names, host range, and geographic distribution. Plant Health Progress doi:10.1094/PHP-2008-0318-01-RS.

 

Cooper, D. C., and Porter, C. L. 1928. Phytophthora blight of peony. Phytopathology 18:881-899.

 

de Bary, A. 1881. Zur Kenntniss der Peronosporeen. Bot. Ztg. 39:521-625.

 

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

 

Hartig, R. 1876. Die Buchencotyledonen-Krankheit (Cotyledon disease of beech). Z. Forst- Jadgwes. 8:117-123. (In German)

 

Ho, H. H., Ann, P. J., and Chang, H. S. 1995. The genus Phytophthora in Taiwan. Inst. Bot. Acad. Sin. Monogr. Ser. 15.

 

Lebert, H., and Cohn, F. 1871. Ueber die Fäule der Cactusstämme. Beitr. Biol. Pflanz. 1:51-57.

 

Rebollar-Alviter, A., Madden, L. V., and Ellis, M. A. 2005. Efficacy of azoxystrobin, pyraclostrobin, potassium phosphite, and mefenoxam for control of strawberry leather rot caused by Phytophthora cactorum. Plant Health Progress doi:10.1094/PHP-2005-0107-01-RS.

  

Schenk, A. 1875. Neue Peronospora: P. sempervivi (New Peronospora: P. sempervivi). Bot. Ztg. 33:690-693. (In German)

 

Schröeter, J. 1889. Gattung Phytophthora de Bary (The genus Phytophthora de Bary). Kryptogamenflora Schlesien. 3:235-236. (In German)

 

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.

 

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.

 

Waterhouse, G. M., and Waterson, J. M. 1966. Phytophthora cactorum. CMI Descr. Pathog. Fungi Bact. 111:1-2.