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,
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
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,
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
Lebert, H., and Cohn, F. 1871.
Ueber die Fäule der Cactusstämme.
Beitr. Biol. Pflanz. 1:51-57.
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,
Waterhouse, G. M.
1963. Key to the species of
Phytophthora
de Bary. Mycol. Pap. 92.
Waterhouse, G. M., and Waterson, J. M. 1966.
Phytophthora cactorum.
CMI
Descr. Pathog. Fungi Bact. 111:1-2.