Introduction
Phytophthora cryptogea Pethybr.
& Laff. (1919)
Phytophthora cryptogea
was first described as the cause of foot rot of tomato in
Cultural Characteristics
Cultures grow uniform with fairly fluffy mycelium (Figs. 2 and 4A). Some isolates display a slight to distinct floral pattern on V-8 agar medium. The minimum temperature for growth is less than 1°C, the optimum temperature for growth is 22–25°C, and the maximum temperature for growth is 31–33°C, with no colony growth at 35°C or higher. Isolates with morphological traits similar to P. cryptogea and that grow above 35°C should be classified as P. dreschleri.
Reproductive Structures
Asexual Structures
Sporangiophores:
Sporangiophores proliferate internally through empty sporangium (nested) or develop sympodially from below previously formed sporangia (Fig. 1). Sporangiophores are 2–3.5 µm in diameter. Sympodia form in water.
Sporangia:
Nonpapillate sporangia are produced only on liquid media. They are noncaducous and the first-formed sporangia are regularly ovoid or obpyriform. Later-formed sporangia can be elongated and variable in shape. Sporangia are 24–35 × 35–63 µm (average 30 × 52 µm) (Ho et al., 1995) (Figs. 3 and 4C–E). The length–breadth ratios are greater than 1.6. Sporangia are nonpapillate, without an obvious apical thickening. the exit pore is more than 8 µm wide. Sporangia collapse after zoospore release.
Chlamydospores:
Chlamydospores are not formed.
Hyphae:
Hyphae are very uneven or irregular in width, up to 8 µm in diameter. There is a conspicuous network of small, catenulate hyphal swellings abundant on aqueous cultures (Fig. 4B).
Sexual
Structures
P. cryptogea is heterothallic and requires mating with an opposite mating type to form oogonia and oospores. It sometimes forms in single cultures of aged cultures.
Antheridia:
Antheridia are amphigynous and spherical, sometimes oval or cylindrical, and unicellular. Antheridia are 12–17 × 13–17 µm.
Oogonia:
Oogonia rarely form in single cultures but develop when paired with isolates of opposite mating types. Oogonia measure 28–37 µm in diameter (average 31 µm). Oogonia have tapered bases and become yellow with age.
Oospores:
Oospores nearly fill the oogonia, are plerotic, and are 24–32 µm in diameter (average 27 µm). Oospores are thick walled, with the walls measuring 3.5 µm.
Host Range and Distribution
See Table 20.1 in Erwin and Ribeiro (1996) for a complete host range list.
Symptoms
Foot and Root
Rot of Tomato:
The disease was first described causing a foot rot on tomato. It is severe in wet, waterlogged soils. A brownish black discoloration and lesion occurs at the base of the stem. Secondary adventitious roots can emerge above the lesions. Lesions advance, causing the plant to fall over. The root system becomes entirely rotted. Foliage does not display symptoms until the plant stem is girdled. It is a limiting factor in the greenhouse production of tomatoes at low temperatures of 15°C.
Root and
Collar Rot of Ornamental and Woody Plants:
P. cryptogea is an important pathogen of many ornamentals. Plant roots with collar rot exhibit a decline in foliage (Fig. 5), leading to defoliation and the dieback of branches on many hosts (Fig. 6). Plants appear chlorotic and stunted. The pathogen is spread with poorly drained soils under wet conditions. Cankers can also girdle the stem. Pinus species show a general decline and die prematurely.
Diagnostics:
The pathogen must be isolated on selective media for accurate identification. Species identification requires morphological and molecular identification (Cooke et al., 2000; MacDonald et al., 1990).
References
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.
Cooke, D. E. L., Drenth, A.,
Erwin, D. C., and Ribeiro, O. K.
1996.
Phytophthora Diseases Worldwide. American Phytopathological
Society,
Ho, H. H., and Jong, S. C.
1986. A comparison between
Phytophthora cryptogea and
P. drechsleri. Mycotaxon 27:289-319.
Ho, H. H., Ann, P. J., and Chang, H. S. 1995. The genus
Phytophthora in
Pethybridge, G. H., and Lafferty, H. A. 1919. A disease of tomato and other
plants caused by a new species of
Phytophthora. Sci. Proc. R.
MacDonald, J. D., Stites, J., and Kabashima, J. 1990. Comparison of serological
and culture plate methods for detecting species of
Phytophthora, Pythium, and
Rhizoctonia in ornamental plants.
Plant Dis. 74:655-659.
Stamps, D. J.
1978.
Phytophthora cryptogea. CMI Descr. Pathog. Fungi Bact.
592:1-2
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,