Introduction

Phytophthora phaseoli Thaxt. (1889)

Phytophthora phaseoli was first described by Thaxter (1889) and amended by Clinton (1906) as a causal organism of a destructive disease of lima beans in Connecticut, United States. A synonym is P. infestans var. phaseoli (Thaxt.) Leonian (1925) (Tucker, 1931). The pathogen causes a downy mildew on lima bean and at least six races have been identified, including D, E, and F, leading to serious disease outbreaks in the eastern United States, including Maryland and Delaware (Evans et al., 2002). P. phaseoli is a group IV Phytophthora species (Stamps et al., 1990) and is a member of the clade Ic species (Cooke et al., 2000) (Fig. 1).

Cultural Characteristics

The optimum temperature for growth is 15–20°C, the minimum temperature for growth is 5°C, and the maximum temperature for growth is 25–30°C (Fig. 2). P. phaseoli can be maintained for long periods of time on sterile lima bean seeds in water (Goth and Wester, 1963). Some higher temperature strains have been reported (races E and F).

Reproductive Structures

Asexual Structures

Sporangiophores:

Sporangiophores are sympodially branched and slightly swollen at the base (Fig. 3).

Sporangia:

Sporangia are semipapillate and can be oval or elliptical with a truncate base (Fig. 4). While the size may vary by substrate on which the sporangia are produced (Hyre and Cox, 1953), Thaxter (1890) reports sporangia with dimensions of 20–24 × 3–50 µm. Sporangia are nonproliferating and caducous with short pedicels 5–20 µm long (Fig. 5).

Chlamydospores:

Chlamydospores are not produced.

Hyphae:

Hyphal swellings are not produced.

Sexual Structures

P. phaseoli is homothallic.

Antheridia:

Antheridia are amphigynous and shaped like a flattened sphere.

Oogonia:

Oogonia are subspherical and 16.2–35.6 µm in diameter (average 23.7 µm).

Oospores:

Oospores are produced abundantly on lima bean or host tissue and are 14.4–26.6 µm in diameter (average 19.2–22.5 µm) (Hyre and Cox, 1953) (Fig. 6).

Host Range and Distribution

The pathogen has been found in Africa (Congo, Zaire), Asia (Philippines, Sri Lanka, USSR), Europe (Italy, Romania), North America (Mexico, United States), Central America and the Caribbean Islands, and South America (Brazil) (Commonwealth Mycological Institute, 1983). The primary host is Phaseolus lunatus (lima bean). It is capable of infecting plants from other families after inoculation (Cline et al., 2008; Erwin and Ribeiro, 1996).

Host	Common Name	Disease	Geographical Distribution
Brassica juncea	Brown mustard	Seedling blight	Philippines (artificially inoculated)
Hevea brasiliensis	Para rubber	Seedling blight	Philippines (artificially inoculated)
Lycopersicon esculentum	Tomato	Seedling blight	Philippines (artificially inoculated)
Phaseolus lunatus, P. limensis, P. vulgaris	Lima bean, butter bean, common bean	Downy mildew	United States, Russia, Puerto Rico, Congo, Philippines, Italy
Raphanus sativus	Radish	Seedling blight	Philippines (artificially inoculated)
Sandoricum indicum	Santol	Seedling blight	Philippines (artificially inoculated)
Solanum melongena	Eggplant	Seedling blight	Philippines (artificially inoculated)
Vigna unguiculata subsp. sesquipedalis	Asparagus bean	Seedling blight	Philippines (artificially inoculated)

Symptoms

Downy mildew of Phaseolus lunatus (lima bean) is most destructive on pods, but the pathogen also infects shoots, leaves, and petioles (Fig. 7). Symptoms include early lesions on the leaves that are irregularly shaped and purplish, and reddish bands often surround infected areas of the plant (Fig. 8). A white felt of mycelium and sporangia develops on the pod and eventually progresses to cover the entire pod (Figs. 9 and 10). Infected pods eventually shrivel (Fig. 11). The disease appears most commonly under humid conditions. Disease forecasting systems have been developed.

References

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

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.

Clinton, G. P. 1906. Downy mildew, Phytophthora phaseoli Thaxt., of lima beans. Conn. Agric. Exp. Stn. Annu. Rep. (1905):278-303.

Commonwealth Mycological Institute. 1983. Phytophthora phaseoli. Distrib. Maps. Plant Dis. 201. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

Cooke, D. E. L., Drenth, A., Duncan, J. M., Wagels, G., and Brasier, C. M. 2000. A molecular phylogeny of Phytophthora and related oomycetes. Fungal Genet. Biol. 30:17-32.

Evans, T. A., Davidson, C. R., Dominiak, J. D., Mulrooney, R. P., Carroll, R. B., and Antonius, S. H. 2002. Two new races of Phytophthora phaseoli from lima bean in Delaware. Plant Dis. 86:813.

Evans, T. A., Mulrooney, R. P., Gregory, N. F., and Kee, E. 2007. Lima bean downy mildew: Impact, etiology, and management strategies for Delaware and the Mid-Atlantic region, U.S. Plant Dis. 91:128-135.

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

Goth, R. W., and Wester, R. E. 1963. Culture of Phytophthora phaseoli on living and sterilized media. Phytopathology 53:233-234.

Hyre, R. A., and Cox, R. S. 1953. Factors affecting viability and growth of Phytophthora phaseoli. Phytopathology 43:419-425.

Leonian, L. H. 1925. Physiological studies on the genus Phytophthora. Am. J. Bot. 12:444-495.

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

Thaxter, R. 1889. A new American Phytophthora. Bot. Gaz. 14:273-274.

Thaxter, R. 1890. Report of the mycologist. Conn. Agric. Exp. Stn. Annu. Rep. (1889):127-177.

Tucker, C. M. 1931. Taxonomy of the genus Phytophthora de Bary. Univ. Mo. Agric. Exp. Stn. Bull. 153.


Figure 1. Morphology of Phytophthora phaseoli. Upper row, Ovoid to elliptical sporangia with short pedicels. Lower row, Subspherical, globose oogonia with amphigynous antheridia. Right, Mycelium and sympodially branched sporangiophores. (Courtesy A. Vaziri; Reproduced from Erwin and Ribeiro, 1996) Click image to see larger view.
Figure 2. Culture of Phytophthora phaseoli grown on lima bean agar. (Courtesy Jean B. Ristaino)
Figure 3. The compound sympodial sporangiophore of Phytophthora phaseoli (×350). (Reproduced, by permission of CAB International, from Blackwell, 1949)
Figure 4. Caducous ellipsoid sporangia of Phytophthora phaseoli with short pedicels. (Courtesy Jean B. Ristaino)
Figure 5. SPhytophthora phaseoli. A, One sporangium attached to its sporangiophore and the characteristic shape and very short pedicel of a detached sporangium (×630). B, Simple sporangiophores with sporangia attached on a leaf surface (×200). (Courtesy Robert P. Mulrooney; Reproduced from Evans et al., 2007) Click image to see larger view.
Figure 6. Oogonium of Phytophthora phaseoli with attached amphigynous antheridia, isolate Phyp15. (Courtesy Nancy Gregory, University of Delaware)
Figure 7. Signs and symptoms of downy mildew of lima bean, caused by Phytophthora phaseoli, on petiole and shoots. (Courtesy Robert P. Mulrooney; Reproduced from Evans et al., 2007)
Figure 8. Signs and symptoms of lima bean downy mildew, caused by Phytophthora phaseoli. A, Healthy flower racemes; B, infected flower racemes; and C, close-up of typical cottony mycelium and sporangia on flat pods. (Courtesy Robert P. Mulrooney; Reproduced from Evans et al., 2007) Click image to see larger view.
Figure 9. Lima bean downy mildew, caused by Phytophthora phaseoli. (Courtesy Robert P. Mulrooney; Reproduced from Evans et al., 2007)
Figure 10. Effect of Phytophthora phaseoli infections on lima bean pod production and color. Infected pods with various degrees of drying and necrosis. (Courtesy Robert P. Mulrooney; Reproduced from Evans et al., 2007) Click image to see larger view.
Figure 11. Lima beans with symptoms caused by Phytophthora phaseoli. (Courtesy Robert Wick, University of Massachusetts-Amherst)