Symptoms, Life Cycle, and Infection Process of Asian Soybean Rust (ASR)

Source of information is the APSnet

Monte R. Miles
USDA-ARS, Urbana, IL 61801

Reid D. Frederick

Glen L. Hartman
USDA-ARS and the Department of Crop Sciences
University of Illinois, Urbana, IL 61801


Life cycle and infection process
Two spore types are known in P. pachyrhizi. The urediniospore is the common spore type found throughout the season. The abundantly produced urediniospores are readily wind dispersed and multiple spore cycles occur throughout the season. Telia and teliospores have been found on infected plants late in the season in Asia as well as in the greenhouse (2, 12). Under laboratory conditions, teliospores have been germinated and basidiospores produced (11). Since no alternate host has been identified, there has been no further characterization of the life cycle.

The infection process starts when urediniospores germinate to produce a single germ tube that grows across the leaf surface, 5 to 400 µm, until an appressorium forms. Appressoria form over anticlinal walls or over the center of epidermal cells, but rarely over stomata (1, 6, 9). Penetration of epidermal cells is by direct penetration through the cuticle by an appressorial peg. When appressoria form over stomata, the hyphae penetrate one of the guard cells rather than entering the leaf through the stomatal opening. This rust and related species are unique in their ability to directly penetrate the epidermis; most rust pathogens enter the leaf through stomatal openings and penetrate cells once inside the leaf. The direct penetration of the epidermal cells and the non-specific induction of appressoria (5) in the infection process of P. pachyrhizi may aid in understanding the broad host range of the pathogen and may have consequences in the development of resistant cultivars.
Uredinia can develop 5 to 8 days after infection by urediniospores. The first urediniospores can be produced as early as 9 days after infection, and spore production can continue for up to 3 weeks (6, 7). Uredinia may develop for up to 4 weeks after a single inoculation, and secondary uredinia will arise on the margins of the initial infections for an additional 8 weeks. Thus, from an initial infection, there could be first generation pustules that maintain sporulation for up to 15 weeks. Even under dry conditions this extended sporulation capacity allows the pathogen to persist and remain a threat. If conditions for re-infection are sporadic throughout the season, significant inoculum potential still remains from the initial infection to reestablish an epidemic. Successful infection is dependant on the availability of moisture on plant surfaces. At least 6 hours of free moisture is needed for infection with maximum infections occurring with 10 to 12 hours of free moisture. Temperatures between 15 and 28°C are ideal for infection (3, 4, 8, 10).



The most common symptoms of soybean rust on cultivated soybean are 2 to 5 mm2 tan to dark brown polygonal lesions. Within each of the lesions is one to many erumpent, globose uredinia (Fig. 3 A-F).


  Fig. 3(A). Early infection by Phakopsora pachyrhizi on a susceptible soybean       Fig. 3(B). Phakopsora pachyrhizi: susceptible (TAN) lesions.   Fig. 3(C). Phakopsora pachyrhizi: red brown (RB type) lesion with some sporulation.            
  Fig. 3(D). Phakopsora pachyrhizi: red brown (RB type) lesion with some sporulation.       Fig. 3(E). Phakopsora pachyrhizi: RB lesion with circular ostiole and spores.   Fig. 3(F). Phakopsora pachyrhizi RB lesion with no sporulation.            
Urediniospores are released through the circular ostiole. Lesions are found on petioles, pods, and stems but are most abundant on leaves (Fig. 4). As rust severity increases, premature defoliation and early maturation of plants is common.
  Fig. 4(A). Soybean rust pustules caused by Phakopsora pachyrhizi on the upper side of a soybean leaf.     Fig. 4(B). Soybean rust pustules caused by Phakopsora pachyrhizi on a soybean cotyledon.     Fig. 4(C). Soybean rust pustules caused by Phakopsora pachyrhizi on petioles.        
  Fig. 4(D). Soybean rust pustules caused by Phakopsora pachyrhizi on stems.     Fig. 4(E). Soybean rust pustules caused by Phakopsora pachyrhizi on the under side of a soybean leaf.                   .  


Acknowledgement and Disclaimer
Photo credits: USDA-Agricultural Research Service except where noted from collaborators in the figure legend.
Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable

Literature Cited
1. Bonde, M. R., Melching, J. S., and Bromfield, K. R. 1976. Histology of the suscept-pathogen relationship between Glycine max and Phakopsora pachyrhizi, the cause of soybean rust. Phytopathology 66:1290-1294.
2. Bromfield, K. R. 1984. Soybean rust, Monograph (American Phytopathological Society), No. 11. American Phytopathological Society. St. Paul, MN.
3. Desborough, P. J. 1984. Selection of soybean cultivar and sowing date as a strategy for avoidance of rust (Phakopsora pachyrhizi Syd.) losses in coastal New South Wales. Aust. J. Exp. Agric. Anim. Husb. 24:433-439.
4. Hartman, G. L., Saadaoui, E. M., and Tschanz, A. T., Scientific eds. 1992. Annotated bibliography of soybean rust (Phakopsora pachyrhizi Sydow), AVRDC Library Bibliography Series 4-1, Tropical Vegetable Information Service. Taipei: Asian Vegetable Research and Development Center.
5. Koch, E., and Hoppe, H. H. 1988. Development of infection structures by the direct-penetrating soybean rust fungus (Phakopsora pachyrhizi Syd.) on artificial membranes. J. Phytopathol. 122:232-244.
6. Koch, E., Ebrahim Nesbat, F., and Hoppe, H. H. 1983. Light and electron microscopic studies on the development of soybean rust (Phakopsora pachyrhizi Syd.) in susceptible soybean leaves. Phytopathol. Z. 106:302-320.
7. Marchetti, M. A., Uecker, F. A., and Bromfield, K. R. 1975. Uredial development of Phakopsora pachyrhizi in soybeans. Phytopathology 65:822-823.
8. Marchetti, M. A., Melching, J. S., and Bromfield, K. R. 1976. The effects of temperature and dew period on germination and infection by uredo spores of Phakopsora pachyrhizi. Phytopathology 66:461-463.
9. McLean, R. J. 1979. Histological studies of resistance to soybean rust, Phakopsora pachyrhizi Syd. Aust. J. Agric. Res. 30:77-84.
10. Melching, J. S., Dowler, W. M., Koogle, D. L., and Royer, M. H. 1989. Effects of duration, frequency, and temperature of leaf wetness periods on soybean rust. Plant Dis. 73:117-122.
11. Saksirirat, W., and Hoppe, H. H. 1991. Teliospore germination of soybean rust fungus (Phakopsora pachyrhizi Syd.). J. Phytopathol. 132:339-342.
12. Yeh, C. C., Tschanz, A. T., and Sinclair, J. B. 1981. Induced teliospore formation by Phakopsora pachyrhizi on soybeans and other hosts. Phytopathology 71:1111-1112.

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