Life Cycle, and Infection Process of Asian Soybean Rust (ASR)
of information is the APSnet
Monte R. Miles
USDA-ARS, Urbana, IL 61801
Reid D. Frederick
FDWSRU, USDA-ARS Frederick, MD
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).
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