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Epidemiology

For information on the disease cycle, see the pathogen biology page.

Soybean rust epidemics begin with the arrival of airborne inoculum (urediniospores). This pathogen is unique among rusts because it has many alternative hosts (Table 1), which may serve as sources of inoculum. Alternative hosts are other plants that can become infected with the same pathogen, but are not required to complete the pathogen’s life cycle. Alternative hosts are not to be confused with alternate host, which is a plant other than the principal host, that is needed for a pathogen to complete its life cycle. In frost-free areas, such as South America, Central America, the Caribbean basin, southern Texas, and Florida, the inoculum source could be nearby on volunteer soybean plants, kudzu, or some other alternative host. In areas that experience frost, such as the Midwestern United States, inoculum must be blown in from over-wintering sources that may be hundreds of miles away. Re-introduction of obligate pathogens into a distant region occurs with several other diseases, such as wheat stem rust and downy mildews, e.g. blue mold on tobacco. Because spores of P. pachyrhizi are sensitive to ultraviolet radiation, long distance movement of these rust spores probably occurs in storm systems where clouds protect the spores from the sun.

Once viable spores have landed on the leaf surface of a suitable host, infection and subsequent epidemic development are dependent on environmental conditions. Generally, infection occurs when leaves are wet and temperatures are between 8°C and 28°C, with an optimum of 16°C to 28°C. At 25°C, some infection occurs in as little as 6 hours of leaf wetness, but 12 hours are optimal. After infection, lesions and pustules with urediniospores can appear within 7 or 8 days, and the next infection cycle is set to begin. This short life cycle means that, under the right conditions, soybean rust epidemics can quickly build up from almost undetectable levels to very high levels. Soybean rust epidemics can progress from below detectable levels to defoliation within a month. Epidemics may seem to progress even faster than that, because early infections occur in the lower canopy and are hard to find. Besides the environment, plant age affects soybean rust epidemics. Usually, rust lesions are not found on soybean until flowering, unless there are high inoculum levels early in the season. This may be due to greater susceptibility of plants to rust as the host enters the reproductive stages, it may be because in lower parts of the canopy spores are more protected from UV radiation, or it may be because conditions in the canopy become more humid as the canopy closes. In any event, lesions can form at any growth stage, but major increases in disease do not occur until after flowering.

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