Sexual propagation on barberry and its role in stem rust pathogen virulence and diversity
Yue Jin: USDA-ARS Cereal Disease Laboratory
<div>In the past two decades, a limited number of races have been involved in wheat stem rust and stripe rust epidemics in East Africa and elsewhere. These highly virulent and aggressive races rose from obscurity, however. Investigations on a few known sexual populations of <em>Puccinia graminis </em>f. sp. <em>tritici</em> indicated that sexual cycles can generate diverse virulence recombinations, but only a few of which have the necessary virulences and aggressiveness enabling them to establish and thrive in the vast landscapes of cultivated hosts. There is little doubt that the Ug99 race group was originated in the highlands of eastern Africa, but how it came about is unclear. More complex virulence combinations in the Ug99 race group have been detected in recent years, indicating a continuous evolution. Our research has established that <em>Berberis holstii</em>, a native of East Africa highlands, is functional as the alternate host for <em>P. graminis </em>f. sp. <em>tritici</em> in Ethiopia. Its presence in proximity to wheat production in the highlands of East Africa may have had a role in the origin and evolution of the Ug99 race group. <em>Berberis</em> spp. also serve as the alternate host for the stripe rust pathogen (<em>P. striiformis</em>). A number of <em>Berberis </em>spp., including <em>B.</em> <em>holstii </em>and <em>B. vulgaris</em>, is known to be susceptible to <em>P. striiformis</em> f. sp. <em>tritici</em> in controlled inoculation. These species may have played a role in the origin of contemporary aggressive races that have been responsible for the sweeping epidemics around the world since the mid 1980s. Finding ecological niches where active sexual cycles take place may offer some predictability on novel virulences that we may anticipate in the future.</div>
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