Satellite uredinia: an important trait related to Phakopsora aggressiveness
Isabela Primiano: University of Sao Paulo
<div><em>Phakopsora euvitis</em> (<em>Pe</em>) and <em>Phakopsora pachyrhizi</em> (<em>Pp</em>) cause severe rust epidemics on grapevine and soybean, respectively. In contrast to <em>Uromyces appendiculatus</em> (<em>Ua</em>), the bean rust pathogen, both are highly aggressive, causing significant reduction on CO<sub>2</sub> assimilation even at low disease severity. A lesion is visible to the naked eye as a discrete area within which uredinia are produced. Comparative studies on lesion expansion were carried out to determine a possible correlation with secondary inoculum generation. Single-lesions of <em>Pp</em> on soybean, <em>Pe</em> on grapevine, and <em>Ua </em>on bean were measured every 3 days after the first disease symptoms developed on leaves. Lesion area was evaluated under the stereomicroscope before and after staining with trypan blue, which enabled visualization of mycelial growth inside the leaf tissue. The number of uredinia was counted. All pathosystems showed lesion expansion. Staining showed that mycelial colonization did not extend beyond the lesion border. The number of uredinia of <em>Pp </em>and <em>Pe </em>within the lesions increased over time (9 to 19 fold), whereas the number of uredinia of <em>Ua </em>did not change. Aggressiveness of both <em>Phakopsora</em> species correlated with the high number of satellite uredinia produced within lesions rather than with the total colonized leaf area. Formation of new uredinia within an existing lesion, without the need for a new infection site, seems to be an adaptation strategy of <em>Phakopsora</em> rust.</div>
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