Dikaryon formation in Ustilago syntherismae: A key life-history trait in its interaction with Digitaria sanguinalis.

Loose smut sterilizing annual large crabgrass has occurred since 2004 in a field surrounded by crops near Barcelona (Spain); no other patch of smut-diseased plants has been seen in the nearby area. In order to understand the coexistence of both partners over the years, (i) the formation of the dikaryotic hyphae were investigated under fluorescence microscopy and (ii) spore viability was assessed by quantifying the proportion of germinated ustilospores from sori collected during three consecutive fall seasons in the same field. The dikaryon is formed by intratetrad mating; in none of the media tested could the formation of sporidia be observed. On average, 7.1% of ustilospores were able to germinate in a liquid medium without any organic carbon source following 2 years of storage. The role that these findings could play in disease escape and thus in the plant-pathogen coexistence at local field scale is discussed in light of the results.