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Release and dispersal of ascospores of Stagonosporopsis citrulli from colonized watermelon debris

Gabriel Rennberger: Coastal Research and Education Center, Clemson University

<div>Gummy stem blight (GSB), caused by 3 species of <em>Stagonosporopsis</em>, is a serious disease of cucurbits worldwide. A watermelon field inoculated with <em>S. citrulli </em>in Sep 2016 was used as a spore source for the following experiments. Every 2 weeks from Jan to Jul 2017, vine sections from 4 randomly chosen spots in the field were collected, rehydrated, surface disinfested and attached to lids of petri dishes containing agar for 4 days to monitor spore release. Release of ascospores from pseudothecia on vine sections was observed until 19 Jun, almost 10 months after the field was inoculated, and decreased significantly over time in a linear fashion (<em>P < </em>0.0001; <em>R<sup>2 </sup></em>= 0.58). From 25 Oct until 19 Jun seedlings of watermelon (Charleston Grey) and cucumber (Straight 8) were placed at distances of 0, 15, 31, 61, 122 and 244 m from the center of the field to trap spores on 10 dates. After 48 hours of exposure, trap plants were moved to greenhouse chambers, incubated for 4 days at >90% RH and examined for symptoms of GSB. Symptoms on trap plants placed 0 to 122 m from the center were observed on both cucurbits. Disease incidence on trap plants decreased significantly with distance and over time (<em>P </em>< 0.0001). Symptoms on trap plants were detected from 25 Oct until 19 Jun. The identity of isolates recovered from spore release plates and trap plants was confirmed as <em>S. citrulli</em> using a multiplex PCR. This study demonstrates the importance of ascospores in the dispersal of <em>S. citrulli</em>.</div>