Spatial and Temporal Localization of Serratia ureilytica Causing Cucurbit Yellow Vine Disease in Cucurbits Indicates Phloem-Associated Colonization and Systemic Movement

Cucurbit yellow vine disease, caused by the bacterium Serratia ureilytica , is a phloem-associated disease of cucurbits. This study characterized the spatial and temporal distribution of S. ureilytica in Cucurbita pepo cultivar ‘Delicata’ plants under greenhouse conditions using a GFP-tagged isolate (P01). Seedlings were sampled weekly for 4 weeks. Transverse sections from the stem, petiole, leaf, shoot apex, and root were imaged by laser scanning confocal and fluorescent dissecting microscopy. In parallel, bacterial abundance in each plant tissue was assessed by quantifying colony-forming units (CFUs) via droplet plating over a 4-week time course. Across plant tissues and time points, the S. ureilytica fluorescent signal was primarily concentrated in the inner and outer periphery of the bicollateral vascular bundles, with higher magnification images revealing mainly symplastic localization within phloem-associated cells. Consistent with the imaging results, bacterial quantification data showed a high abundance of CFUs in the main stem across weeks, with an irregular pattern of presence in the distal tissues at later time points. These results suggest that S. ureilytica is predominantly localized within phloem-associated cells and spreads both acropetally and basipetally during infection.