Localization of Acidovorax citrulli in watermelon seed and its influence on survival and seedling transmission of bacterial fruit blotch of cucurbits

Abstract

Bacterial fruit blotch (BFB) is caused by Acidovorax citrulli, a Gram-negative seedborne bacterium that can cause up to 100% fruit yield losses. The goal of this research was to determine the effect of A. citrulli invasion pathways on bacterial localization in watermelon seeds and to investigate the effect of bacterial localization on pathogen survival, seed health testing and exudates produced by seeds during germination. Pistil-inoculation of A. citrulli resulted in seeds in which the bacterium was localized in the embryo, whereas while pericarp inoculation resulted in bacterial localization under the testa but outside the perisperm-endosperm (PE) layer. In addition, significantly higher percentages of seeds became infested when A. citrulli was inoculated via the pistil than via the pericarp by 7 days post inoculation (DPI). We observed that bacterial location in seed is critical for A. citrulli survival. Acidovorax citrulli survived stress (surface and sub-surface seed treatments) better in pistil-inoculated seeds than in pericarp-inoculated seeds. We also observed that the localization of A. citrulli in seed affects seed health testing. The recovery of A. citrulli by seed washing was more effective for pericarp-inoculated lots than for pistil-inoculated lots. Additionally, it was observed that A. citrulli infestation affected the release of xylose in germinating watermelon seeds. Finally, we examined the effect of A. citrulli seed inoculum load on frequency of BFB seedling transmission and spatio-temporal spread among watermelon seedlings in the greenhouse. The proportion of seedlots that yielded BFB-infected seedlings was considerably higher for lots with one seed inoculated with ≥1 × 10^5 colony forming units (CFU) (>95%) than for lots with one seed harboring ≤1 × 10^3 CFU (<16.7%). The temporal and spatial spread of BFB in 128-cell seedling trays increased linearly with A. citrulli seed inoculum load.