Abstract

Bacterial fruit blotch (BFB), caused by Acidovorax citrulli , is a devastating seed-borne disease of cucurbit crops. The chemical form of nitrogen fertilizer is a key factor modulating host plant defense and pathogen virulence. In this study, we investigated the effects of three nitrogen regimes on watermelon resistance to BFB by assessing disease severity, seedling biomass, stem vascular morphology, expression of defense-related genes, and in planta growth of A. citrulli . Our results showed that sole ammonium nitrogen application led to a significantly higher disease index compared with nitrate nitrogen alone or a 1:1 nitrate-ammonium mixture. Moreover, nitrate nitrogen application enhanced watermelon seedling growth, calcium uptake, development of stem vascular tissue, and expression of disease resistance-related genes. Temporal dynamics revealed that A. citrulli populations in cotyledons were initially lower under sole ammonium but surpassed nitrate and mixed nitrogen treatments at later infection stages. Furthermore, the expression of type III secretion system genes hrpG and hrpE in A. citrulli from watermelon cotyledons initially decreased and then increased over time, with this fluctuating expression pattern accelerating as the ammonium proportion in the fertilizer increased. This finding suggests that A. citrulli establishes infection more rapidly in watermelon seedlings under sole application of ammonium nitrogen. Together, nitrate nitrogen mitigates BFB severity in watermelon, whereas sole ammonium exacerbates disease likely via combined effects of weakened plant vigor, altered tissue nutrition, and accelerated pathogen infection. This study offers a theoretical foundation for nutrient−based management of watermelon BFB.