For this
study, we generated a type III secretion system mutant of A. citrulli,
AAC00-1ΔhrcC, that was nonpathogenic on watermelon but retained
its ability to colonize germinating watermelon seed. With watermelon
seed naturally infested with A. citrulli, AAC00-1ΔhrcC reduced BFB
seedling transmission by 81.8% relative to control seed. In comparison,
another A. citrulli antagonist, A. avenae strain AAA 99-2, reduced BFB
seedling transmission by 74.6% for seed samples from the same lot.
Additionally, when female watermelon blossoms were protected with
AAC00-1ΔhrcC and subsequently challenged with AAC00-1, the resulting
seedlots displayed 8% BFB seedling transmission. This was not
significantly different than seed from blossoms protected with AAA
99-2 (4%) but significantly less than those from blossoms protected
with 0.1 M phosphate-buffered saline (36%). These results suggest that
nonpathogenic A. citrulli has potential as a biological control seed