Overall, our results suggest that the location of A. citrulli in
watermelon seeds is influenced by the pathway of bacterial ingress.
In cool, dry conditions that generally exist in commercial
seed production fields, BFB fruit symptoms are not generally
observed but infested seed lots are still produced (4,38). This
suggests that pistil invasion might be involved in natural seed
infestation by A. citrulli. While the PE layer might block bacterial
penetration into the embryo in pericarp-inoculated fruits, the same
is not true for seed from pistil-inoculated fruits. Using the two
seed inoculation methods reported in this study, we can begin to
investigate how bacterial localization influences A. citrulli survival
in seeds. Insight gained from these studies may improve the
management of BFB and other seedborne phytobacterial diseases.
In particular, this information may lead to improved techniques
for A. citrulli extraction from seeds for improved seed health
testing and for developing improved methods for effectively
decontaminating seeds.
We also found that pistil-inoculation circumvented the PE layer
and deposited A. citrulli cells in the embryos of watermelon
seeds. This conclusion was supported by the observation that
significantly higher percentages of embryos were infested with A.
citrulli in pistil-inoculated seeds compared to pericarp-inoculated
seeds. Additionally, removal of PE layers and testae did not affect
BFB transmission for pistil-inoculated seeds, while BFB transmission
was significantly reduced for pericarp-inoculated seeds.
At present, details on the specific pathways of A. citrulli ingress
through watermelon blossom tissues are not available; however,
bacterial deposition might be influenced by the timing of development
of the different seed layers. To date, PE layer development
has not been described in detail for watermelon seed, but in
cucumber seeds the PE layer begins to form by 15 days post-