The pathogen that causes bacterial spots is very good at what it does. Forming small lesions on the skin of pumpkins, melons, cucumbers and other cucurbits, it spoils the appearance of the fruits and introduces secondary pathogens that lead to rot and severe yield loss. The bacterium, Xanthomonas cucurbitae, is so successful that it has had no reason to evolve through time or space. That’s according to new research from the University of Illinois, Urbana-Champaign, which characterizes the pathogen’s genetic diversity in the Midwest.
“Earlier we sequenced the series Xanthomonas cucurbitae genome from a sample taken in New York in 1926. In our current study, we sequenced the genomes of samples taken by our collaborators in the Midwest in 2012 and 2013. All the genomes were actually quite similar, more than 99% identical, but one isolate from Michigan was 99.9% identical to that 1926 isolate. Our results show that there has been very little pressure on this pathogen to evolve,” said Sarah Hind, assistant professor in the Department of Crop Sciences, part from the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.
The surprising result suggests the pathogen is a one-trick pony. It peaks reliably under specific conditions – warm and wet – and otherwise lurks quietly in the background. But the genetic uniformity could be good news for breeders looking to develop resistant crops.
“If we could deploy a resistant plant population, it should be quite effective against what they would likely encounter, at least in the Midwest and probably much of the US, because there isn’t a lot of diversity in the pathogen populations,” says she. said.
Hind says there is currently no known resistance in commercially available cucumber crops, but she and her collaborators found bacterial spot resistance in experimental pumpkin and squash lines in a 2021 study. Still, she says they need to screen many more varieties before breeding efforts for resistance can begin in earnest.
In addition to opening potential avenues for the development of disease-resistant cucurbits, the findings may inform current and future management strategies. For example, Hind says bacterial stains don’t particularly respond to industry-standard copper antimicrobial sprays. Knowing more about the genetic capabilities of the pathogen can help predict whether such management strategies will be effective in the long term.
