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One Pumpkin Pathogen Is so Successful It's Stopped Evolving, Say Scientists

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The pathogen that causes bacterial spot is very good at what it does. Forming small lesions on the rinds of pumpkins, melons, cucumbers, and other cucurbits, it mars the fruits’ appearance and ushers in 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 University of Illinois Urbana-Champaign research characterizing the pathogen’s genetic diversity across the Midwest.

“Previously, we sequenced the Xanthomonas cucurbitae genome from a sample taken in New York in 1926. In our current study, we sequenced genomes of samples taken by our collaborators across the Midwest in 2012 and 2013. All of the genomes were really quite similar, greater than 99% identical, but one isolate from Michigan was 99.9% identical to that 1926 isolate. Our results show there's been very little pressure on this pathogen to evolve,” said Sarah Hind, assistant professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.

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The surprising result suggests the pathogen is a one-trick pony. It surges reliably under specific conditions — hot and wet — and lurks quietly in the background otherwise. But the genetic uniformity could be good news for breeders looking to develop resistant crops.

“If we were able to deploy a resistant plant population, then it should be pretty effective against what they would likely encounter, at least across the Midwest and probably much of the U.S., because there's not a lot of diversity in pathogen populations,” she said.

Hind says there’s currently no known resistance in commercially available cucurbit crops, but she and her collaborators found bacterial spot resistance in experimental pumpkin and squash lines in a 2021 study. Still, she says they’d need to screen a lot more varieties before breeding efforts for resistance could begin in earnest.

In addition to opening up potential opportunities for developing disease-resistant cucurbits, the findings could inform current and future management strategies. For example, Hind says bacterial spot isn’t particularly responsive to industry-standard copper antimicrobial sprays. Knowing more about the genetic capabilities of the pathogen could forecast whether such management strategies will be effective long-term.

Reference: Rai R, Vittore KM, Pasion J, et al. Comparative genomics of Xanthomonas cucurbitae isolates collected from Midwestern United States pumpkin fields. Plant Pathology. 2024. doi: 10.1111/ppa.13965

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