Conquistadors brought new human diseases like smallpox and cholera when they invaded North and South America. They also brought infections to local agriculture.

Bacterial wilt is a disease that can infect Curcurbita plants, like pumpkins, squash, and gourds. It also infects plants in the genus Cucumis, things like cucumber and muskmelon. Caused by the bacteria Erwinia tracheiphila, infected plants wilt, turn yellow and brown around the edges, and die off. The disease progresses down the vine until an entire crop could be lost. It is only transmitted from bites of the leaf beetle (the bacteria survives winter by living in the beetles' guts). Economic losses from bacterial wilt numbers in the millions of dollars.

E. tracheiphila is an unusual bacteria: its range is today limited to the Midwestern and eastern portions of North America, even though susceptible plants are distributed all over the world. When they analyzed the genetic diversity of the Erwinia bacteria, in search of an explanation for this unusually limited range, scientists last year found something unusual.

They found that Erwinia can be assigned to three related groups, but that overall there wasn't much genetic diversity in the bacteria, and genetic traits that are usually rare were overrepresented. These are two clues that the bacteria went through a "bottleneck," where a large percentage of the population died off, with the survivors eventually rebounding. They also found that cucumbers, a plant native to Europe, Asia, and Australia but absent from North America until the 1500s, could be infected by all three Erwinia groups, but pumpkins and squashes couldn't.

The scientists infer that the newly introduced cucumbers, brought by conquistadors in the 1500s, acted as a reservoir for Erwinia. Moving from Europe bottlenecked Erwinia, but it grew and diversified into infecting local plants native to the Americas. It had safe harbor in cucumbers until it developed the ability to spread elsewhere. In the same way that colonizers brought human diseases like smallpox and measles, they brought bacterial wilt to the crops.

So, if your squash is sick this Thanksgiving, you'll know who to blame.