For many of us, bumper crops of zucchinis and cucumbers conjure up the sweltering days of summer, while pumpkins and gourds decorate our holiday tables throughout the fall. However, these iconic fruits and vegetables – known collectively as cucurbits – can also help us understand the spread of plant diseases that pose a significant risk to crops.

by Lori R. Shapiro

I recently led a study that used crop species from two genera of closely related plants belonging to the family Cucurbitaceae – zucchini, pumpkin and squash, which are all in the same genus (Cucurbita) and are native to the Americas, as well as cucumber and muskmelon, which both belong to a genus (Cucumis) that is originally of Eurasian origin. I used these closely related, native and introduced Cucurbitaceae crop plants as a model system to understand how and why plant pathogens emerge.

For this study, I collected 88 samples of Erwinia tracheiphila from across these two host plant genera in the eastern United States and sequenced their entire genomes. E. tracheiphila is a pathogen that causes bacterial wilt in these species of cucurbits, resulting in tens of millions of dollars in crop losses and prevention costs to U.S. farmers each year. Yet, lack of basic knowledge about this pathogen means there are few control methods available for farmers.

E. tracheiphila’s genome is like Frankenstein’s monster – cobbling together lots of different pieces of genes and DNA from different sources, suggesting that it has recently undergone dramatic changes. There was also very little genetic difference between the genomes of the 88 different samples. From these genomic data, I determined that E. tracheiphila evolved as a pathogen only recently.

My colleagues and I also found three distinct bacterial lineages. Surprisingly, we also found that cucumbers are the only host plant susceptible to all three lineages, and the most susceptible host plant species overall. This is notable because, while pumpkin and zucchini are native to eastern North America and have been present throughout the Americas for millions of years, cucumbers and muskmelon were only introduced to North America after European colonialization in ~1500. This strongly suggests that the introduction of a foreign crop plant, followed by widespread cultivation in eastern North America, inadvertently caused the emergence of a new, very damaging plant pathogen.

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