Natural warfare between a virus and a Pseudomonas bacterium is helping scientists learn about the ability of the bacterium to kill rocket, broccoli, and several other cruciferous vegetables.

Carolee Bull, a plant pathologist with the US department of agriculture’s Agriculture Research Service (ARS) and her fellow researchers use the virus, known as PBSPCA1, as the basis for a lab test that helps quickly identify Pseudomonas cannabina pv. (pathovar) alisalensis.

In ongoing research that dates back to 1998, Bull and her co-investigators have detected and identified the family tree of the bacterium and determined that it is the culprit behind a costly disease called bacterial blight of crucifers. The blight causes water-soaked spots to appear on plant leaves. Eventually, the spots coalesce and turn brown, giving the leaves an unattractive, burnt appearance that makes the vegetable unmarketable.

Early on, the scientists' field, greenhouse, and laboratory studies indicated that the crucifer blight bacterium could be easily confused with a close cousin, P. syringae pv. maculicola, which causes pepper spot disease. The two different pseudomonads kill some of the same vegetable crops, and several standard lab tests cannot reliably tell which bacterium is which.

Bull and her colleagues chose the PBSPCA1 virus as the basis for a lab assay that reliably sorts out the two confusing bacteria. Because it can kill the crucifer blight bacterium, but not the pepper spot pseudomonad, the virus can be used to differentiate one from the other.

The California research team helps growers and plant pathologists in the US and abroad identify the crucifer blight bacterium. Positive identification matters, especially when growers are deciding what to plant. For example, Bull’s studies have shown that broccoli, cabbage, and cauliflower are vulnerable to both the crucifer blight and the pepper spot bacteria, while some other crops, such as lettuce, are not.

Bull said: “Correct identification and classification is the first step toward controlling plant diseases…We showed that the host range of the blight pathogen is broader than the host range of the pepper spot pathovar,” says Bull. “Our host-range data helps growers plan strategic crop rotations, such as planting lettuce, a non-host of the blight pathogen, in blighted fields.”

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