Pseudomonas

Pseudomonas [soo-doh-MO-nas] is a genus of bacteria that most of us gardeners end up fighting.

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These bacteria reside in plant debris, soil, and water. They also hide out in many dicot seeds. But don’t worry, Pseudomonas only infects plants with leaves and stems. The rest of your garden is safe. *wink*

Tomato leaf infected with bacterial speck (Alan Collmer, Cornell University) Public Domain

Pseudomonas plant pathogens

Scientists have sequenced the DNA of more than 500 strains of Pseudomonas. Here is a list of the most common bacterial diseases caused by Pseudomonas:

• Angular leafspot
• Bacterial blight
• Bacterial brown spot
• Bacterial canker
• Bacterial dieback
• Bacterial leaf spot
• Bacterial soft rot
• Bacterial speck
• Citrus blast
• Halo blight
• Leaf spot
• Olive knot

 
Nearly all these diseases feature small dark spots that expand into odd-shaped dead areas. It’s all downhill from there.

Bacterial leaf spot on a beet leaf (Howard F. Schwartz, Colorado State University, Bugwood.org) CC BY 3.0 US

Managing Pseudomonas

These are some tough SOBs. They have evolved to survive rugged conditions. Their cell walls are equipped with pumps that eject antibiotics and other unwanted materials before they can do anything, so chemicals are often ineffective. Because of this, prevention is your best management tool. Spacing plants far enough apart so they dry off rapidly is the best prevention. And avoid overhead watering.

Bean leaves with bacterial brown spot (Nancy Gregory, University of Delaware, Bugwood.org) CC BY 3.0 US

Pseudomonas isn’t all bad

As handy as it would be to say that all Pseudomonas are trouble, it ends up that some of these soil bacteria help plants stay healthy. They practically make life possible on Earth. Life sure is messy.


Some Pseudomonas protect plant roots against disease-causing Fusarium fungi and Pythium oomycetes. They also protect against plant-eating nematodes. Another strain helps activate disease resistance within wheat and other cereal crops. Some Pseudomonas can metabolize pollutants and are part of bioremediation efforts.


Finally, Pseudomonas are responsible for most snowflakes and raindrops that fall on Earth.


Now you know.