Pseudomonas

Researching yesterday’s post on bacterial blight, I was astounded at the number of diseases caused by Pseudomonas. Pseudomonas [soo-doh-MO-nas] is a genus of bacteria that most of us gardeners end up fighting.

These bacteria are commonly found 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

To date, more than 500 strains of Pseudomonas have been sequenced. 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

 
Small dark spots appear and expand into odd-shaped dead areas in nearly all these diseases. 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. These bacteria 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. Most importantly, space plants far enough apart so they can dry off rapidly. 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 evil, it ends up that some of these soil bacteria help plants stay healthy. In fact, they practically make life possible on Earth. Life sure can be messy, eh?


Some Pseudomonas protect plant roots against disease-causing Fusarium fungi and Pythium oomycetes. They also protect against plant-eating nematodes. And other strains help activate disease resistance within wheat and other cereal crops. Some Pseudomonas can metabolize pollutants and are used in bioremediation.


Finally, Pseudomonas helps form most snowflakes and raindrops that fall on Earth.


Now you know.