Hairy Root Disease

Hairy root disease is a mixed bag.
 
Formerly known as Agrobacterium rhizogenes, recent research has reclassified this soil bacterium within the Rhizobium genus, changing its name to Rhizobium rhizogenes and establishing it as a cousin to the beneficial soil bacteria that fix atmospheric nitrogen.

Hairy root host plants
 
Blackberries, blueberries, mulberries, raspberries, and fruit and nut trees are susceptible to hairy root. This disease is also a serious problem for cucumbers, eggplant, bell peppers, and tomatoes grown hydroponically in European greenhouses. In my opinion, it’s only a matter of time before this condition appears in all gardens. And that might not be all bad.

​Hairy root symptoms
 
Hairy root disease (HRD), also known as crazy roots, is a bacterial disease.
 
These bacteria infect plant cells with their own genetic material, reprogramming them to produce a proliferation of clustered, adventitious roots, hence the name. These deformed roots often result in an unsustainable surge of vegetative growth, leading to crop reductions of 10% or more. Because hairy root alters the DNA of the host plant, the effects are irreversible.

Hairy root of apple (Annual report of the Commissioner of Agriculture, 1900)

Hairy root management
 
If hairy root disease becomes a problem in your landscape, soil solarization may be your only option.
 
It is much easier to prevent this problem in the first place by buying certified pest- and disease-free plant stock from reputable sellers. As tempting as it may be, do not add grocery store plants or seeds to your landscape. Those plants are certified safe to eat, not to grow. If you must grow plants from the grocery, keep them in containers and place them in quarantine to avoid putting other plants at risk.
 
Also, be gentle with your trees and shrubs when transplanting. Damaged roots are more susceptible to infection.

All that being said, research at Oregon State back in 1979 found that infected asymptomatic apple trees allowed to grow were better able to respond to drought. And that stems were “equal to or slightly larger” than uninfected wood.

​Hairy root grows so readily on carrot slices without the use of plant hormones that it is commonly used to study plant metabolic and phytoremediation processes.
 
So it isn’t all bad, but I still don’t want it in my garden.

Carrot slice infected with hairy root bacteria (FruitDefendu) CC BY-SA 3.0