I just have to say - even if it weren't for all of the valuable bonsai info I get from this group; it's worth being a member just for the occasional posts like this one from Nina and similar ones from Brent. I have learned more about plants in general from those two individuals than in reading all of the many books I've read on the subject. Thanks! And keep 'em coming.

Jeff Isom
Cleveland, OH / Sunset Zone 39


-----Original Message-----

I'm sure people on this list know how to ignor off-topic posts and how
to delete spam, and I'm sure you know, if you see a post by me, that
you can safely ignor it if you don't want to hear the ravings of a
plant pathologist.

Almost no root rots are caused by bacteria. After root rot has set in,
bacteria will follow, but they're almost never the causal agent. The
causal agent is usually a fungus or a water mold. Even these may be
coming in after wounding of the roots caused by insects or nematodes
(or pruning). Often we refer to "root rot syndromes" to indicate that
the etiology is complicated.

Often in plant pathology we refer to the "Disease Triangle" whose sides
are "pathogen", "susceptible plant" and "disease-conducive
environment". Obviously, you can have a plant but no disease if the
pathogen is absent. However, you can have the pathogen present but no
disease if the environment is nonconducive. You can have the pathogen
and a conducive environment, but no disease if the cultivar is
resistant. All three sides of the triangle must be present.

Sides of the triangle:
The pathogen: The problem with root rot is that the organisms are
almost always nearby, if not actually present. Bonsai growers use
nonsoil mixes rather than soil to reduce the possibility that root rot
organisms will be present, but if a potting mix is stored on the
ground, organisms can invade. Commercial peat can contain water molds
(I've isolated pythium from new bags of peat).

Conducive environment: Another problem is that some root rot organisms
are encouraged by warm weather (Fusarium, Phytophthora) and others by
cool weather (Pythium). In bonsai, the most common cause of root rot
is overwatering. The organism is almost invariably already in the soil
of a bonsai kept outdoors or which has spent some time in a greenhouse
(a colleague who is a world authority on Phytophthora in nursery
environments says you can isolate Phytophthora from the pots of 20-30 %
of symptomless plants in a nursery. My work with Pythium in
greenhouses has shown similar results). So when the environment
becomes conducive (soggy), the stage is set for disease. Well-draining
soil and the "chopstick" technique of watering are the best defenses.

Susceptible host: Most species used in bonsai are susceptible to root
rot. However, we can make a few generalizations. Mature plants are
less susceptible than seedlings. Overfertilized plants are more
susceptible than normally-fertilized plants. Plants that have been
recently pruned or repotted are more susceptible than ones that
haven't.

This spring when you repot, make sure to look for signs of root rot: a
foul smell, slimy, soft discolored roots. Remove all such roots.
Repot in well-draining mix and keep the plant free from all stresses.
A healthy plant is a resistant plant.

Nina.

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