Re: Thoughts on root diseases (was California poppies & root rot

"M. & L. Doster" wrote:
> 

> 
> Both Rhizoctonia and Phytophthora have a very wide host range and could
> attack almost any plant in a garden.
> 
> To complicate things, if the soil is too wet, then the plant can have
> problems from lack of oxygen. I have heard debates between plant
> physiologists (who think lack of oxygen for the roots is causing the
> symptoms) and plant pathologists (who think that pathogens like Phytophthora
> are causing the symptoms). Probably both factors are involved.
> 
Mark

Evn though I was trained as a pathologist  (many many years ago) as a
result of a lifetime's experience I definitely side with the
physiologists. I think it is very rare for non-obligate parasites at
least to attack perfectly healthy individuals, but any severe stress on
a plant can make it suceptible. In the case of partly oxygen-starved
roots, such as one finds in overwet soil, it is not uncommon for patches
of cells to die of suffocation even if the root largely survives and
such leisions are very good at letting in infections. 

(However, although the case of roots damaged by suffocation is pretty
clearcut, sometimes severe epidemics of disease do occur in which the
predisposing conditions are not so easy to untangle. Nevertheless I
believe that most if not all of these mysterious epidemics ARE based
ultimately on some unusual stress to the victims which has not yet been
diagnosed. Man is frequently responsible for a change in the fortunes of
whole tracts of country, and weather over several years now seems to
have become increasingly unpredictable, and either of these could
produce "cruel and unusual" stress in some species rendering them
susceptible to attack). In the case of root diseases the organism
involved is very often one which has co-existed with the tree for ages,
but only attacks when some change in circumstances (which could
admittedly on occasion  be a mutation for increased aggressivenes in the
fungus) starts off the cycle of infection.)

Apart from attending to such things as drainage, if you can create a
soil rich in humus plants are  less likely to host root diseases,
because the activity of benign soil organisms defending their territory
tends the protect the root zone and keep away many of the potential
pathogens. I have seen this in action in my own vegetable garden which
originally was host to another wide-ranging pathogen Sclerotinia
sclerotiorum, which was a scourge of my tomatoes every year and also
attacked many other crops from time to time. Once I ceased to use
artificial fertilizers and built up the life in my soil with compost and
mulches Sclerotinia virtually vanished. in fact I had not seen it for
mre than fifteen years until one autumn when it appeared on the pods of
a few late beans (but has not been seen in the four years since) It
appears that it must be still lurking, but the benign organisms round my
plants mostly keep it in check. (This is but one example out of several
on natural disease control I could quote from my own experience), 

As to the control of phytophthora. I have never had to try it myself but
I do know that the related Pythium which attacks seedlings can be
stopped from spreading by watering the soil with camomile tea. People
who have used it apparently simply steep the appropriate teabag in
water!  Of course this is only a treatment to prevent new plants being
attacked. Once the fungus has a hold on an individual nothing can save
it and the wisest course is to pull it up as soon as possible and
dispose of it, so the fungus does not get back into the soil.

For future control the main thing is obviously to remove the cause of
stress (in this case presumbaly poor drainage (??). I don't think it
would make much diference what is planted there next as it seems more
likely to be a general problem rather than specific to the poppies, but
almost any plant might be liable to attack if the conditions are not
improved.

Moira
-- 
Tony & Moira Ryan
Wainuiomata - at the Southern tip of North Island, NZ,
Lat 41°15'S, Long 174°58'E (Antipodes of Spain/Southern France)
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