> > I apologise for the impromptu lecture on plant pathology,
> > and thank you for your attention.
> Don't apologize, plant pathology is very interesting.
> > There are (as Dale points out) certain chemicals that
> > cause plants to turn on these universal defenses (Actiguard,
> > by Novartis, is one. Salicylic acid is another component,
> It would be nice if organic growers could turn on systemic acquired
> resistance (SAR) using non-synthetic chemicals. Can salicylic acid do this
> in a practical sense?
It would be nice. Salicylic acid is very difficult to get into plants, and I
don't think it is UV stable. It is a signalling molecule in the SAR pathway,
and can be found in the Xylem and intracellular matrix (I think).
Phosphoric acid (found in Coca Cola) is better at turning on the SAR. I think
that it is one of the major ingredients in Alloette (sp).
One problem with turning on the SAR pathway is that the plant devotes a _large_
amount of energy producing compounds to prevent infection, and not on fruit
> > Mildews are just too specific.
> Since SAR is a fairly generalized response, it might be possible to turn it
> on in a plant using a non-specific, sub-virulent pathogenic assault. Are
> their less fastidious pathogens that one could brew up and spray on to
> trigger SAR, or would that be playing with fire? I am thinking of
> facultative saprophytes that will germinate and "attempt" to infect almost
In theory, yes. Are you playing with fire? Yes. The best thing to do might be
to brew up a batch of _highly_ pathogenic bugs, and then boil the batch, and use
the extract to spray. The extract may contain materials that could induce SAR.
Of course, the extract could also contain heat-stabile enzymes that would kill
your plants! I would do this with fungi, but not bacteria, and with _great_
caution. Better yet, I would let a university experiment station do this, and
watch to see what happens to their fields!
> > The important thing to remember is that most plant pathogens
> > don't cause plant disease on most plants. Disease is always
> > the exception. Reactions to pathogens go from none to resistant
> > reactions to susceptible reactions (disease).
> Don't certain bacteria colonize the leaf surface? It is amazing to me how
> seed-borne Xanthomonads and Pseudomonads can survive and propagate in plant
> canopies, causing disease even under arid conditions. They must have a very
> intimate relationship with the leaf surface. Of course, in the grand scheme
> of things fungi are much more important pathogens Could fungal triggers of
> SAR be put into leaf-surface bacteria to get generic SAR triggers?
Sorry, I didn't mention bacteria or nematodes (or viruses for that matter) .
Both of which can cause disease, and induce SAR (in fact the SAR phenomenon was
first observed in TMV infection and subsequent TMV challenging to different
leaves on the same plant).
Most plant pathogenic bacteria (except for most of the Erwinia spp.) do tend to
survive epiphitically, and will cause SAR to occur. Most often hypersensitive
responses occur (a different phenomenon all together), and are usually caused by
a direct genetic response. Once SAR is induced (by fungi, virus, herbivory, or
bacteria) it can be effective against all pathogens.
-- Russ Bulluck Ph.D. Candidate Department of Plant Pathology North Carolina State University PO Box 7616 Raleigh, NC 27695-7616
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The soil population is so complex that it manifestly cannot be dealt with as a whole with any detail by any one person, and at the same time it plays so important a part in the soil economy that it must be studied. --Sir E. John Russell The Micro-organisms of the Soil, 1923 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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