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Re: Plant Resistance to Herbivory

Hi to all sanets,
Joel Gruver's posting regarding plant resistance to attack by herbivores
is a most interesting one. Many involved in organic farming would
suggest that a Healthy soil->Healthy Plant is less prone to attack by
pests and diseases. This is fine but what evidence is there to suggest
that this might be the case.

There are many scientists that accept that a healthy fertile soil does
result in stronger healthier plants. This is not however altogether

A soil of high fertility is generally accepted as one that will result
in a plant being able to maximise both its physical and nutritional
capabilities  . This means that the soil will be stocked with a wide
range of the 13-23 elements required for plant nutrition while having
adequate organic matter that will not only support an active microbial
population but on decomposition will provide a range of inorganic
nutrients and importantly organic substances that will assist in
mobilising nutrients through chelation to assist in plant nutrition and
protect the plant from many soil borne disease causing pathogens.

The result of this fertility is suggested in the literature as providing
a plant with stronger cell walls assisting in pest and disease invasion(
Schuphan 1965, Chaboussou, 1986) and a more balanced nutritional make-up
of the plant(Chaboussou 1986, Eigenbrode & Pimental 1988). Chabouussou
suggests in his theory of trophobiosis (protein synthesis) that
imbalanced nutrition leads to free amino acids and other substances
circulating in the sap stream of plants that attract pests and disease.

Eigenbrode & Pimental studied collards in the field that were fertilised
with organic and chemical fertilisers and found that although the
mechanisms remained unclear, resistance of collards to insect attack was
greater when fertilised with organic manures.

I would encourage all those interested in this area of research to also
search out the paper by Chaboussou as it really does provide some
insight into why plants are attacked by pests and diseases and why also
importantly that pesticides often increase the likelihood of attack by
pests and diseases. Probably the most important paper in my library on
plant nutrition and soil fertility.

The references are:
Chaboussou, F. 1986 The Ecologist Vol. 16, No. 1, How Pesticides
Increase Pests
Eigenbrode & Pimental 1988 Agriculture & Ecosystems & Environment, Vol.
20 P109-125

Chris Alenson
Organic Advisory Service
Organic Retailers & Growers Association of Australia