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Re: Joel Grossman's Excellent Post
Another possible explanation for reduced effectiveness of fumigants/soil
pesticides over time is the phenomenon of enhanced pesticide
biodegradation. This is the accelerated dissipation of a chemical
from soil after repeated exposure of the soil to that chemical. The
cause is an increase in the population of microorganisms that use
the chemical as a nutrient or carbon source. A good review of this
topic was written by A.S. Felsot, 1989. (Enhanced biodegradation of
insecticides in soil: implications for agroecosystems. Annual Review of
Entomology: 34:453-476)
On Wed, 12 Apr 1995, Charles Benbrook wrote:
> Geez, SANET has had some really good posts lately. Joel -- thanks for
> adding some important details to discussion of disease suppressive
> soils. I too have looked long and hard through the literature in about
> 20 disciplines, talked to many experts, spent lots of time with farmers
> who know the difference between a disease suppressive soil and one which
> just does not seem able to slow down nematodes and pathogens.
> My research/contacts lead me to a plausible explanation of why
> the unfumigated trees catch up with the fumigated ones. Scientists have
> now documented at several levels, in many crops, a phenomenon called
> systemic acquired resistance. This is the mechanism whereby a plant
> attains a strong, or high degree of capacity to express its inherent
> potential immune response, its ability to withstand or overcome pest or
> stress attacks. Systemic acquired resistance (SAR) is sort of like a
> mammals response to allergy shots; to "work", or to happen, a plant must
> be exposed to some level of a pathogen early in its growth, when its
> immune system is, in effect, being turned on and stretching to
> accomodate, as best it can, the threats it thinks it will encounter in
> its environment. A plant or tree's immune response is, in effect, fully
> formed after it goes through early maturation. If the plant/tree is not
> subjected to a pathogen when its immune system is "growing", or gaining
> the capacity to "kick in" in response to particular pathogen pressure, it
> will never be fully able. It is a "need it early and use it early,
> or lose the capacity to develop it" phenomenon. This much we know.
> So, my guess is that plants/trees growing up in fumigated soils
> are not exposed to the low levels of pathogen attack needed to stimulate
> SAR. Hence such plants may grow well early on in the abscence of pest
> pressure, but later on when they SHOULD NATURALLY BE ABLE TO WITHSTAND a
> degree of pressure, they are immunological weaklings, having lacked the
> chance to "grow up with" the pathogens that are a normal part of their
> environment.
> I have some other more cmplex ideas/theories about mechanisms
> through which SAR is triggered, and how different management systems
> affect it, but this is not the time or place. Anybody encountered such
> an explanation before? Bob Goodman at Univ. Wisconsin plant path. dept,
> Joseph Kuc at Kentucky are two of the brightest, most broadly
> knowledgeful people on SAR and farming systems. Several excellent papers
> have been published in Science and elsewhere describing the mechanism.
> It is fascinating science, and lies at the heart of disease suppressive
> soils. My guess is that the capacity of a soil to suppress disease has
> as much to do with how soil microorganisms trigger plant physiological
> processes, especially SAR, as it does about microbial biocontrol of plant
> pathogens.
>