Soil Immunization, Pathogens & Plant Health

I’m sure you’ve heard it repeatedly – the advice to burn, bag up and dispose of (bury?) diseased plant parts. Keep reading for a different perspective on disease and what to do with those plant parts…

Pathogenic organisms (the ‘bad guys’) are everywhere – in the soil, on the wind, or carried around on the feet of insects. It is impossible to burn or bury them all. What to do? Consider that, in good soil, there are 250,000 + different species of microbes. Somewhere, in that crowd, there is a predator for whatever pathogen is feeding on the plants. Not just predators, but natural enemies, parasites, competitors and antagonists too!

So, instead of trying to vanquish the bad organisms (which is impossible anyways) consider creating favorable habitat for their predators, enemies, parasites, competitors and antagonists. One way to lure such allies into a system is to provide them lots of food…the diseased plant parts you’ve been told to burn and bury! One way to achieve this is ‘in situ’. John d’Hondt, an Irish potato farmer has eradicated potato blight through a practice he calls ‘Soil Immunization’. Instead of panicking when ‘Phytophthora infestans’ (potato blight) made an appearance in his potato field, he mulched, put a layer of compost out and let the diseased crops rot where they lay. He wanted those ‘Phytophthora infestans’ predators, enemies, parasites, competitors and antagonists to come right to where they were needed – in the field!

Another way to ‘grow’ pathogen predators, enemies, parasites, competitors and antagonists, is to put the diseased plant parts into a thermophilic compost pile (aerobic thermal compost). Really! The pathogens on the plant parts are food for their predators and lure the ‘good guys’ into the pile. Now we have our allies in the pile. Then, the heat and microbial activity of the composting process, acting together, reduce the pathogen level. Perfect! That compost can be used on the soil where the pathogens fed on the plants or to make a ‘compost tea’ for foliar treatment.

Why are the pathogens there in the first place?
Over 100 years ago, Rudolph Virchow, Father of Pathology said: “If I could live my life over again, I would devote it to proving that [pathogens] seek their natural habitat—diseased tissue—rather than being the cause of the diseased tissue; that is, mosquitoes seek the stagnant water, but do not cause the pool to become stagnant.”

In soil, plant pathogens are simply microbes that eat unhealthy plants. They are not the cause of the disease but a symptom of unhealthy plant tissue. The ‘bad guys’ will always be there and they have an essential ecological role – to remove the unhealthy, nutrient deficient plants from the gene pool! What an incredible service to those of us higher up the food chain – ensuring we consume the most nutrient dense food possible.
The physical symptoms we call disease (black spot, rust, wilt, etc.) are just the visual aftermath of the pathogens feeding on the unhealthy plant tissue. Like the mosquito to stagnant water, diseased tissue is the natural habitat for pathogens. They seek it out, but are not the cause of it. Burning or burying the pathogens is akin to shooting the messenger who is communicating that a plant is unhappy and un- healthy where it is growing.

So, why is the plant so unhealthy?
Healthy plants are actually toxic to the pathogens. The ‘bad guys’ can digest the diseased plant tissue, but not healthy tissue. So, instead of declaring war on the ‘bad guys’, heed their message and practice what Eliot Coleman calls a ‘plant-positive approach’: creating health, not by fighting and destroying disease, but by creating optimal growing conditions.

Some common causes of unhealthy plant tissue are: toxicity (chemical overload), compaction (lack of oxygen for microbes and roots), nutrient imbalances, too much or too little water (allowing saturation or wilting point conditions) and an unhappy microbe population (lack of beneficial organisms, no competition for the ‘bad guys’, not enough food, water or air).
In conclusion, deal with plant disease by growing pathogen predators in a thermophilic com- post, figure out why the pathogens were attracted to the plant in the first place, and work towards creating a healthy ecosystem where plants are happy, healthy and pathogen-feeding free!

Please visit to contact Kathleen Millar or for more information about the soil foodweb.

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