One idea that is getting some play lately in the local scientific community is that of a disease suppressive soil.  A disease suppressive soil is one in which specific soil pathogens no longer inflict accustomed levels of crop damage due to the inhibitory activities of other soil microbes.   

Of note is that many of these microbes, generally bacteria and bacteria-like organisms, can be actively managed by the plant, which provide them, through the roots, with photosynthates thus influencing microbial activity and diversity.   The net result is that this soil, very close around the roots in most cases, inhibits fungal pathogen invasion of the plant and is said to be a disease suppressive soil.

The question then to further understand this marvel of nature is what are the key species and genetic mechanisms of these microbes which result in the suppressive property of a soil?  In other words, can we identify what makes a soil suppressive? 

A few key points from "Deciphering the Rhizosphere Microbiome for Disease-Suppressive Bacteria", the ground-breaking paper in this field from the journal “Science” (attached below):

1. The soil investigated in this study was suppressive, but this only came about after years of being severely affected by the fungal pathogen, Rhizoctonia solani.   Consistent with previous research, disease outbreaks are essential to get a suppressive soil – to gain immunity you have to get sick first.

2. Most suppressive soils lose their disease suppressive capability when pasteurized (in other words exposed to high temperatures; 80^oC in this paper).  Disease suppressiveness is transferable from a suppressive soil to a disease-conducive (non-suppressive) soil by mixing them together. These results indicate the microbial nature of the suppressiveness of the soil in this study.

3. An eye-popping 32,346 taxonomic units (classification groups) of bacteria and bacteria-like organisms were isolated from the soil associated with plant roots in this study.   No significant differences in numbers of taxonomic units were noted between suppressive and conducive soils, nor were any differences noted in any of the blended mixes of the two. 

4. However, and this is the key point, the study found that abundance of several bacterial taxa corresponded to suppressiveness of a soil.  In other words, it's not so much the species of bacteria present which confers suppressiveness to a soil, it's actually the numbers of individuals of specific bacterial populations which confer suppressiveness or lack thereof to soil.

The findings in this paper are quite illuminating in that they state soil suppressiveness cannot be attributed to the presence or absence of a taxonomic group of bacteria, but can rather be attributed to the combined effect of a number of microbial groups working together in differing abundances. 

Because of the tremendous potential such a technology could offer to agriculture, soil suppressiveness has been a bit of a holy grail for scientists.  However, in my mind, the sort of interactive complexity described in this paper between possibly hundreds of microbial species, soil environment, plant roots and the pathogen needed to obtain satisfactory disease suppression will confound all but the most determined attempts at comprehension.

Hat tip to Stefanie Bourcier of Farm Fuel, Inc. for this paper, provided below: