For soil-borne pathogens, there is no correlation between the presence of the organism in the soil and the expression of the disease in the crop. Infections severe enough to produce crop loss are correlated with the absence of suppressive organisms more than the presence of the pathogen.
Soil colonizing organisms are usually dependent on crop residue for nutrition and generally have higher nutrition requirements. Soil inhabiting organisms have much lower nutritional requirements and remain present in the soil more or less constantly.
Both groups can be effectively managed with cultural management practices to prevent any infections from occurring. From the podcast interview with Don Huber.
John: That’s a very impressive statement. We can manage disease and pathogenicity based on how we manage our soils, from a cultural perspective. That’s a very, very important perspective that I think we don’t commonly hear in agriculture.
You mentioned a number of different management tools: crop rotations, using cover crops, tillage, the impact of moisture, etc. Earlier you spoke of the differences between soil-borne pathogens and soil-inhabiting pathogens. It’s fairly well understood that we can use crop rotations to manage soil-inhabiting pathogens. Are you suggesting that it’s also possible to use these tools to manage and suppress soil-borne (colonizing) pathogens?
Don: Very definitely. Most of our soil-borne (colonizing) pathogens have very limited genetic resistance. We rely on those management techniques to control them. Sometimes we don’t recognize it as much as we need to, but soil-borne pathogens have a much more limited relationship as far as population dynamics. We may measure the population of spore load and other things for organisms like Fusarium, but the organism is there in a high-enough population that regardless of what we do—if we didn’t have the other organisms associated with it—it would take our crop.
Soil colonizers colonize only as long as they have a nutrient base to function with. So we can either extend the time between susceptible crops—which we typically do with most of our potato pathogens, for instance—we see them building up in two or three crops and we want to break that population down. Same thing with anthracnose on corn. It’s a soil colonizer. Cephalosporin on wheat. All of those organisms survive in the residue. Many of them even produce an antibiotic, so they slow down residue degradation to extend their lifetime in the soil—so that other organisms aren’t able to colonize that food base.
This is quite different from Rhizoctonia or Fusarium—many of the Basidiomycete-type pathogens are very excellent soil inhabitants. They don’t require the base of nutrients that many of our colonizers do.