Soilless, by its definition, is not hydro - because the substrate provides nutrient. I define coco as hydro - en evthough it is not an entirely inert substrate. I'm sorry, but by your definition, every time you water soil plants with a nutrient solution, it's "hydro". I think we both know it's not And for the record, plant pathogens such as pythium are in most water sources and spores are easily carried by wind and insects, meaning it is virtually ubiquitous. Plant roots have natural defences against root rot, only allowing it to take hold when they start to die or stress in anerobic conditions, and when temperatures are conducive to rapid multiplication of the pathogen, which start to colonise the roots in large numbers and enter the plant through damaged tissue. Which all means it's pretty much everywhere (except in the most sterile environments) and will attack the plant only when conditions are right.