vs something in the air? Most likely, could even be the water source and plants/cuttings can also contain virus/pathogens imho..but to know what the problem exactly is without lab tests we're poking around in the dark. Stem nematodes..bacterial virus..the list is long on what could cause duds..plant disease..deteriorating health could be brought on by salt stress..environmental stress..heavy metals, causing depletion in auxins and plants become more susceptible to pathogens. Chitosan,
Regalia, Salicyclic Acid are used to create a SAR response in the plants so they can secrete their own chemicals to help fight pathogens. Chitosan and
Regalia have bio-fungicide properties as well. EM1/Bokashi take the fight more directly to unwanted micro-organisms and assist in breaking down components in the soil so that they are more available to a plant.
Basically you want a foliar as well as soil drench approach.