We are cautioned against keeping soil (and, as a result, the plant roots) constantly wet in order to avoid rot and fungus from harming the plant.
I can understand why ebb and flow systems would have some resistance, but why don't raft tanks and NFT hydroponic setups suffer from ceaseless blight? The constantly moist roots seem the perfect breeding ground for fungi.
Since you're starting with soiless systems, the time for severe fungal infections takes longer to develop. But they do. And it may be that the seedlings get infected before transplantation into the hydroponic systems as they may not be grown in sterile media.
Run to waste systems seem more prone to symptomatic infection then recirculating systems seemingly because the other microbes can compete against pathogenic fungi.
Not all of the root diseases occurring in hydroponic systems are readily dispersed or transported by recirculating nutrient solutions, but those fungi that are well adapted to living in water provide the most severe disease problems. Pythium, Phytophthora and Rhizoctonia species are often referred to as water moulds as they are particularly well adapted to water borne dispersal with the first two having motile (swimming) zoospores as well as other spores and fruiting bodies that can be carried by water. The corky root fungus ( Pyrenochaeta lycopersici ) is not spread in NFT solutions, but Didymella lycopersici can be spread in NFT solutions, although such spread does not normally result in stem lesions (7). Most diseases caused by Fusarium species, and by some Verticilium species, and black dot disease of tomato roots (Colletotrichum sp.) appear to spread well through nutrient solutions, as do many bacterial and virus diseases. Nematodes are not general a problem in hydroponically grown food crops in NZ, but are readily spread in recirculating nutrient solutions.
