However, in crops where culture and virus indexing is done, the probability of serious losses due to organisms carried on or in the cuttings is greatly reduced. The fungus Pythium , which can cause damping-off, root and stem rots, and cutting rots is probably the main pathogen that can be brought into the greenhouse in water. Surface water such as lakes, ponds, rivers, and streams contain Pythium primarily in the bottom sediment. Run-off can carry the fungus from the soil into wells. Care should be taken to avoid pumping bottom sediment from water supplies into the greenhouse irrigation system.
Pythium is a major problem in hydroponic systems.
It becomes and important problem in ebb and flow systems when the system is heavily contaminated. The spores of powdery mildew fungi, Botrytis , rust fungi, Alternaria , and others can be carried by air currents from outdoor plants into the greenhouse. Infected plants in nearby gardens and weeds close to the greenhouse can supply enormous numbers of spores. Thus, even if great care is taken to eliminate other sources of pathogens, the air we breath may carry certain disease-causing organisms. During the warm months, it is critical to maintain greenhouse conditions that inhibit pathogen activity and to eliminate non-crop sources of pathogens around the greenhouse.