Controlled ecological life support systems (CELSS) which include higher plants for food and oxygen production are proposed for permanent manned space stations and long-duration space flights. A primary concern in the design and operation of such a life support system is maintaining plant health and maximizing plant growth rates. It is inevitable that a variety of microorganisms such as fungi and bacteria will be introduced into CELSS. A potential problem for plant growth systems is plant pathogenic microorganisms. Pathogens could cause major perturbations which would be highly undesirable in a closed life support system. In CELSS plant growth systems, microorganisms can not realistically be excluded, but they can be managed to favor beneficial microorganisms and exclude undesirable microorganisms. There are 5 principal methods of managing microorganisms in plant growth systems.