FILTRATION AS A METHOD FOR CONTROLLING PYTHIUM ROOT-ROT OF HYDROPONICALLY GROWN CUCUMBERS

To test the efficacy of filtration of zoospore-infested water for the control of Pythium root rot of cucumber (Cucumis sativus) growing in recirculating hydroponic greenhouse culture, cucumber seedlings were transplanted into separate hydroponic tanks. Each tank received water from a zoospore-infested source tank. The infested water was recirculated three times through a 20-mu-m filter or through the 20-mu-m filter and then a 7-mu-m filter. Within 24 hr after the first recirculation cycle, 67% of the plants in the tank receiving infested water passed through only the 20-mu-m filter were infected; within 3 days, all plants were infected. None of the plants in the tank receiving water passed through the 20-mu-m and 7-mu-m filters were infected until 1 day after the third and final recirculation cycle. The fungus was recovered from the surface (0 mm) and middle (8 mm deep) of the 7-mu-m filter but not from the inner core (16 mm deep). Thus, the 7-mu-m filter effectively removed the fungus from infested water. Although plants in the tank receiving water passed through the 7-mu-m filter eventually became infected, shore flies (Scatella stagnalis) were suspected as introducing the pathogen into this tank.