Aerial transmission and strategies for control of Pythium on hydroponically grown cucumbers.

Abstract

Recirculating hydroponic cultural systems facilitate continuous dissemination of introduced plant pathogens. Chemicals are unavailable for the control of diseases in hydroponically-grown vegetables, thus alternate methods of control need to be investigated. An experiment was conducted to test the efficacy of filtration of infested water for the control of root rot of cucumber caused by Pythium aphanidermatum. Germinated cucumber seedlings were transplanted into separate hydroponic tanks. Each tank received water from a common source tank which was (a) recirculated through a 20-μm filter or (b) through a 7-μm filter. Pythium aphanindermatum was introduced into the source tank on two infected plants. Recirculation of infested water was conducted every other day for 5 days at a flow rate of 114 L/min for 30 min. One day after recirculation, 67% of the plants in the tank receiving water passed only through the 20-μm filter were infected and all plants were infected within 3-days. None of the plants in the tank which received water passed through the 7-μm filter were infected until 5-days post-recirculation. A repeat experiment yielded similar results, however there was a delay in the onset of disease expression. The fungus was recovered from the surface (0 mm) and middle (8 mm depth), but not from the inner core (16 mm depth) of the 7-μm filter. It was concluded that the 7-μm filter was effective in removing zoospores from infested water. Since plants in the tank receiving water passed through the 7-μm filter eventually became infected, an alternate source of introduction and spread of the pathogen was investigated. Aerial transmission of Phythium aphanidermatum by shore flies (Scatella stagnalis) was documented for the first time. Shore flies which were thought to feed only on blue-green algae and diatoms, also fed on cucumber roots colonized by the fungus. Ninety-seven percent of the first and second instar larvae, 20% of the pupae/third instar larvae, and 10% of adult flies carried mature oospores in their gut. Oospores excreted by larvae and adults were germinable. Phythium aphanidermatum was transmitted to healthy cucumber plants by naturally infested larvae and adult flies. Adult flies infested with P. aphanidermatum may account for pathogen introduction and spread within commercial greenhouse facilities.