Fungi associated with roots of cucumber grown in different greenhouse root substrates

Abstract

Hydroponic soilless culture systems are commonly used in the greenhouse industry to mitigate soilborne root pathogens, but root diseases continue to be a major problem. A thorough knowledge of the root zone ecology of greenhouse crops is needed for the successful biological control of root diseases. The objectives of this study were to characterize the fungi associated with symptomless cucumber roots grown using different greenhouse substrates and to determine the effects of these fungi on plant growth. A total of 1250 fungal isolates were collected from cucumber roots grown in soil, rockwool, sawdust, or nutrient film and identified to genus and species. There was a greater density of fungal colonies on roots grown in soil than on roots grown in other substrates. Penicillium (87.2% of all isolates), Trichoderma (4.6%), and Pythium (3.0%) were the most common genera isolated from the cucumber roots. The two most common species were Penicillium oxalicum Currie & Thom (69.9%) and Penicillium janthinellum Biourge(13.4%). Pythium group F, Pythium group G, Pythium aphanidermatum (Edson) Fitzp., and Pythium irregulare Buisman were also isolated from healthy cucumber roots and all except Pythium group F were pathogenic to germinating cucumber seedlings. Treatment of cucumber seeds with Penicillium spp. resulted in significantly greater shoot length, root length, and seedling fresh weight of the resulting cucumber seedlings than did treatment with Trichoderma spp. One isolate of Penicillium janthinellum and one of Aspergillus sp. significantly improved survival of cucumber seedlings growing in a Pythium aphanidermatum-infested medium.Key words: hydroponics, Penicillium, Trichoderma, Pythium, Cucumis sativus.