Biological control of Fusarium and Pythium root rots on greenhouse cucumbers grown in rockwool

The two most important root rot pathogens on greenhouse cucumbers (Cucumis sativus L.) in British Columbia are Fusarium oxysporum f.sp. radicis-cucumerinum and Pythium aphanidermatum, for which there are few chemical pesticides registered for disease control. Biological control agents (BCA) may therefore provide a potential means of disease control if efficacy data were available to support their use. Several commercially available BCA were evaluated for their ability to reduce disease due to these two pathogens. Mycostop (Streptomyces griseoviridis strain K61), RootShield Drench (Trichoderma harzianum strain T-22), SoilGard (Gliocladium virens strain GL-21), and Prestop WP and Mix (Gliocladium catenulatum strain J1446) were each evaluated for their potential to reduce root and stem rot (RSR) due to Fusarium and root rot and damping-off (DO) due to R aphanidermatum on cucumbers grown in rockwool under growth room and greenhouse conditions. Each BCA was added to the seeding cavity of rockwool blocks as a drench, or was incorporated into vermiculite and then added to the cavity at seeding time. In growth room trials against RSR, the pathogen was applied 24 hr after seeding as a drench and disease development was evaluated 37 days after seeding. Against DO, the BCA were applied at seeding and again 11 days after seeding, followed by the pathogen 24 hr later. Disease development was evaluated after 19 days. The most effective BCA in reducing mortality due to RSR and DO in growth room trials (by up to 50%) was Prestop WP, while Prestop Mix also significantly reduced RSR. Mycostop also reduced RSR, but to a lesser extent. RootShield Drench and SoilGard did not reduce RSR or DO significantly under these experimental conditions. In greenhouse conditions, with two applications of the pathogens and the biological control agents, Prestop WP, Mycostop and RootShield Drench all reduced RSR and Mycostop and Prestop WP and Mix reduced DO. The application of commercially available BCA at seeding time has the potential to reduce the severity of Fusarium and Pythium root rots on cucumbers grown under greenhouse conditions.