Reducing infection and secondary inoculum of Phytophthora ramorum on Viburnum tinus roots grown in potting medium amended with Trichoderma asperellum isolate 04-22

Abstract

Phytophthora ramorum is a concern of the ornamental nursery business because it is implicated in the movement on ornamental plants such as rhododendron and viburnum. Since there is a soil phase that infects the roots, above-ground symptoms are not often manifested and easily detected. Government agencies require that a P. ramorum-positive nursery be placed under some type of quarantine until it can demonstrate that it has mitigated P. ramorum. Typical methods of mitigation are often expensive, harmful to the environment, and impractical. Biological control using Trichoderma asperellum isolate 04-22 (Ta 04-22) has been demonstrated to reduce P. ramorum soil populations to below detectable limits when incorporated into soil or potting mix. The purpose of this study was to determine whether Ta 04-22 would reduce Viburnum tinus root infection and reduce secondary sporulation from infected roots. When rooted V. tinus cuttings were planted into potting mix amended with three different final concentrations (1×105, 1×106, and 1×107CFU/cm3 potting mix) of Ta 04-22 and drenched with either a P. ramorum sporangia or chlamydospore suspension, plants with any infected roots declined as the Ta 04-22 concentration increased. This was significant (P=0.039) when P. ramorum chlamydospores were used as the inoculum propagule but not (P=0.143) when sporangia were used. A second application of Ta 04-22 as a top-dressing after 4weeks did not result in a decrease in plants with infected roots. When P. ramorum-infected V. tinus roots were treated with Ta 04-22 grown on wheat bran, Ta 04-22 formulated as a wettable powder, or RootShield® Plus WP, flow-through of P. ramorum secondary inoculum was significantly reduced compared to non-treated controls (P<0.001). A second application did not reduce the secondary inoculum any further. These results demonstrate that various formulations of Ta 04-22 and, to a lesser degree, RootShield® Plus WP can be used as a microbial biological control treatment to reduce infection of V. tinus roots by P. ramorum and reduce plant-to-plant spread by reducing secondary inoculum.