In vivo studies evaluating commercial biofungicide suppression of blight caused by Phytophthora ramorum in selected ornamentals

Phytophthora ramorum is a regulated pathogen in North America, which causes ramorum blight on nursery stock resulting in the implementation of costly quarantine and eradication measures. Earlier studies showed some biofungicides could inhibit growth and reduce the disease in vitro. The objective of this study was to evaluate the effect of commercial biofungicides on in vivo disease development and plant growth on four nursery species when inoculated with three isolates representing each of the NA1, NA2 and EU1 lineages of the pathogen. The plant species were Gaultheria shallon, Rubus spectabilis, Rhododendron caucasicum x R. ponticum var. album and Cornus sericea. Prior to pathogen inoculation, plants were pretreated with Aliette (R) (standard fungicide), Actigard 50WG Plant Activator (R), Actinovate (R) SP, Sonata (R), Serenade (R), Plant Helper (R), SoilGard (R) 12G and Pro Mix BX Biofungicide (TM). A suspension containing 7000 sporangia ml(-1) was used to inoculate plants. Disease severity and foliar biomass were recorded 14 days after inoculation. Actinovate was the only product able to reduce disease severity by about 50% and improve plant growth of the susceptible hosts. Its effectiveness differed by isolate. Several other biofungicides and the standard fungicide provided suppression of disease and improved plant growth but their effectiveness was dependent on host species, product and isolate. None of the products prevented disease and generally the level of control obtained was lower than would be acceptable for a commercial nursery. In addition to assessing the disease control products, it was also observed that rhododendron, salal and salmonberry were highly susceptible hosts of P. ramorum with PR05-001 (NA2) being the most aggressive isolate on them.