S. Masri

Evaluation of several commercial biocontrol products on European and North American populations of Phytophthora ramorum

Abstract Five commercially available biological control products were tested in vitro with seven isolates of Phytophthora ramorum from North American (NA1, NA2), and European (EU1) populations. The in vitro tests included dual culture methods and detached leaf assays on wounded Rhododendron and Camellia leaves. Variability in response to biocontrol agents among isolates of P. ramorum from North American and European populations was examined. In dual culture tests, both Bacillus subtilis products (Companion® and Serenade®) resulted in better inhibition of the NA1 group than NA2 and EU1. Actinovate® (Streptomyces lydicus) was the least effective of the three bacterial biocontrol agents and there was no difference in percent inhibition among P. ramorum lineages. Two products containing Trichoderma spp. were tested: Plant Helper® (T. atroviride) caused 100% inhibition of all lineages of P. ramorum, while SoilGard™ (T. virens) was only about 30% effective. There was great variability among P. ramorum isolates in their response to biocontrol agents. All treatments reduced P. ramorum lesion size on both Rhododendron and Camellia. Combined treatments of Actinovate® with one other BCA did not perform as well as either treatment used individually. Best results were obtained with Serenade® on Rhododendron and Camellia foliage, especially against the NA1 group. Lack of a linear relationship between percent inhibition of P. ramorum by BCAs in vitro and foliar treatments on detached Rhododendron and Camellia leaves indicates that in vitro testing is a poor predictor of BCA performance on plant material.

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

Abstract 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® (standard fungicide), Actigard 50WG Plant Activator®, Actinovate® SP, Sonata®, Serenade®, Plant Helper®, SoilGard® 12G and Pro Mix BX Biofungicide™. 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.