Occurrence of Phoma argillacea on Rubus spectabilis in British Columbia and an evaluation of its potential as a forest weed biological control agent

Abstract

A survey was conducted on Vancouver Island and in coastal mainland British Columbia of naturally occurring fungal pathogens of salmonberry (Rubus spectabilis), an endemic plant species considered to be undesirable in conifer regeneration sites. Fungi were recovered from necrotic lesions on foliage and stems and identified to genus level. An in vitro pathogenicity test was performed on 281 fungal isolates using detached salmonberry leaves. Nineteen isolates were found to be moderately to highly aggressive, causing >50% leaf area necrosis within 14 days. These included Botrytis cinerea (17 isolates), Phoma sp. (3 isolates), and Septoria rubi (1 isolate). The three isolates of Phoma, identified as P. argillacea, were selected for further testing on salmonberry plants under greenhouse conditions. Mycelial or conidial inoculum were applied to the foliage of plants, which were incubated for 48 h at 20–22 °C and 100% relative humidity (RH) and then grown at 20–22 °C and 55–65% RH, with a 16 h photoperiod, under greenhouse conditions. Extensive necrosis of leaves and stems developed within 14 days for all isolates using mycelia and conidia as inoculum. A minimum of 18 h of continuous leaf wetness and mycelial inoculum of 2.6 × 103 colony forming units/mL was required for infection to occur. In a small host range test, three economically important conifer species, namely Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata), showed tolerance at the seedling stage with <5% disease severity. On red raspberry (Rubus idaeus), P. argillacea caused some initial damage (leaf spots and chlorosis) but failed to cause cane blight. These results demonstrate the pathogenicity of P. argillacea on salmonberry and the conditions required for establishing disease.