METHODS USED IN FRANCE TO STUDY STRAWBERRY POWDERY MILDEW IN LABORATORY CONTROLLED CONDITIONS

Abstract

Strawberry powdery mildew caused by the ascomycota fungus Podosphaera aphanis (Wallr.) U. Braun and S. Takamatsu, reduces fruit production and quality. It is becoming the most damaging disease of strawberry in the soil-less systems which have been recently widely adopted in France. A collaborative project has been developed by several French technical and research institutes to address this issue, with four main objectives: better understanding of the epidemiology of the disease, evaluating the susceptibility of populations of the pathogen towards fungicides, identifying sources of host plant resistance, and assessing new methods for disease management. Several methods were developed and applied to study this obligate parasite. Monoclonal strains were obtained from deposition of single conidia on detached strawberry leaves. Strains were maintained for several months without manipulation on vitro plants at low temperature (7-9°C). One- to two-week-old leaves from one- to two-month-old strawberry plants provided the most susceptible material, and were used to produce inoculum. Methods have been developed to measure epidemiological characteristics of the asexual cycle of P. aphanis: conidia germination, infection efficiency, mycelium growth, latent period, and sporulating intensity. Leaf disks and seedlings from strawberry cultivars with varying levels of partial resistance were assessed according to these epidemiological characteristics. Resistance of the fungus towards fungicides was evaluated by inoculating leaf disks previously sprayed with several concentrations of fungicides. Inhibiting concentrations were calculated relative to untreated controls and suggested a range of resistance levels to fungicides.