Grape downy mildew spread and mite seasonal abundance in vineyards: evidence for the predatory mites Amblyseius andersoni and Typhlodromus pyri

Mite populations, in particular those belonging to the Phytoseiidae family were monitored in a commercial vineyard comprising two varieties (Prosecco and Pinot gris). On Prosecco, densities of Amblyseius andersoni, and to a lesser extent of Typhlodromus pyri, increased when downy mildew symptoms occurred on a high number of leaves. On Pinot, A. andersoni was the dominant species and its densities markedly increased when downy mildew spread. Amblyseius andersoni populations were significantly larger on leaves with downy mildew symptoms than on leaves without symptoms and their densities were positively correlated to the extent of leaf surface showing symptoms. The effect of downy mildew on T. pyri populations was less marked. There were no relationships between downy mildew spread and spider mite abundance (Panonychus ulmi). Untreated plots or plots treated with folpet to control downy mildew were monitored in an experimental vineyard colonized by A. andersoni. Amblyseius andersoni populations increased when downy mildew symptoms spread and their densities were significantly higher in the control than on folpet plots. In another experimental vineyard, T. pyri densities were significantly higher in the control (severely infected by downy mildew) than on copper hydroxide plots. Isoelectric focusing electrophoresis (IEF) was employed to detect downy mildew in mites and Glucose Phosphate Isomerase (GPI) was selected from among different enzymes. A stain corresponding to downy mildew in addition to the phytoseiid intrinsic stains was detected in a high proportion of A. andersoni and T. pyri collected from infected leaves. Downy mildew stains were not detected in P. ulmi females collected from infected leaves. Most A. andersoni and T. pyri females confined on symptomatic leaves became positive to IEF a few hours later.