This is what damage by the grape erineum mite or grapeleaf blister mite, Colomerus vitis (Pagenstecher), looks like. We’ve found it in a few of our vines now that they’re bursting into life. It produces deformations of blister-like galls on the upper surface of the leaves and whitish patches on the underside.
This is inevitable, especially since we’re using no fungicidal or pesticidal treatments on the vines, organically-approved (copper and sulphur-based) or otherwise. We are removing clusters of infected leaves as we find them and will see what develops as the growing season progresses. This year I hope we’ll be able to devote more time to the vines, managing the canopy more effectively to minimise the effect of this and other vine diseases such as downy mildew (Plasmopara viticola) and harvesting at a more optimum time. We paid neither enough attention last year, which left us with a very low yield.
We’ll be removing leaves from around the ripening bunches of grapes to encourage more air flow and sun exposure and thinning shoots only where necessary. Last year we found that too much shoot thinning stimulated even more shoot formation!
For reference, here is a summary of the results of experiments conducted in Hungary in the 90s. There are some interesting conclusions, among which are that canopy management gives much the same results in terms of disease incidence and impact as conventional or organic spraying, and that the incidence of grey or noble rot (Botrytis cinerea) owed almost nothing to the management of the vines and everything to the rainfall patterns during the growing season.