I think I’ve got downy mildew on my grapevine that circles half of my house. For the last 3 years the leaves become mottled, turn brown and fall off. The Interweb suggests downy mildew but there does not appear to be a treatment available to non professional growers. Any suggestions on how I can treat it?

Potassium bicarbonate (baking powder) 5 grams per litre of water plus a wetter somthing like non coloured non scented washing up liquid spray on all leaf surfaces and fruit. It has eradicant action apply 2-3 times aweek avoiding the hottest part of the day.
we use it on commercially grown fruit there is no issue with harvest intervals as it is not an agrochemical.

Thanks I will give it a go, although the vine is looking pretty dire at the moment and maybe on its way out. Shame as it’s taken me over 20 years to grow it.