I've always been aware of this notion, and when I planted my vines I put a rose bush every few rows but I have found them to be useless in their canary in coalmine role. Mildew/Odium comes with no warning at all...(in my experience)...the temperature drops below 25C (77F) ...perhaps some rain...and Bob's your uncle, it can zap you out of nowhere in a couple of hours.. I can't see how a rose bush can forecast three weeks in advance...it's not as if the disease is slowly making its way through the vines. Early on in my deranged vineyard experiment I lost row after row of vines...I was buying standard fungicide from my farmers' coop...Metalaxyl and Mancozeb. Our local crop is pimiento (famous Pimenton de la Vera) and tobacco...maybe it works with them, but I would strongly advise to buy specialised grapevine fungicides such as Vivero, Enervin, etc equivalent. After ringing the changes with fungicides so as to avoid the fungus developing immunity while overwintering...I now rarely lose more than a few vines a year.