Treat with sulfa right away. Let sit 12-24 hours and add a well started yeast starter using domesticated yeast. The domestic yeast can tolerate higher sulfa levels so you can add sooner to prevent reinfection. If you really do not want to use domestic yeast get a galleon or two of fresh raw apple cider. That will be loaded with wild yeast. Let it start to ferment before adding to your grape must. But keep the grape must dosed with sulfa and cold while you wait to kill off the infection/mold and prevent it from reinfecting.
Pesticides kill off lots of stuff. However an early aplication followed by months of rain should have washed off enough of it for the wild yeast to make a come back. This is my uneducated guess anyway.
Check the temp. Too cold and mold will grow but the yeast will not.
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