Good point LarryGene.
I visited my friend who has a Concord grape growing in her GH and compared notes. She is at the same altitude as my place but there are some significant differences between our setups. She has E and W doors which are kept open for venting during the days, and since the prevailing winds here are from the W, this keeps her GH considerably cooler and less humid than mine. Her grape vine is doing beautifully, and has never suffered from any diseases (I know this started as BER on a zuke, but my grapevine in the GH also has some rust like problem).
So I decided to keep the vent fan on all night for a night. It made quite a difference in temps and humidity (our nightime air is ~60F and 5-10% humid). The cooling also carried over to the next day, with humidity and temps significantly lower than before.
I think that running the vent fan only when the temp is >90F is inadequate. I am going to continue this experiment with the thermostat set to 55F (essentially run all night unless it gets really cold like 50F or below). I'll keep monitoring and see if the increased airflow and lower temps and humidity have the desired effect on these diseases.