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Just to add to Tom's points - materials like sulfur, certain salts and oils work against powdery mildew because the fungus resides primarily on the surface of the leaf, where these materials can act on the organism directly. These materials have almost no effect on downy mildew or other fungal diseases because they live inside the tissue. To get at those diseases, you need something that is more 'systemic' - something that can get into the plant tissues as well and possibly be translocated to some extent. The phosphite products are an example of this. The 'hot days, cool nights' idea primarily comes from warmer grape growing climates, like California, I think. Studies have shown differences in the amount of malic acid in grapes from vines grown at the same daytime temperature but different night temperatures. More malate remains with cooler evening temps due to lower respiration rates (malate is burned up in respiration by the berries). In climates where they can have too little acid in their fruit (i.e., hotter growing regions), having cool nights that will help to retain some acidity would be beneficial. Thus why so many vineyards have developed in "cooler" areas of California like Carneros and Santa Maria. Here in NY, we usually don't have to worry about loss of too much acid (2007 was a bit of an anomaly), so I'm not sure that it makes as much of a difference for us - from this particular standpoint at least.