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UC Pest Management Guidelines - Grape - Powdery Mildew
University of California - Integrated Pest Management Online
http://www.ipm.ucdavis.edu/PMG/r302100311.html
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Tips on Spraying Sulfur for Powdery Mildew on Winegrapes

Sulfur only protects against new mildew infections. Normal spray applications of sulfur will not eradicate existing mildew colonies. Apply at least one dormant spray before bud break to kill mildew spores left over from last year.

On the foliage, sulfur particles oxidize and slowly disappear. So, sulfur must be applied often and at regular intervals. Spray at 10 day intervals under normal conditions. Spray at 14 day intervals when mildew pressure is low. Spray at 7 day intervals when mildew pressure is high.

Sulfur alone will not control powdery mildew in “bad mildew “years, so besides sulfur, use a systemic material such as Rally, Rubigan, etc. Most systemic materials can be tank mixed with sulfur, and the two materials can be sprayed together.

Mixing
Use about half a cup of wetable sulfur in a 4—gallon, backpack sprayer. However, the amount of sulfur needed depends on particle size, so read the label and use the specified amount,

Add a commercial “wetting agent” or a little dish washing detergent to the tank to help the spray wet the foliage.

Be sure the sulfur is mixed well in the tank.

Application
Sulfur is one of the more benign mildew sprays. Even so, it is a controlled material. If you have an AG number, you may receive a citation if you spay and are not wearing approved spray apparel. Regardless, don’t spray with bare arms (or remove your shirt on hot days).
Sulfur can burn foliage when the temperature exceeds 90 degrees, so spray early in the morning when it is cool.

Don’t spray in the wind or much of the applied spray will be deposited on the ground or on you. Spray early in the morning when the wind isn’t blowing.

Sulfur is a contact material. Only those parts of the vine covered by the spray will be protected, so complete coverage is very important.

Use high pressure to get better spray penetration. With backpack sprayers, maintaining high pressure requires much pumping and is hard work.

Spray from both sides of each row of vines. Try to spray through the canopy and apply spray material on the foliage on the opposite side of the vines.

Complete coverage is essential, so apply enough spray material to get a significant amount of dripping. On the other hand, large amounts of spray material dripping on the ground do little good. In general, the amount of material applied depends on how fast you move down the row.

Spray into the foliage ahead of you, into the foliage behind you, up into the foliage and down into the foliage as you move down each row, Complete coverage is essential.

Clean Up
Sulfur residues can clog spray nozzles. Remove any remaining material from the sprayer when finished. Wash the tank, hose and nozzle with clean water and rinse three times.
Leave the prayer open and drain the water from hose.

Sulfur stinks and after spraying sulfur you will stink.

After spraying, my wife makes me undress in front of the washing machine. I put my clothes directly in the machine, and then I take a hot shower.