Wilt-Pruf is a spray that creates a protective coating over the leaves of the plant. It was originally designed to hold in moisture in plant foliage and stems, substantially reducing water loss during periods of plant stress; and it still does that job nicely.
In this case however, experts have discovered that when a fungal disease is expected on an ornamental plant, a coating of Wilt-Pruf prior to the arrival of the fungal spores, prevents those spores from entering the plant's leaves thus preventing the incidence of the disease. The timing of this spray is important. You need to know when the plant was infected last year then apply Wilt-Pruf about two weeks prior to that date this year. The coating will last for more than a month which is more than the period of the activity of any specific group of spores.