The brown spots on the blooms is just from the blooms getting wet. It is more obvious because the blooms are white. When you water, try not to get the water on the blooms, especially if you water early in the day when the sun is hot.
Many people are confusing mite damage with aster yellows. These mites can cause deformed blooms that look just like the aster yellows blooms. Often the blooms without petals are just mite infested. The foliage of aster yellows infected plants has clearing of the veins and a yellowish tint. We have lots of info at the Echinacea Cubit about these problems and photos of them all too.
