It looks to me like petal blight. Some varieties are more susceptible. It gets worse as the season progresses. Early blooms are usually spared. The it hit starting with mid season or late bloomers. It hits some and spares others.

Flowers exhibit small spots which enlarge rapidly and appear water-soaked is a symptom of Petal Blight, Ovulinia azaleae Petal Blight. The flowers turn brown and wilt. They remain on the plant. Wet weather in spring often results in serious damage from the petal blight fungus. Some fungicides provide control if applied when or just before flower buds open. The best fungicides are a combination of chlorothalonil and benomyl or Mancozeb or Bayleton or triadimefon. During moist seasons, control may be difficult for late blooming varieties. Sanitation practices such as deadheading and removal of diseased material my improve control. In the northwestern US, Ovulinia is present, but similar symptoms can also be caused by Botrytis cinerea which is less damaging to the flowers and easier to control.

Petal Blight, Ovulinia azaleae: This fungal disease, caused by Ovulinia azaleae, primarily affects the flowers of azaleas, but mountain laurel and rhododendron flowers can also be infected. Indian and Kurume azaleas are especially susceptible. The disease starts on the flower petals as tiny, irregularly-shaped spots, giving a "freckled" appearance. On colored flowers the spots are white, and on white flowers the spots are brown. The spots quickly enlarge and become soft and watery. Flowers rot and stick to the leaves. Infection is easily spread from flower to flower by wind, rain and insects. The fungus survives the winter in the soil. The most important things that you can do to control this disease in the home landscape are to pick and destroy infected flowers and avoid overhead watering. This fungus survives in the soil, so it is important to replace the ground litter with uncontaminated mulches. Rake and remove flower debris from beneath plants and, if possible, remove old flowers still attached to plants. Apply new mulch around the base of plants to serve as a barrier to new infection. On large azalea plantings, where it is not practical to remove infected flowers, make weekly fungicide applications beginning just before bloom and continue until the last buds open.