The fungus lives from one season to the next in very small, thin, black sclerotia on the ground or in mulch under the bushes. During a rainy period in January or February the sclerotia produce cuplike reddish-tan apothecia, less than 1/8 inch across, probably not discernible to the untrained eye but diagrammed in Figure 11. The apothecia are lined with asci, shown in insert, and spores from these are literally shot into the air and carried by air currents to the lowest petals. A little rain or heavy dew allows for spore germination and penetration into the petal. In a few hours a large white spot is formed which is distinctly wet to the touch, and the tissue literally disappears if the spot is placed between thumb and finger. This wet or slimy feel is an excellent diagnostic character and differentiates fungus blight from frost spots, insect injury, etc. Secondary spores, conidia, are formed inside infected petals, and are spread by wind, rain, and insects to other flowers, bushes, and gardens.

Distribution is extremely efficient, and in moist weather colored flowers are literally peppered with small white spots, each marking where a spore has landed and started to grow. White petals have numerous brown spots. By the next day these flowers may be reduced to slimy mush, hanging brown and limp by the thousands on every bush. With two or three more days of moist weather, black, slightly curved, sclerotia form in the petals, but if a period of dry weather intervenes it may be some time before sclerotia are produced. These either drop to the ground out of the petals in a few days or remain in the dried flowers. Healthy azaleas, except for some Kurume varieties, shed their flowers as soon as they fade. Those infected with petal blight hold their unsightly, dried brown flowers for months.