Raspberries are very susceptible to Botrytis fruit rot which is one of the most common and serious diseases of raspberries. It is usually worse on red raspberries than on black raspberries.
Infection can happen during the blooming period or when fruit is nearly or fully ripe.
Fruit can show this rot after it has been harvested, even when it looked healthy at harvest time. Open flowers can become infected, especially if they have been damaged by frost. They become dark and shriveled. During humid or wet times, they are covered with a mass of fuzzy gray fungal growth. The fungus then grows into the fruit stems, and may cause them to turn black.