没经验但是搜索了一下，估计是这个

Gray mold or bunch rot is caused by a nearly ubiquitous fungus (botrytis) that infects flowers in bloom, as well as ripening fruit. Infections that occur during bloom may go unnoticed because the pathogen remains latent until the fruit’s sugar levels rise as ripening occurs. Once the fruit rot symptoms develop, the pathogen moves through the cluster stem infecting all or most of the berries in a cluster. Infections also can become established in ripening fruit when this fungus enters berries injured by the grape berry moth.