Bunch rot

As berries ripen and sugar content exceeds 8%, injured fruit become increasingly susceptible to invasion by a wide variety of naturally occurring fungi. Invasion occurs at the point of berry injury caused by insect or bird feeding, mechanical or growth cracks, or lesions resulting from powdery mildew infection or esca (black measles) berry damage that results in cracking. The resulting rot can be severe as it progresses beyond the original injury. Masses of black, brown, or green spores develop on the surface of infected berries. Bunch rots often culminates in sour rot, primarily in the central and southern San Joaquin Valley. Sour rot is caused by a variety of microorganisms, including acetic acid bacteria, which are spread by drosophila flies attracted to the rotting clusters.

Melting decay or Non Botrytis Slip Skin (NBSS) of Redglobe and Crimson grapes is caused primarily by Hanseniaspora spp. These yeasts colonize the sugary and nutrient rich epidermis of berries after they are covered by the oozing liquid resulting from sour rot infections. Symptoms include hairline cracks in the berry skin, watery discoloration of berries, and general berry breakdown. Decay continues to develop slowly under cold storage conditions.