Summer Bunch Rot (Sour 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.

Rotting fruit clusters present during veraison are indicative of summer bunch rot. Management of this disease complex is based on reducing injury or damage to the fruit, thus preventing invasion by bunch rot organisms. Basal leaf removal at or after berry set has given excellent control of summer bunch rot in the San Joaquin Valley. In warmer growing areas, be careful not to remove excessive numbers of leaves, which can lead to sunburned fruit. Remove leaves only from the side of the vine that receives afternoon shade. Also, leafhopper populations and damage caused by omnivorous leafroller have been reduced by this cultural practice. Treat at preclose and veraison if summer bunch rot has been a problem in the past.

To reduce growth-related damage to the berries, follow proper irrigation, fertilizer, fruit thinning, and canopy management practices. Trellis and prune to achieve vine balance between vegetative growth and cluster number. Also control powdery mildew and damaging populations of omnivorous leafroller and other berry feeders.

In table grapes, look for symptoms of summer rot on fruit during harvest to assess this year's management program and to prepare for next year. The presence of drosophila flies may indicate summer bunch rot infections. Control of NBSS can be achieved by controlling sour rot in the vineyard and in table grape vineyards, by not harvesting "dripped on" clusters.

Copper sprays are acceptable on organically certified grapes; check with your certifier for details.

For more information on choosing a fungicide for grapes, refer to General Properties of Fungicides used in Grapes, Fungicide Efficacy—Conventional Products, Fungicide Efficacy—Biologicals and Natural Controls, Treatment Timing for Key Diseases, and Fungicide Resistance Management.

• Photos of fruit damage at harvest