Veraison can set the stage for masses of black, brown or green spores to develop on the surface of infected berries, posing a threat of severe rotting of fruit clusters.
Known as summer bunch, it begins as fruit, which has sustained some sort of injury, begins to soften and sugar content exceeds 8 percent. That’s when the damaged berries become more and more vulnerable to a wide range of naturally-occurring fungi. They include Aspergillus niger, Alternaria tenuis, Botrytis cinerea, Cladosporium herbarum, Rhizopus arrhizus, Penicillium sp., among others. These pathogens invade at the site of injury caused by insect laying eggs or feeding, bird damage, mechanical or growth cracks, lesions resulting from powdery mildew or black measles (esca) or rubbing from leaves, wood or wire. Tight clusters also are susceptible to summer bunch rots, since the compacted mid-cluster berries may be broken or cracked, providing an opening for the pathogens.
As noted in the University of California IPM Guidelines for grapes, bunch rot often leads to sour rot, especially in the central and southern San Joaquin Valley. Sour rot is characterized by a strong acetic acid (vinegar) smell and leaking clusters. A variety of microorganisms, including acetic acid bacteria, are spread by vinegar flies attracted to the rotting clusters.
Slip skin of Red Globe grapes is also associated with summer bunch rot organisms. 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.
The key to reducing injury or damage to the fruit by this disease complex is to prevent invasion by the bunch-rot organisms. The UC IPM Guidelines report that removing basal leaves at or after berry set has produced excellent control of summer bunch rot in the San Joaquin Valley. In warmer growing areas, these guidelines caution against removing too many of leaves and increasing the risk of sunburned fruit. Remove leaves only from the side of the vine that receives afternoon shade. In addition, this practice reduces leafhopper populations and damage caused by omnivorous leafroller.
To reduce growth-related damage to the berries, follow proper irrigation, fertilizer, fruit thinning, and canopy management practices. 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. Also, keep in mind that the presence of vinegar flies may indicate bunch rot infections.