Disease Development

Risk Factors

As berries ripen and sugar content exceeds 8%, injured fruit become increasingly susceptible to bunch rot pathogens. The bunch rot complex of fungi and bacteria are either weak or secondary pathogens that establish infections most often through a wound on the berry. These fungi and bacteria, however, are commonly present on soil debris and most plant surfaces, including fruit, thus the rot process can begin almost as soon as the berry is wounded.

Any injury to the skin of ripening berries, from large to very small, is a potential entry point for these pathogens. Bunch rot can quickly develop in wounds resulting from bird feeding, hail injury, rain splitting, and mechanical cracks or fruit abscission (separation from the pedicel) caused by growth pressure in tight-clustered grape varieties. Small entry holes in ripening berries created by grape berry moth (Endopiza viteana) larvae are a common cause of bunch rot in Texas vineyards. Other insects and diseases can cause much less obvious injury to the fruit that nonetheless provides entry points for bunch rot pathogens. Infection by the powdery mildew fungus creates lesions that may later cause skin cracking leading to bunch rot, but even inconspicuous powdery mildew infections can increase the severity of bunch rot. Similarly, early season feeding injury from tiny thrips causes scaring on fruit skin that reduces its elasticity, resulting in small cracks as the berry grows.

Grape varieties differ in the incidence and severity of bunch rot, which is mostly related to their cluster architecture. Bunch rot is more common in varieties with tight clusters that may experience fruit abscission or cracking from growth pressure. Tight clusters also promote rapid berry-to-berry spread of bunch rot once an infection gets started.

Wet weather during fruit ripening favors bunch rot and the longer the wet period, the greater the amount of rot. Some of the bunch rot pathogens (Botrytis and Alternaria) can directly penetrate intact berry skins under conditions of prolonged moisture or very high humidity. Therefore, grapevines with dense canopies that dry slowly and maintain high humidity have increased risk of bunch rot.

Disease Symptoms

Summer bunch rot begins in one or a few berries, usually at the site of an injury. Appearance of the rot varies somewhat by the associated pathogens and sometimes more than one may be actively involved. Generally, the tissues of rotting berries soften and collapse and the rot can rapidly spread to adjacent berries. Rotting tissues may at first be tan and soft, later turning to brown, firm and leathery. Some pathogens create a wet rot with very soft berries that may drip juice. Specific pathogens can sometimes be identifiable if the fungus produces spores. Penicillium produces greenish and/or white spores, Aspergillus and Rhizopus produce black spores, Botrytis produces gray spores. Sometimes, however, no spores are produced or more than one of these fungi may be present and producing spores on the same fruit. Bacteria do not produce spores, but the common spoilage bacterium Acetobacter produces acetic acid resulting in the typical vinegar smell from which the name Sour rot is derived.

Positive identification of the causal organisms may be inconsequential in many cases because the major contributing factor to bunch rot is previous injury to the fruit.

Photographs of Summer Bunch Rot symptoms

2008 New York and Pennsylvania Pest Management Guidelines for Grapes

Grape Summer Bunch Rot (Sour Rot)
http://www.ipm.ucdavis.edu/PMG/r302100211.html
University of California IPM Pest Management Guidelines.

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