Summer bunch rot is a fruit-rotting disease of ripening grapes involving one or more fungal or bacterial species. The predominant fungal pathogens contributing to bunch rot in regions with hot summers include Aspergillus niger, Alternaria tenuis, Cladosporium herbarum, Rhizopus arrhizus, Penicillium spp., and others. In hot climates, Botrytis cinerea may also be involved with bunch rot, but usually to a lesser degree than other fungi, in contrast to its predominant role as the cause of bunch rot in regions with relatively cool fruit-ripening conditions. Other microrganisms, especially the bacterium Acetobacter, commonly contribute to grape bunch rot. The name Sour rot is often applied to bunch rots involving Acetobacter due to the characteristic vinegar smell of acetic acid produced by the active bacteria. Bunch rots initially caused by fungi can culminate in Sour rot. Crop damage can be extensive because infections that begin in a single berry can rapidly spread to adjacent berries and destroy most or all of a cluster.
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.
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.
Sporulation (green, unidentified pathogen) on
Sporulation (green, unidentified pathogen) on
Sporulation (black, possibly Aspergillus) on
Botrytis sporulation (gray) on Chardonnay
Disease Management Strategy
Management of summer bunch rot should be based on reducing wounds or injury to berries that enable these opportunistic pathogens to get established. Effective management of grape berry moth, birds, powdery mildew, and Botrytis bunch rot should greatly reduce the risk of summer bunch rot in many Texas vineyards. Do not underestimate powdery mildew – even inconspicuous infections can provide tiny cracks for bunch rot to get established. Powdery mildew may be a major contributor to summer bunch rot outbreaks that occasionally develop in the hot, dry climate of west Texas. Excellent control of powdery mildew and, especially, Botrytis will significantly minimize summer bunch rot.
Promoting good air circulation within the grapevine canopy also reduces the risk of bunch rot. Canopy management methods that can be used to improve air circulation and reduce humidity include leaf removal in the fruit zone, shoot positioning, shoot thinning, and hedging. Avoid excessive vegetative growth through irrigation scheduling and cover crop management.
The fungicide Pristine has a label for grapes stating that it “Aids in Control Only” of Summer bunch rot (Sour rot) caused by Cladosporium spp. and Aspergillus spp., and that it provides “suppression only” of Botrytis gray mold. The 2008 New York and Pennsylvania Pest Management Guide states that there are indications that broad-spectrum fungicides used for Botrytis control (e.g., Flint, Pristine) may provide some additional control of the wound-invading sour rot fungi; however, any product that gives good Botrytis control will help greatly to prevent sour rot.
Flaherty, et al., 1992. Grape Pest Management Guide. 2nd edition. University of California.
Pearson, R.C. and A.C. Goheen. 1988. Compendium of Grape Diseases. St. Paul,
MN: APS Press.
Grape Summer Bunch Rot (Sour Rot)
University of California IPM Pest Management Guidelines.
This publication may contain pesticide recommendations. Changes in pesticide regulations occur constantly and human errors are possible. Questions concerning the legality and/or registration status for pesticide use should be directed to the appropriate Extension Agent/Specialist or state regulatory agency. Read the label before applying any pesticide. The Texas A&M University System and its employees assume no responsibility for the effectiveness or results of any chemical pesticide usage. No endorsements of products are made or implied.