MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

There are several late-season bunch rots that can affect wine grapes in Michigan. The most common of these is Botrytis bunch rot caused by Botrytis cinerea, the same fungus that causes gray mold in a number of other crops. Botrytis bunch rot got an early start this year in Michigan vineyards. Tight-clustered varieties such as ‘Pinot gris’, ‘Pinot noir’, and ‘Vignoles’ are particularly susceptible to Botrytis bunch rot. Another, more sporadic bunch rot disease is sour rot, which smells distinctly of vinegar due to the presence of acetic acid bacteria. Often other organisms are also involved in sour rot, including various fungi and yeasts. 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. While under cool, dry conditions, Botrytis bunch rot sometimes can be beneficial for wine quality (“noble rot”), sour rot is very undesirable.

Disease symptoms

Bunch rot often begins in one or a few berries, usually at the site of an injury, and then spreads rapidly throughout the cluster. Generally, rotting berries turn brown and become soft, collapse or shrivel up. Specific pathogens are sometimes identifiable by their appearance. Botrytis produces gray spores, whereas Penicillium produces green spores, and Aspergillus and Rhizopus have dark brown or black spores. Sour rot often has a wet look to it, with berries collapsing and leaking juice and no visible mold growth. It also results in the typical vinegar smell from which the name sour rot is derived. Fruit flies are often present and are thought to spread the disease. Botrytis bunch rot can sometimes also be confused with Phomopsis fruit rot; however, Phomopsis usually does not produce visible mold (only small black pimples) on berries, does not smell, and is characterized by a black or dark brown necrosis on the rachis and a browning and shriveling of the berries, much like a balloon that is being deflated. Phomopsis-infected berries are not leaky and tend to drop off when the berry stem is killed.

Factors that favor disease development

Tightness of fruit clusters as well as injury to the berries are the primary factors influencing bunch rot development. As berries ripen and sugar content increases, injured fruit become increasingly susceptible to bunch rot pathogens. Other than Botrytis cinerea, which can directly penetrate intact berry skins under conditions of prolonged moisture or very high humidity, most other bunch rot organisms are opportunistic pathogens that live on plant surfaces and can only cause infections if they gain entry to the berry through wounds. Examples are injuries from fruit splitting due to internal pressure, bird and insect feeding, hail, and rain splitting. Entry holes created by grape berry moth larvae are a common cause of bunch rot as well. Similarly, early-season feeding injury from thrips can cause scarring on fruit skin that reduces its elasticity, resulting in small cracks as the berry grows. Fruit flies that are attracted to rotting or overripe fruit may also play a role in development and spread of sour rot. In addition, infection by the powdery mildew fungus creates small dead spots on the berry skin, which can lead to cracking of the berry and possible invasion by bunch rot pathogens. Even inconspicuous powdery mildew colonies resulting from late-season infections can increase the severity of bunch rot. We have to remember that these microorganisms operate on a very small scale, so that even microscopic wounds that are invisible to the naked eye can lead to infection. Bunch rot is more common in grape varieties with tight clusters that may experience fruit abscission or splitting from growth pressure. Any juice leakage from cracked or abscised berries also boosts growth of fungi, yeasts, and bacteria. In addition, wet weather during fruit ripening also favors bunch rot, and the longer the wet period, the greater the amount of rot. Therefore, grapevines with dense canopies that dry slowly and maintain high humidity have an increased risk of bunch rot.

Disease management strategy

One of the main goals in managing bunch rot diseases is to minimize injuries to berries that enable bunch rot pathogens to get established. Reducing the number of berries per cluster on tight clustered varieties would be the best method and may be possible by mechanical or chemical means. However, research is currently being done at MSU to determine a safe way to do this. In addition, effective management of grape berry moth and powdery mildew will greatly reduce the risk of bunch rot in many vineyards. In addition, promoting good air circulation within the grapevine canopy also reduces the risk of bunch rot. Canopy management methods aimed at improving air circulation and reduce humidity include leaf removal in the fruit zone, shoot positioning, shoot thinning, and hedging. Limit excessive vegetative growth by balance-pruning and avoiding excess nitrogen fertilization.

There are a number of fungicide options for control of Botrytis bunch rot, including Rovral, Vangard, Scala, Endura, and Elevate. However, these are generally ineffective against sour rot organisms. Captan, as a general broad-spectrum fungicide, is sometimes used to control a variety of bunch rot fungi, but does not have activity against bacteria.

There are two products that have sour rot listed on the label: BlightBan A506 (Pseudomonas fluorescens A506) and Serenade Max (Bacillus subtilis QST 713). Both are biological control products containing bacteria that are antagonistic to a variety of other microbes. Both are strictly protectants, so thorough coverage is important. Small plot and on-farm trials have not shown much efficacy of Serenade against sour rot and BlightBan A506 is being evaluated this year for efficacy.