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. 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. Botrytis bunch rot can sometimes be confused with Phomopsis fruit rot; however, Phomopsis usually does not produce visible mold (only small black pycnidia) 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 usually leaky and tend to drop off when the berry stem is killed.

Factors that favor disease development

Injury to the berries is the primary factor influencing bunch rot development. As berries ripen and sugar content increases, injured fruit becomes 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 in the soil or on plant surfaces and can only cause infections if they gain entry to the berry through wounds. Examples are injuries from bird and insect feeding, hail, rain splitting and mechanical cracks or fruit abscission (separation from the pedicel) caused by growth pressure in tight-clustered grape varieties. 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. For instance, 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 (do this soon after fruit set if possible – late and excessive leaf removal can result in sun scald), 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 and other bunch rot fungi. 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.

While we have no experience testing products in Michigan for control of sour rot, there are two products that have sour rot listed on the label: BlightBan A506 (Pseudomonas fluorescens A506) and Serenade Max (Bacillus subtilis QST 713). There are several formulations of Serenade available, but Serenade Max is an improved, concentrated version. We cannot comment on their efficacy, but they may be worth a try since there is not much else available. Both BlightBan A506 and Serenade are biological control products containing bacteria that are antagonistic to a variety of other microorganisms. In fact, BlightBan A506 is used for reduction of frost damage and russetting on tree fruit and small fruit crops. It works by suppressing the growth of frost-forming and fire-blight inducing bacteria on plant surfaces. Both are strictly protectants, so thorough coverage is important. It would be important to start sprays well before veraison. BlightBan A506 is recommended at bloom and again prior to bunch closure (5.3 oz per acre). Serenade Max is recommended at bloom, before bunch closure, at veraison and pre-harvest (1-3 lb per acre). Both have a 0-day PHI. Serenade is OMRI-listed.

Dr. Schilder's work is funded in part by MSU's AgBioResearch.