Bunch Rot
Gabriel Torres, UCCE Tulare & Kings Counties
Image 1. Advanced bunch rot.
Botrytis cinerea, also known as gray mold, is a common disease found in all grape growing areas worldwide. Gray mold is the causal pathogen in bunch rot; however, other fungi such as Aspergillus, Alternaria, Rhizopus as well as other saprophytic species can also be involved. Advanced infections lead to large portions or entire clusters starting to rot (Image 1). Vinegar flies are attracted to the rotting berries bringing bacteria, specifically acetic acid producing bacteria that progress bunch rot into sour rot.
Table grape, wine grape and raisin grape production are all affected by bunch rot. Berry infections above 3% can result in significant quality reduction of wine. In raisin production the disease also causes quality reduction at a 5% threshold, which can lead to postharvest mold problem. Table grapes have a zero- tolerance policy for bunch rot. As a single infected table grape berry in postharvest storage can result in an infection that compromises all the fruit in a box leading to the entire box being rejected. This is because Botrytis can continue to spread and infect berries in temperatures as low as 32 F°.Botrytis infects both actively growing and decaying tissue. For this reason, it can be observed affecting different parts of the plant. This includes the leaves, flowers, canes, buds, and fruits. Cane and leaf infections, also known as botrytis shoot blight, is unusual in the San Joaquin Valley except during rainy springs as we had in 2019.
Conditions leading to bunch rot may start at bloom (Image 2). Botrytis takes advantage of the dying flower parts, including calyptras and stigmas to initiate infection. After berry set, the pathogen will continue infecting new berries. This spread will continue if temperatures are below 86 F°. Above 86 F° the pathogen becomes quiescent, and new infections are not observed. At veraison, the increasing sugar content of the berries, the reduction of peel resistance and cooler temperatures under a bigger canopy, again produce good conditions for the pathogen to resume infection.
Image 2. Decaying flower parts in a cluster at berry set. The decaying tissues is a preferred substrate for Botrytis to initiate growth on.
After veraison infected berries from green varietals become brown-colored, while infected berries on red cultivars, quickly develop a dark red color. As the fungi grows within infected berries it increases internal pressure. Once the internal pressure exceeds the peel’s resistance, peel cracking can occur, releasing juices from the berries (Image 3). These cracks, along with powdery milder scars (Image 4), berry damage from insects and birds, or mechanical damage all serve as new colonization sites for more Botrytis infections. Released juice speed up the process giving the invading fungi an easily available source of nutrients. These sites also allow for infections of other secondary pathogens involved in bunch rot. Vinegar flies, as well as other insects, are attracted to the damaged and rotting berries. The vinegar flies carry more pathogens to the infection sites as well as spread them within and between clusters (Image 5). When temperatures stay below 86F° for extended portions of the day, a gray fluffy mycelium is produced on the surface of affected berries. After harvest, leftover clusters, canes, and other tissues that were colonized by Botrytis may serve as over wintering sites. On these tissues, the pathogen overwinters as sclerotia or chlamydospores. After budbreak, spores can be produced from the chlamydospores under the correct conditions. These conditions include free water drop, such as rain drops or dew drops, temperatures between 58° and 82°F, and a preference for relative humidity exceeding 92% for more than 2 hours. These spores can then start infecting green growing tissues (Wilcox et al. 2017).
Image 3. Berries spliting and bleeding at veraison (left) and at maturing (right). These cracks provide easy entry for all pathogens involved in bunch and sour rot.
Management of bunch rot requires an integrated pest management approach, working first on preventive measures and then on curatives. Before planting, row direction, trellis system selection, and cultivar susceptibility (Table 1) are important decisions to consider. Any action taken to improve air flow will be beneficial. Airflow reduces humidity and allows water droplets to dry faster, which are both important steps to preventing initial infection and managing bunch rot.
Image 4. Powdery mildew scars on table grape. Note the compactness of the cluster. Tight clusters are more prone to develop bunch rots.
In established vineyards it is important to practice good hygiene in order to reduce the inoculum that pass from one season to another. For this, it is important to remove all unpicked clusters and incorporate them into the soil prior to budbreak. Canopy management techniques such as shoot, and cluster positioning are beneficial for disease management. The objective here is to increase air flow, improve superficial water drying and reduce spore germination. In addition, these canopy management practices can improve fungicide spray efficacy.
Irrigation also plays a role in the disease cycle. Over-irrigation can result in continually growing shoots. This extra growth will reduce temperature under the canopy and will lead to an increase of humidity, providing better conditions for fungal growth. Excessive vegetative growth can lead to tighter clusters increasing internal humidity. In addition dense canopies reduce spray coverage efficiency, which can lead to reducing fungicide efficacy.
Biological fungicides, including Trichoderma, Aurobasidium, Ulocladiumand and Bacillus, have some efficacy against Botrytis, especially when they are applied in a preventive manner. Synthetic fungicides dicarboxamides (FRAC 2), anilinopyrmidines (FRAC 9), hydroxyanilides (FRAC 17), strobilurines (FRAC 11) and SDHI (FRAC 7) fungicides, can be used in conventional agriculture. However, selection of systemic fungicides needs to be done carefully since the pathogen can generate resistance towards fungicides easily. The use of fungicides to control Botrytis is advised in at least 4 critical stages: 1) mid-bloom; 2) before cluster closure, 3) at veraison; and 4) 2-4 weeks after veraison. On table grapes, another spray before harvesting is recommended to help maintain post-harvest quality.
In the Eastern United States, the control of Drosophila Spotted Wing flies (fruit flies) has been identified as part of the strategy to manage bunch rot. UCANR and the USDA are investigating if Drosophila control is beneficial for the San Joaquin Valley.
Image 5. Vinegar flies and other insects are attracted to rotten berries (A and B). They feed and reproduce there (C) and then move to berries with fresh damage (D) transmitting the pathogens involved in the complex.
Key points
What can trigger Bunch rot?
Clusters in contact with wire or trellis system
Poor cluster thinning (tight clusters)
Botrytis infection
Powdery mildew scars
Overwatering
Bird damage
Poor pruning
Mechanical damage
How can Bunch rot be treated?
Bunch rot is a complex of fungi, bacteria, yeast, and insects. There is not a “silver bullet” product that can reduce the disease has started. All treatments must be preventive with an integrated pest management approach.
When does bunch rot normally start?
Bunch rot pathogens normally use entries of scars left by Botrytis or powdery mildew. Infection takes place normally after veraison.
References
Wilcox W. F., Gubler W. D., and Uyemoto J. K., eds. Compendium of Grape Diseases, Disorders, and Pests. Second Ed.. St Paul, Minnesota, USA.: The American Phytopathological Society; 2017.
Bettiga L. J., and Gubler W. D. Bunch Rots. 2013. In: Grape Pest Management. 3rd Ed. University of California, Agriculture and Natural Resources publication 3343. 93-103.