Dr. Jay W. Pscheidt, Professor and Extension Plant Pathology Specialist, OSU Dept. of Botany and Plant Pathology
Dr. Patty Skinkis, Associate Professor, Viticulture Extension Specialist, OSU Dept. of Horticulture

As we get into fall with a little rain, we wanted to highlight the potential for various bunch rots. These bunch rots are weather-, disease- and insect-related. Botrytis bunch rot and sour rot are the two most frequently encountered in this region, but others that are important around the world are not common here.

Botrytis Bunch Rot
We in Extension have written about the ubiquitous Botrytis bunch rot off and on over the years. Water in the form of rain or irrigation drives this disease, especially at bloom and near harvest. The fungus can infect (gain entrance to) ovaries and colonize floral tissue at bloom. It then becomes inactive (quiescent) and does not reactivate until berries begin to ripen in the fall. Open training systems and cluster zone leaf removal help create an environment that does not favor the disease. Fungicides are less effective than canopy management but are useful in wet years. Fungicide use can be challenging since sprays need to go on well before you know whether it will be a wet season, and fungicide resistance is common and complicated by fungicides used in your powdery mildew program. Read more about Botrytis bunch rot here:

Powdery Mildew
Powdery mildew is not really a bunch rot. Depending on how early infection occurs, the result may be poor fruit set or small and split berries. By the time véraison rolls around there is not much of a cluster to rot.  Small or light infections of the berry, however, can also allow Botrytis to get a foothold. Good powdery mildew control will aid Botrytis bunch rot control. 

Sour Rot
New research out of New York has defined sour rot and given us clues as to how to manage it in the vineyard. Very specifically, sour rot occurs when the berry becomes brown AND has both ethanol and acetic acid accumulation, which gives it the characteristic sour vinegar smell. The ethanol is no surprise as it comes from yeasts, but the acetic acid comes from bacteria. There is a sequence of events that is required for sour rot to occur, and it starts with wounding.

Somehow the berry skin breaks, allowing entry of these organisms. This can happen through berry growth, rainy weather during ripening (as we had a few years ago) and/or insect or bird damage. The yeasts produce ethanol that is then converted to acetic acid by the bacteria. This is still not enough to get sour rot symptoms. In New York, fruit flies were critical for sour rot symptom development. They do not need to introduce the microorganisms but are a factor all in themselves, and that factor is unknown at this time. It is unknown whether other insects, such as yellow jackets, can also induce symptoms. Targeting fruit flies with insecticides in the vineyard did result in less sour rot development. Interestingly, targeting the microbes with anti-microbial sprays alone was not effective. You can learn more by reading:

Other Grape Rots
A few other grape rots have been reported or observed in the PNW. Several more have been described in other viticultural regions of the world, including the following list. (We mention these various rots because it is always possible for new exotic organisms to be introduced into our region. They may just be a temporary “flash in the pan” problem or could establish as an annual concern over time)

  • Phomopsis: I have seen Phomopsis fruit rot only once in my 30 years here in Oregon and that was in an unmanaged vineyard used for nursery stock. A disease with similar symptoms from the southeastern USA is called bitter rot. The only way to tell the difference is by taste, which I had enough of during my postdoctoral research in New York!
  • Black rot has been reported from eastern Washington on Concord grapes but is not a common problem.
  • Anthracnose (or better named “bird’s eye rot”) and ripe rot are also fungal fruit rots more commonly found in the southeast USA.
  • White rot is a real fungal disease of grape and not someone just joking around about bird doodoo on a leaf!
  • Downy mildew: This is not a problem here but is common in many other regions of the world.

In the Winery
Grapes affected by fruit rot diseases can cause problems in the cellar as well.  Dr. James Osborne wrote this article titled, Dealing with Compromised Fruit in the Winery, for Wines & Vines magazine in August, 2014.

Bottom Line
It is most important to manage powdery mildew and Botrytis bunch rot, and to scout for fruit flies around harvest. Also, keep an eye out for unusual problems or rots. If you find some suspect diseases or unusual rots, contact your local Extension team member. We hope that the harvest will go smoothly with few problems.

Dr. Patty Skinkis, Viticulture Extension Specialist & Associate Professor, OSU
Dr. R. Paul Schreiner, Research Plant Physiologist, USDA-ARS

Véraison marks the start of fruit ripening in the vineyard, and it is one of the two main time points to consider evaluating vine nutrient status. Sampling petioles at bloom was the long-time standard recommendation for measuring grapevine nutrient status. However, more recently we have been suggesting that growers consider sampling vine tissues at véraison for macronutrient assessment, particularly nitrogen (N), phosphorous (P), potassium (K), and magnesium (Mg). Sampling should be conducted at approximately 50% véraison (50% color change/softening), and leaf blade or petiole samples can be taken. However, leaf blades are better for diagnosing many nutrients compared to petioles (Schreiner and Scagel 2017). Recent studies conducted in Oregon by both of our labs show that leaf blade samples at véraison serve as a good indicator of vine N status and fruit N levels (yeast assimilable N). To learn more about research into leaf and petiole nutrient guidelines and to learn about how to collect tissue samples and interpret results, see the articles listed below.

Further Reading

Schreiner RP and Scagel CF. 2017. Leaf blade versus petiole nutrient tests as predictors of nitrogen, phosphorus, and potassium status of ‘Pinot Noir’ grapevines. HortScience. 52: 174-184.

Schreiner P. and Skinkis P. 2014. Monitoring grapevine nutrition. eXtension.org.

Dr. Laurent Deluc, Associate Professor, Dept. of Horticulture, OSU Oregon Wine Research Institute

As part of the research project studying the role of the regulatory protein Auxin-Response Factor 4, namely ARF4, on the ripening initiation of grape berries, our group has lately invested time and research efforts in promoting the microvine system at OSU, which was developed by our Australian partner Dr. Mark Thomas at the Commonwealth Scientific and Industrial Research Organization (CSIRO) (Chaïb et al. 2010). In early May, we received the first plant materials from Australia (see Figure 1 & 2), which includes in vitro material from white and red-grape varieties. One question that we are often asked about the microvine system is “what is the microvine system and why is it important to research?” If recent advances in sequencing technologies and genomic tools are very helpful to build new hypotheses on complex molecular processes, such hypotheses still need to be validated in planta in order to prove the concept. When one gene from genomic data has been identified as potential link with a trait of interest (fruit quality, disease resistance, stress tolerance), one way to prove the relationship between the genetic marker and the trait of interest is to perform “genetic engineering” or molecular breeding. To do so, this approach requires the use of a reliable model system that must combine several advantages that include small space requirements for growth, short generation time (constant flowering trait), tractable system for genetic engineering (genetic transformation), and small-size genome. The microvine system offers all of these advantages.

Dr. Satyanarayana Gouthu, Research Associate in the Deluc lab, is currently visiting Dr. Mark Thomas’s lab at the CSIRO in Australia to receive the necessary training for the different steps related to microvine propagation and genetic transformation. From my interaction with Dr. Gouthu, it is clear to me that he is learning a lot about microvine, which is essential for him to “master” when he eventually initiates the genetic engineering work at OSU. Meanwhile, another aspect of his research project is also in its final phase. By the end of the summer, we hope to identify a series of potential “interactors” with ARF4. This information is necessary to understand how a protein (ARF4) is regulated and what ARF4 interacts with during the process of berry ripening? We also made significant progress in terms of adapting a new method called Atmospheric Pressure Gas Chromatography Mass Spectrometry (APGC-MS) for metabolite identification in grape berries, in collaboration with the OSU Mass Spectrometry Center. Our colleague from the center has built a database containing around 75 individual analytes belonging to different classes of metabolites (organic acids, amino acids, sugar-related compounds, and pigment- related compounds). Our goal is not only to use this database for routine metabolite analyses in our lab, but also over time to improve the depth of the database by adding new metabolites. We are currently running samples from another experiment with promising results. We are very excited using this new analytical method for our current research project on ARF4 and future research projects as well.

By using the microvine, we expect to connect the function of proteins to important traits for grapevine production. Our goal is to specifically connect the timing of ripening initiation to the protein ARF4. This information could be useful by providing the industry with a potential genetic marker associated with véraison that could serve be used to validate new practices in the field, and to identify new or existing cultivars/clones for advanced or delayed ripening more amenable to local changes in the environment due to climate change. We hope to collaborate with OWRI, OSU and other external partners in pursuit of our research objectives.

Figure 1. Microvine embryonic cells of white grape variety


Figure 2. Microvine plantlet of red grape variety

Literature cited:

Chaïb J, Torregrosa L, Mackenzie D, Corena P, Bouquet A, Thomas MR. 2010. The grape microvine- a model system for rapid forward and reverse genetics of grapevines. Plant J. 62(6):1083-92. doi: 10.1111/j.1365-313X.2010.04219.x

James Osborne, Enology Extension Specialist, OSU, Oregon Wine Research Institute

The impact of Grapevine red blotch associated virus (GRBaV, commonly referred to as red blotch) on wine quality is largely unknown, with most of the information available focused on fruit composition. A recent study on how GRBaV interferes with grape ripening at the molecular level (Blanco-Ulate et al., 2017) has been published, which may provide insights on how to mitigate the impact of the virus on fruit development in the vineyard. There are very few peer reviewed publications that have reported on winegrape compositional changes due to red blotch and most information regarding the impact on wine quality is anecdotal. A number of studies are currently being conducted in the US to determine the impact of red blotch on wine composition but results from these experiments have not yet been published. Early data from other studies suggest that the impact of red blotch is affected by site and year more than cultivar by cultivar, indicating that impact needs to be evaluated over multiple growing seasons. Based on the few published reports the two main effects on fruit quality have been:

  • A decrease in sugar accumulation leading to reduced Brix levels in grapes at harvest compared to grapes from non-infected vines. The reduction in Brix has been reported to range from 1 to as high as 5 with some varietal differences being noted (Poojari et al 2013), though in this publication the vines were co-infected with Grapevine fanleaf virus. To date the sample size is too small to make any conclusive statements about consistent differences between varietals but early reports indicate this may be the case. Other anecdotal information suggests site and season are more important than cultivar in the degree of impact GRBaV has on grape quality.
  • Lower anthocyanin concentration in grapes from red blotch infected fruit (Poojari et al 2013). Early results from studies being performed in Washington State and California also indicate lower Brix in fruit from red blotch infected vines as well as higher titratable acidity and lower anthocyanins.

While it would be expected that lower Brix will lead to wines with lower alcohol, the impact on other wine parameters such as flavor, aroma, mouthfeel, color, and sensory is relatively unknown. An upcoming presentation by Anita Oberholster (UC Davis) at the OWRI Grape Day will discuss results from some of the trials she has been conducting in California. This includes data regarding changes in wine anthocyanins and tannins as well as sensory attributes. This type of information will be vital for the development of strategies to manage this issue in the winery. If the only significant impact of GRBaV is lower Brix and higher acidity then that can be amended in the winery. However, if red blotch significantly impacts concentrations of tannins and flavor and aroma compounds then red blotch fruit will be more challenging to manage in the winery.  Sensory studies also need to be conducted to determine the specific sensory impact across different wines as well as what percentage of red blotch fruit can be used before sensory impacts become noticeable. It is likely that the percentage of red blotch fruit needed before sensory differences are noted will vary between different red wines as is seen with other taints/faults such as Brettanomyces taint where higher concentrations of volatile phenols are required in a Cab. sauvignon compared to a Pinot noir to be noticeable. We are really only at the very starting line when it comes to understanding both the specific effects of red blotch on wine quality and how these could be managed at the winery.   

Literature cited:

Blanco‐Ulate, B., Hopfer, H., Figueroa‐Balderas, R., Ye, Z., Rivero, R.M., Albacete, A., Perez-Alocea, F., Koyama, R., Anderson, M.M., Smith, R.J., Ebeler, S.E. and Cantu, D. 2017. Red blotch disease alters grape berry development and metabolism by interfering with the transcriptional and hormonal regulation of ripening. J. Exp. Bot. 68:1225-1238.  doi:10.1093/jxb/erw506

Poojari, S., Alabi, O.J., Fofanov, V.Y., and Naidu, R.A. 2016. A leafhopper-transmissible DNA virus with novel evolutionary lineage in the family Geminiviridae implicated in grapevine redleaf disease by next generation sequencing. Plos One. 8(6): e64194. doi:10.1371/journal.pone.0064194

Now available through OSU Extension is the 2017 Pest Management Guide for Wine Grapes in Oregon. This guide is co-authored by viticulture, horticulture and pathology extension faculty at Oregon State University and updated annually. It provides chemical and cultural control information for insects, weeds, and diseases based on grapevine phenology (growth stages throughout the year). Updated information from fungicide efficacy trials is included as well as other resources and an air blast sprayer calibration worksheet.