Tracking Down the Causes of Bunch Stem Necrosis

Annie Klodd – UMN Extension Educator

Shriveled grape cluster on the vine
Late bunch stem necrosis (LBSN) is a complex physiological disorder of grapevines, where the bunch stems (rachises) shrivel during ripening, followed closely by berry shrivel. This sudden change is frustrating for growers, when seemingly healthy vines produce unusable clusters. LBSN affects vineyards worldwide, but may be caused by a number of environmental stresses. This means that developing treatment recommendations is a complex, long-term task that may vary by vineyard.

The University of Minnesota viticulture team is working to understand why LBSN is happening on Minnesota vineyards so that we can provide appropriate treatment recommendations.

Two important points to understanding bunch stem necrosis:

1) LBSN is not a disease, so fungicides are not effective against it. LBSN should not be confused with diseases that cause mummies, such as black rot and bunch rot.

2) LBSN is a physiological problem that happens in response to some stress in the environment. Stress causes the plant’s xylem (the “veins” of the plant that transports water and nutrients) to shut down in the cluster during ripening. With a dead xylem, water cannot be transported to the rest of the cluster. So the bunch stems dry, and then the berries dry up.

A Widespread Problem

I wrote an article about LBSN in February in the UMN Grape Breeding and Enology Blog. Between then and now, at least 22 vineyards in Minnesota have reported LBSN symptoms to me, as well as growers in Wisconsin, Ohio, California, Virginia, Vermont, and Nova Scotia. These reports included crop losses between 5-90%. This widespread response highlights the problematic nature of the disorder and the need to determine causes and treatments for it.

Searching for the Causes of Bunch Stem Necrosis  

The causes of LBSN are not well understood globally. Previous research has associated several different environmental stresses with LBSN, so we are working to determine what is causing the problem in Minnesota. Potential stresses include cool, wet weather after veraison; excessive pruning; unhealthy vines; and soil nutrient imbalances.

A shriveled grape cluster

It is more likely that LBSN in Minnesota is associated with wet, cool conditions and the overall health of the grapevines, rather than soil nutrient availability. This hypothesis is based on preliminary research this season, previous studies in other regions, and state-wide weather data from 2016-2018.

Cool, Wet Fall: Cooler-than-average temperatures and heavy rainfall during ripening have been associated with LBSN in previous studies in Europe and Australia. Weather data from 2016-17 in Minnesota do show relatively wet, cool weather during August and September in many parts of the state, when LBSN was reported to be most severe. LBSN in our area may be worse during seasons when weather is cool and wet following veraison. However, further research is necessary to examine this more thoroughly.

Faster ripening may potentially reduce the impact of LBSN, by allowing the fruit to mature before symptoms set in. In at least one vineyard I visited this season, fruit that was mature and ready for harvest prior to Sept. 18 was essentially able to “outpace” the peak development of LBSN. When temperatures are cooler than average during veraison and ripening, clusters tend to ripen less quickly. Practices to help accelerate ripening, such as shoot thinning, cluster thinning, and increasing sunlight exposure to the fruiting zone, can all be used to help accelerate ripening.  Future research should explore how these canopy management practices may help minimize LBSN.

Site Selection: Excess soil moisture has been associated to LBSN in previous studies. Sites with better water drainage are less likely to stay wet following heavy rains. Heavy soils with high clay content, and flat or low-lying sites are at higher risk of retaining excess soil moisture.

Nutrients: A handful of studies in other regions linked LBSN to nutrient deficiencies like Mg, Ca, and N. However, this does not appear to be the case in Minnesota. My preliminary research from this season yielded no evidence of nutrient deficiencies in any of the vineyards I studied that reported LBSN. In fact, we found that all four vineyards had plenty of these soil nutrients. Fertilizer applications are not recommended unless soil tests reveal deficiencies. Unneeded fertilization can cause run-off and excessive vine canopy.

An Interesting Observation: Aerial Roots

While visiting vineyards exhibiting LBSN symptoms in mid-September, I began noticing that almost all affected vines also had aerial roots. Aerial roots (roots forming on the cordons) can happen in response to winter injury, late frost damage, or excessively wet, humid conditions. While they do not negatively affect the plant, they do serve as a useful clue that the vine is experiencing these conditions.

Free Webinar: 2018 Grape Grower Season Re-Cap, Nov. 15

Vineyard in fall

Click here to watch the recording of the webinar from Nov. 15: Grape Grower Season Re-Cap Webinar

______________________________________________________________________

How did the 2018 season go for grape growers in the upper Midwest? What challenges did we tackle, what went well, and what should growers think about for next year? This one-hour webinar will cover these topics and leave plenty of time for questions and comments. Hosted by the University of Minnesota Extension and U of MN Grape Breeding and Enology, with collaboration from the Southern MN Wine Growers Alliance and the MN Grape Growers Association.

If you are new to webinars: A webinar is just like watching someone give a seminar, but online instead of in-person. That way, you can participate from anywhere in the world without leaving the house. You will hear us talk while also seeing a powerpoint presentation. There is also a text box where you can submit questions.

When: Thursday, November 15 at 6:00pm

Where: Your computer, anywhere you get internet. If you don't have internet, you can still listen in by phone but won't be able to see the powerpoint slides.

How To Participate: It's easy! Just click “Join WebEx meeting” below, on Nov. 15 by 6:00pm, and it will take you to the webinar. 

Join WebEx meeting (click here on Nov. 15 by 6pm to join the webinar)

Meeting number: 819 673 521

Meeting password: R5Y4QxY3

If you do not have internet, join by phone:

+1 210 606 9466 US Toll

+1 866 282 7366 US Toll Free

Access code: 819 673 521

Add this meeting to your calendar. (Cannot add from mobile devices.)

Spotted Wing gDrosophila (SWD) Alert for Wine Grapes

Recent rains have caused berry splitting for grape growers around Minnesota, leading to spotted wing Drosophila infestations. Insecticide applications are warranted to prevent damaging affects on wine quality, and must be done at least one day prior to harvest.

Bill Hutchison, Dominique Ebbenga, and Matt Clark
UMN Entomology, UMN MN Extension IPM Program, UMN Horticulture

With the wine grape harvest upon us, and a recent increase in Spotted Wing Drosophilla (SWD) counts, via our weekly trapping network, growers should be aware of the potential contamination of juice or wine, if berries are exposed to SWD just prior to harvest.  The concern with late-season SWD in grapes is two-fold, including a) the potential for direct contamination of fruit/juice via egg-lay and larval infestations, and b) the surface exposure of adults and/or larvae contributing to increased levels of volatile acidity (VA) at either the juice stage or during fermentation --- which is responsible for a vinegar taint to the wine. 

Regarding the risk of direct berry infestation, our 2017 research, via a graduate student (Dominique Ebbenga), we found that with intact (non-damaged) berries, the vast majority of MN grown varieties were resistant to egg-lay. Among 34 varieties tested, only 2 commercial varieties (Swenson red, Vanessa) experienced egg-lay and larval infections. However, the complication this year with recent heavy rainfall, is that excess splitting is underway with several varieties, and splitting allows SWD to easily lay eggs and generate larval infestations.

Regarding concerns with VA, Ebbenga's research found statistically significant, and higher levels of VA in Frontenac, Marquette and La Crescent juice, following exposure to 20 SWD adults (vs zero adults) for 10-14 days prior to harvest. For more information on VA, and how this contributes to a vinegar taint, see the enclosed fact sheet from Penn State. This study is being repeated this year, and will be extended to assess VA levels in wine as well. 

In brief, in preparation for this year's harvest, if growers are in an area where trap catches, or adult numbers are high, or have their own traps to confirm, and one or more varieties are splitting, an insecticide application prior to harvest is warranted. Unfortunately, there are few options available with a short pre-harvest interval (PHI). Mustang Maxx is the primary pyrethroid product to consider, with a 1-day PHI and 12 hr re-entry interval; this is a Restricted Use Pesticide (RUP), and the label should be reviewed. In other words, growers must wait at least one day after applying Mustang Maxx before harvesting.

For recent SWD counts at selected TC-metro locations, see: https://www.fruitedge.umn.edu/swdtrap

Click here for additional SWD management information and labelled insecticides via our UMN guide.