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Are you even sap flow pruning if you don’t do a little “summer pruning?” One of the most critical aspects of sap flow pruning principles has to do with "summer pruning," or as most would call - shoot thinning and bud rubbing. While bud rubbing is standard practice for most vineyards, shoot thinning is not something that is necessarily seen as required. We’re here to highlight the benefits of shoot thinning as it relates to sap flow pruning and inevitably trunk disease prevention.
Junk in the Trunk – Production Vineyard Shoot Thinning
If there is one undeniable truth about trunk diseases, it’s that the surface area of cuts made during winter pruning is one of the largest factors in the spread of trunk disease. Therefore, the principle of shoot thinning in the summer is so important. Removing small unwanted shoots during the season not only leaves a much smaller wounds on the trunks, but the timing has some added benefits (e.g. reduced conditions for new infections, regular fungicide spray programs, etc.).
A quick thin through the heads will improve airflow through one of the most congested areas of the vines, reducing disease pressure and increasing canopy spray penetration. Come winter pruning it also means that there are fewer required cuts in the head, and fewer wrong options for pruners to choose.
Young Vine Training and Establishment
As much as I wish more growers would accept a little “wild” when it comes to vineyard floor management, I draw the line at young vines with “wild” trunks. Investing the resources to keep your vines tidy during establishment benefits the vines, trellis, your staff, and your equipment.
So, as I was out in one of our young vine establishment trial blocks today (check out the blog post), it seemed timely to share one important piece of advice when it comes to young vine training and establishment. In the year you decide to establish your trunk, whether it is the year of planting or the year after 2-budding, remove laterals on the trunk and portion of the vine that will be wrapped on the wire while they are still small and easy to pinch out. This is not only from the perspective of reducing winter pruning cuts, but it also means there are not loads of little nubbins left which make bud rubbing a pain next growing season.
Managing Trunk Disease in Season
There is still a bit of unknown around the risks of summer wounds and trunk disease. Recently Dr Mark Sosnowski from SARDI and Dr Eline van Zijll de Jong from Linnaeus Labs presented on their research in Marlborough and Hawke’s Bay. During their session in Marlborough there were a few takeaways and some questions. Their findings on trunk disease spread in winter largely supported the principles of sap flow pruning (e.g. reduce the number of cuts, reduce the size of cuts, and avoid flush cuts, etc).
As part of the re-trunking project that was presented, trunk disease was assessed at intervals down the length of the removed trunks, and there were a couple of instances where presence was detected high up in the trunk, and then not again until a lower point in the trunk. This raised the question of whether that could have been from inoculation during the growing season through bud rubbing wounds. While it is possible for these pathogens to infect green tissue (Sosnowski et al, 2022), and certainly conditions can be suitable for inoculation at the time of bud rubbing and shoot thinning, after spending a number of years managing crews during both summer and winter, my first guess would be that those lower infections came about from a sucker that was missed during the season, and thus removed during winter pruning. It stands to reason that if bud rubbing was a significant risk factor in trunk disease spread, that efforts such as the re-trunking study above would have uncovered higher incidences of infections that originated lower in the trunk.
That being said, I am as much a plant pathologist as a winemaker is a viticulturist, so I look forward to seeing the research to either support or refute this hypothesis. It does, however, raise a good point about in season infection risk, and an opportunity to look into best practice around in season management. For example, would timely chemical bud rubbing reduce the risk, because it reduces the wound surface area and pathway into the trunk as compared to physical bud rubbing? If you are shoot thinning, should you ensure the fungicide application just after (or within 2 weeks after) has chemistry labelled for trunk diseases? Final Thoughts
Wound protection is an important part of trunk disease management, but let us not forget the power of cultural practices in disease management. It is extremely easy to fall into a pattern of reliance on chemistry to control pests and diseases, but even when using the “best” chemistry, there are management practices that can make or break its efficacy. Sap flow pruning, shoot thinning, leaf plucking, young vine training, proper irrigation, efforts to increase biodiversity - these are some of the most powerful tools to ensure fruit and wine quality – so it’s important to make time for them.
Literature Cited:
Sosnowski, Mark & Ayres, Matthew. (2022). Spring pruning wounds are susceptible to grapevine trunk disease pathogens. To request full text: https://www.researchgate.net/publication/373754430_Spring_pruning_wounds_are_susceptible_to_grapevine_trunk_disease_pathogens
Eline van Zijll de Jong and Mark Sosnowski (2022). New Guidance on Managing Grapevine Trunk Disease With Remedial Surgery. New Zealand Winegrowers Magazine. Accessed 20 November 2024 online at <https://issuu.com/ruralnewsgroup/docs/nzwg_134_june-july>
Other Helpful Resources:
Wine Australia Best Practice Management Guide v2.0. (July 2019). Accessed 20 November 2024 from <https://www.wineaustralia.com/getmedia/6bccff66-018b-4b90-bb12-434094b6c917/RD_BPMG_GrapevineTrunkDisease_Jul2019.pdf>
Eline van Zijll de Jong and Mark Sosnowski (2024). Improving remedial surgery practices for control of grapevine trunk disease to increase vineyard longevity. Final Report Submitted to BRI. Accessed 20 November 2024 from <https://www.nzwine.com/>
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