Trunk disease continues to be a topic that attracts much debate in the wine industry the world over. Unfortunately, as researchers explore further, the lack of major breakthroughs highlights the complexity of these pathogens; the title of a 2012 review paper sums it up well, “Grapevine trunk diseases: complex and still poorly understood” (Bertsch et al. 2012). There has been some progress though and many of the recent papers follow up from the 2012 International Workshop on Trunk Disease (IWGTD) in Valencia which demonstrated some promising lines of enquiry for the future.
Whilst this work was done in the lab rather than the field and used high concentrations of conidia to infect the vines, the authors point out that these levels can exist in the vineyard during the winter season if vineyard hygiene is not good and, for example, mummified berries and dead, infected wood is left as a source of inoculum. This further underlines the case for good vineyard hygiene and, as the authors also point out, potentially the use of fungicidal sprays during the dormant period.
On a more positive note, another New Zealand paper was able to demonstrate that the use of a mustard cover crop can reduce levels of Ilyonectria found in soils (Busch 2012). The mustard crop releases volatile compounds (isothiocyanates) when it is broken down by cultivating into the soil and these compounds can reduce infection levels. In the experiment, the cultivated soil was covered with plastic to reduce the tendency of the isothiocyanates to vaporise and whilst this is not necessarily practical in the vineyard, mustard has long been prized as a bio-fumigant and it is reassuring to learn that it is effective against at least one TD fungi and potentially more.
Mustard is not the only bio-fumigant to be a potential weapon against trunk disease. Research continues into the use of Trichoderma spp. and a piece in the Australian & New Zealand Grapegrower & Winemaker discussed the use of dowels impregnated with Trichoderma harzianum as a potentially curative treatment for Esca (Roberts 2012) when inserted into the vine trunk. Duncan McNeill of MVM Ltd. recently posted to the UKVA forum to highlight the attention Trichoderma has received in Germany as a trunk disease treatment with a wide range of positive effects: this looks to be a promising area of research.
In terms of chemical treatments, options are still restricted. ‘Symptoms and management of bot canker’ (Pitt & Savocchia 2012) was also published in the Australian & New Zealand Grapegrower & Winemaker and gives a good summary of the current state of understanding with regard to Botryosphaeria infections. The article refers to Greenseal, a commercial pruning paint containing the fungicide tebuconazole which is one of the more effective chemicals against trunk disease. We cannot get Greenseal in the UK but we could potentially be able to use the similar produce Bezel. Chris Cooper of Hutchinsons and the UKVA Pesticides Technical Expert advises that Bezel is registered for use in the UK although sales have been so sluggish (Europe-wide) that the manufacturers are not actively marketing the product here.
So with the paucity of effective treatments, a great deal of research is focused on the causes of trunk disease. We know that it is possible to have two vines with the same fungal profile but a differing expression of symptoms and also that trunk disease fungi can exist latently within the vine, i.e. present but not actively damaging the vine. Therefore it is likely that there is a role for external factors in the expression of the symptoms that cause the vine harm. There has been some discussion of the potentially catalytic role of other micro-organisms such as bacteria and this is an interesting area for future research. Stress is also identified as a cause of the expression of TD symptoms in a paper by Brown et al. 2012, who showed that carbohydrate stress caused by partial defoliation of young vines increased the severity of Black Foot infections – a potential concern for the UK where a poor growing year can reduce a vine’s carbohydrate stores. There is little doubt that stress can reduce a vine’s ability to combat infection (see, for example, Mundy & Manning 2011) so especially with regard to new plantings, every effort should be made to reduce vine stress: to quote from Billones-Baaijens et al. 2013 “…it seems likely that young nursery plants with latent infections can decline and die when planted into the more stressful conditions of vineyards.”
The role of nursery infections is a topic that has received considerable attention over the last year or so. At the IWGTD conference, a range of data gathered by differing research bodies from surveys of nurseries globally showed trunk disease infections present in nursery vines, mother blocks and on nursery equipment. The principal weapon to combat TD at the propagation stage is the use of hot water treatment (HWT) although there is reluctance to use this technique on the part of nurseries as it has been linked to a higher rate of vine failure. A paper by two of the leading researchers in this field discussed how best to implement this treatment (Gramaje & Armengol 2012) and highlighted the fact that the management of vines before and after HWT has a significant impact on their survival rate. In particular, long term cold storage without rehydration following HWT may lead to vine failure or poor growth. In general, HWT needs to be adjusted for location and variety from the ‘standard’ 30 minutes at 50 °C and nurseries should be ensuring that the procedure is carried out in the most effective and least damaging way for their products and market.
Following on from issues with infected nursery stock, the IWGTD also saw the launch of a set of draft Planting Guidelines for nurseries and growers. These guidelines cover procedures and best practice in some depth and it is a lengthy and comprehensive document. Some of the most interesting points cover the potential damage caused by the long term cold storage of vines pre-planting and the risks of standing water as a vector for spreading TD infections. Copies of the report are available on request with the permission of the authors - email@example.com