Managing Wood Diseases by Pruning the Vine
Pruning the vine is an essential step in vineyard maintenance. The act of pruning can cause significant wounds on the vine stocks, thus providing many entry points for fungi responsible for wood diseases. Pruned wood and symptomatic stocks are a source of inoculum. It is therefore important, to limit wood diseases, to implement preventive strategies from the planting of a vineyard plot. Several factors can influence contamination through pruning wounds:
- Factors related to the type of pruning: the length of canes and spurs, the number, size, location, aggregation and age of pruning wounds as well as their protection,
- Factors related to the pruning period: weather conditions during pruning and timing of pruning (early or late pruning),
- Factors related to pruning residues and their management (buried, shredded, removed, left on site...)
Type of pruning
Pruning wounds represent an entry point for fungi involved in wood diseases, which are then able to overcome the plant's defense mechanisms due to their virulence. Large and numerous pruning wounds, common in older vineyards or in plots where the pruning system has been modified, constitute an important entry point for fungi because of the large total surface area where spores can land and cause infections. It is possible that part of the fungal inoculum is transmitted through pruning wounds or pruning tools, but the concentration of inoculum thus transmitted is usually negligible.
An experiment conducted in Bordeaux showed that Guyot pruning increases the expression of Eutypa dieback symptoms. Indeed, the development of foliar symptoms of eutypa dieback is higher with short pruning compared to long pruning, but the mortality rate is lower with long pruning. Vines pruned long (Guyot) have grouped pruning wounds on the upper part of the trunk whereas vines pruned short (Cordon, goblet) have a larger surface area of pruning wounds.
Pruning period
To choose the most appropriate pruning period, it is necessary to consider different factors such as the climatic specificities of the region, the development cycles of pathogenic fungi, the spore release and the receptivity of pruning wounds depending on weather conditions and pathogen virulence.
Climatic conditions are decisive in the dispersal of spores of fungi involved in wood diseases, therefore, pruning the vine during dry periods is essential as fungal inoculum is significantly lower.
Management of pruned wood and other inoculum sources
The source of inoculum of fungi causing wood diseases can be on the vines showing symptoms (in the wood and/or on leaves) but also on other crops such as orchards nearby. Inoculum can be found on:
- necrotic wood
- leaves
- dried grape clusters
- bark of old wood (trunk, cordon framework)
- dead wood
- pruning wood debris.
To eliminate these infection sources, different practices are used in European vineyards. The most commonly applied by growers are:
- shredding wood followed by burial in the soil,
- burning (where allowed in vineyards),
- shredding followed by composting
- removal of symptomatic stocks or dead stumps.
It is estimated that pruned wood is a potential source of inoculum for fungi of Black Dead Arm for 42 months, but the infectious power of the inoculum significantly decreases after 24 months and spore viability is reduced to 44%. Pruned wood and other vine fragments can be reintroduced into plots after shredding followed by composting because these processes eliminate fungi of wood diseases. If applied properly, there will be no risk of vineyard recontamination by Eutypa dieback, Esca or Black dead arm.
Mechanical shredding and composting at 40-50°C for a period of 6 months effectively eradicate fungi of wood diseases (for a compost composed of 40% shredded vine wood, 40% sheep manure and 20% green waste). Moreover, some pathogenic fungi of Esca (P. chlamydospora and P. aleophilum) are no longer found in pruning wood fragments after shredding. Researchers suppose that shredding promotes the activity of saprophytic fungi which develop faster than fungi of wood diseases.
Protection of pruning wounds
Adopting preventive methods to control wood diseases shortly after planting a new plot is crucial. The infection rate over a long period is significantly lower if a pruning wound protection strategy is regularly applied 3 to 5 years after planting a new vineyard. If preventive disease management is implemented shortly after planting, it minimizes the development of diseases and additional costs of replacing stocks such as replanting or replanting.
It is important to remember that pruning wounds remain entry points for potential infections for a long time and that protecting new and old pruning wounds is advised to limit the establishment of wood diseases. Protection of pruning wounds can be done with chemical fungicides (when approved) or by biocontrol preventively, taking into account the critical points of each technique (note, in France only the product Phytopast-V based on Cyproconazole and Thiophanate-methyl is approved against eutypa dieback).
One of the main limitations of using chemical fungicides for wound protection, when authorized, is their low persistence. Pruning is usually done at the beginning of the winter season for work organization or weather reasons. However, it is unlikely to achieve efficacy covering this entire period. Some fungicides are effective for 3 weeks after application and if necessary, their application can be renewed. Application to protect pruning wounds is done by spraying or by application with a brush. Sprayable formulations are more practical, less expensive and faster but are easily washed off by rain.
Biocontrol agents (e.g. Trichoderma spp.) and natural molecules (chitosan) have been described as effective for pruning wound protection. Moreover, biocontrol agents can actively colonize pruning wounds for up to 8 months. Treatment 6 hours after pruning with biocontrol agents, in early or late pruning, resulted in high colonization of pruning wounds by Trichoderma spp., even if weather conditions differed during pruning periods. Once the disease is established, it is difficult to eradicate effectively due to the limited means currently available.
Innovative aspects in the fight against wood diseases
Guyot-Poussard pruning
The sap flow-respecting pruning method was adopted by Lafon (1927) from a Guyot pruning system, later named « Guyot-Poussard », after its inventor. The main goal of this pruning is the preservation of the same sap flow from year to year with pruning that positions all pruning wounds on the upper part of the cordon. Guyot-Poussard pruning results in smaller and fewer pruning wounds. Some pruning techniques require rejuvenation of the framework which can be avoided with this pruning method. Moreover, wounds on old wood, common on vines that have been rejuvenated, would be less resistant to infections by fungi of wood diseases than pruning wounds made on one-year-old wood.
Pre-pruning
Pre-pruning is a modification of late pruning recently implemented to preventively fight against wood diseases in short-pruned vineyards. This practice is not applicable for vines pruned Guyot-style, cane-pruned, but in short-pruned vineyards it is an effective technique to delay pruning date and reduce infection rates by pathogens of wood diseases. Pre-pruning is done mechanically, at a uniform height of 30 to 45 cm above the cordon and is not selective. A second, more precise manual pruning is then performed. This pruning technique, which keeps longer wood above the cordon, reduces contamination of the old wood of the framework and trunk, limiting the progression of fungi of wood diseases every year. However, an economic assessment estimates that double pruning is costly compared to simple late pruning, with similar effectiveness.
Minimal pruning
Minimal pruning consists of almost not pruning the vine and has recently been considered as a practice that can potentially reduce the infection rate of pruning wounds by fungi of wood diseases. Although this pruning system reduces labor costs, it is a production system with high yield and lower grape quality. Minimal pruning, compared to cordon pruning, shows less necrotic wood, fewer foliar Esca symptoms, less damage from pathogenic fungi. A study on the impact of pruning systems on Eutypa dieback showed that incidence and severity of attacks are lower for minimal pruning compared to conventional pruning.
Good pruning practices
Reduce inoculum
- Remove infection sources before pruning (pull out and remove dead stocks).
- Prune the vine in dry weather.
- Remove pruned wood as soon as possible (shredding, composting, etc.).
- Avoid depositing pruned wood or dead stocks in or near plots.
Minimize new contaminations
- Protect pruning wounds preventively (physical, biological or chemical protection) quickly after pruning.
- Fungicides (biological or chemical) are only effective in a preventive treatment strategy to limit new contaminations.
- Limit the number of wounds on the vine (injuries during mechanical harvesting, leaf removal, mechanical pruning…)
- Rejuvenation of frameworks, if necessary, should be done with a 2-year-old cane to avoid large-diameter pruning wounds.
- Increase the length of spurs and canes to minimize fungal penetration.
- Pruning symptomatic and asymptomatic stocks one after another is possible because transmission of pathogenic fungi by pruning tools is negligible.
- Disinfection of pruning tools is good hygiene practice but is not a key point to limit the spread of wood diseases.
- Implement pre-pruning and, if not applicable, perform early/late pruning.
Collective management of wood diseases
- Applying a single control method to manage wood diseases has only partial effectiveness. Implementing multiple control methods is essential for comprehensive management of wood diseases.
Limitations
- Technical knowledge.
- Lack of equipment (composting, mechanical pruning...)
- Availability and approval of pruning wound protection products (sealants or fungicides, biological or chemical) on the market.
- Balance between profitability and effectiveness of the technique.
Sources
- This article was written based on the PDF document: https://www.plan-deperissement-vigne.fr/sites/default/files/2020-05/Winetwork-%20les%20bonnes%20pratiques%20de%20taille_0.pdf (Work carried out jointly by the Facilitator Agents of the Winetwork project.)
