The use of synthetic fungicides to control fungal diseases unfortunately also damages the fungal networks in the soil that RV relies on to increase vine resilience to disease. Copper is one alternative that is widely used. However, it is also detrimental to soil health as a high proportion ends up in the soil, where it accumulates over time.
Organic vineyard systems can rely heavily on copper, and in areas, or years, with high fungal disease pressure, it is very hard to avoid some use of copper (or systemic fungicides), if growers are to harvest a viable crop.
Taking the lead from countries like Germany and Austria who have set a lower limit (3Kg/ha/year) for copper use compared to European organic regulations, we believe that all growers should attempt to keep their copper use below this level.
There is no easy solution to fungal disease. Good canopy management can reduce disease pressure. RV practices which improve vine and soil health can build resilience to disease. However, this takes many years.
Products that control fungal diseases without destroying soil-based microbial communities may be the solution, but the effectiveness of these treatments is still under review. Another solution might be to use hybrid varieties that require very little treatment, but these have not yet been widely adopted by growers or consumers.
The RVF is keen to explore and research reliable solutions in this area.
Regenerative viticulture practices work to increase the resilience of the vine to disease through improved soil health, and by extension, upregulation of plant response to fungal pathogens. Mycorrhizal fungi and their hyphal networks assist grapevines in accessing critical nutrients for the biosynthesis of secondary metabolites that help the plant resist infections. It is therefore important not to damage these through the use of fungicides.
Conventional viticulture relies on calendar spraying of synthetic fungicides both preventatively and curatively to avoid the risk of crop loss to fungal diseases. This can be in the region of 20 timetabled sprays per season in damper regions, with associated tractor passes. Weather data used to prompt sprays based on disease pressure can reduce the frequency of sprays. Following models such as the Gubler model for powdery mildew and the Muller-Sleumer model for downy mildew can also inform the best timing for targeted fungicide applications. New technologies such as spore traps deployed in vineyard hot spots can also significantly reduce the number of fungicide sprays by alerting growers to fungal spore release events.
Organic horticultural oils and extracts have shown great efficacy for prevention and control of powdery and downy mildew, but are not registered for organic winegrapes in every region, so practitioners should check the regulations in their region. New organic fungicides/biocontrols have been developed to elicit a defence response in the vine. These colonise the sites where infections could start, outcompeting the pathogens. Organic growers use sulphur, potassium bicarbonate and copper if absolutely necessary.
Copper is not absorbed by the vine but gets washed off into the soil, where it accumulates over time. The condition and pH of the soil determines how toxic it is to the life in the soil. Soils with higher levels of organic matter and higher pH are not as badly affected, but copper has universal negative effects on soil microbes and invertebrates, notably earthworms.
Botanical sprays such as valerian, dandelion, yarrow, nettle and horsetail are also used against fungal diseases.
Canopy management for sunlight and air
The most important tool in preventing disease is a canopy management programme that brings sunlight and air into the vine and onto the fruit: sunlight and air are naturally antifungal.
Therefore it is essential to have best practice canopy management:
- Permanent vineyard floor cover has been shown to reduce canopy vigour and vine nitrogen status when vines are most susceptible to powdery and downy mildew
- Careful and consistent winter pruning to reduce canopy density the following season
- Bud rubbing/summer pruning to reduce canopy density
- Well-timed leaf plucking in the fruiting zone to expose the fruit to sunlight and air. In warmer regions this may be done preventatively on the morning/cooler side only, to avoid sunburn and excessive heat , and in cooler regions double-sided leaf removal may be appropriate
- Trellis redesign to incorporate additional light/airflow and to raise the fruiting zone height if undervine foliage is increasing humidity in the fruiting zone
- Disease resistant varieties such as PIWIs with thicker cuticles or more open cultivars with looser bunches
Vineyard hygiene to reduce inoculum
Good vineyard hygiene is also important:
- Vineyard monitoring and disease scouting is essential to catch disease early
- Removal of overwintering inoculum
- Avoid planting where soil drainage is poor
- Ground cover to reduce the likelihood of downy mildew as there is less standing water for inoculum splashing up from the vineyard floor