Four years of monitoring of disease-resistant grapevine varieties in French vineyards

Resistant cultivars are widely used for arable crops, but remain rarely considered for perennial ones, such as apple and grapevine. This is all the more surprising as these crops often received numerous pesticide treatments. For example, in France, the mean treatment frequency index in viticulture is 15.3 (Agreste, 2019). In this context, the deployment of disease-resistant grapevine varieties is a promising innovation paving the ways toward sustainable viticulture. Various European breeding programs led to the registration of several INRAE and other foreign resistant varieties of grapevine in France. They can now be grown in the vineyards.


Introduction
Resistant cultivars are widely used for arable crops, but remain rarely considered for perennial ones, such as apple and grapevine. This is all the more surprising as these crops often received numerous pesticide treatments. For example, in France, the mean treatment frequency index in viticulture is 15.3 (Agreste, 2019). In this context, the deployment of disease-resistant grapevine varieties is a promising innovation paving the ways toward sustainable viticulture. Various European breeding programs led to the registration of several INRAE and other foreign resistant varieties of grapevine in France. They can now be grown in the vineyards.
The adoption of new disease-resistant grapevine varieties remains challenging for farmers. Indeed, as for any newly registered variety, many agronomic parameters must be assessed in the vineyard. More specifically, disease protection goals and strategies must be adapted to complement the partial genetic resistance conferred by resistant cultivar but also to control the secondary diseases that might re-emerge following the reduction of fungicide use. Long-term cropping system experiments have already been conducted with disease-resistant grape varieties at INRAE in Bordeaux vineyard (Delière et al., 2018), but further evaluations are now required to address these issues in more diverse agro-climatic production situations.
Another major challenge raised by the deployment of diseaseresistant grapevine varieties is the management of resistance durability i.e. the maintenance of resistance efficacy in the long term. Resistance genes do not remain effective forever (McDonald and Linde, 2002). Accordingly, given their scarcity in genetic resources and the time required for breeding new cultivars, resistance genes must be careful manage to ensure that they will remain effective as long as possible (Merdinoglu et al. 2018

Material & Methods
Network of plots INRAE has set up in 2017 a network of plots planted with powdery and downy mildew resistant varieties. The network is named the OSCAR Observatory (https://observatoirecepages-resistants.fr/en/). Plots are located in various agroclimatic conditions. They are planted and managed by the winegrowers according to their own objectives. To join the network, plots must cover an area of at least 0.2 ha (700 to 1500 vines stock), be planted with a single variety and be cultivated with similar cropping practices as those used elsewhere in the estate (pruning, cropping operations, harvest). OSCAR integrates new plots every year. Each plot is described in terms of location, size, variety, year of plantation, density, rootstock, pruning method, and target yield. In 2017, OSCAR consisted of 34 plots located in 14 locations (Guimier et al., 2019). The network has grown and, in 2021, OSCAR is composed of 116 plots (for a total area of 80 ha) distributed in 63 locations. The average size of a plot is 0.7 ha. Twenty-six downy mildew and powdery mildew resistant varieties are monitored, including 14 INRAE varieties and 12 varieties from other European breeding institutes (Table 1).

Fungicide use
Phytosanitary practices are collected from the wine growers. The data collected are dates of treatment, target, product name, active ingredients, dose and decision rules used. The level of pesticide used is estimated by the treatment frequency index (TFI). For each treatment, the amount of fungicide used is calculated as follows: TFI per treatment = ((Dose sprayed / Dose recommended) * (Area sprayed / Area total)). Over the whole cropping season, we then have TFI = TFI per treatment (Pingault et al., 2009 ;OECD, 2013

Fungicide use
The average fungicide TFI obtained on 30 plots in 2017, 32 plots in 2018, 36 plots in 2019 and 61 plots in 2020 amounted to 1.4, 1.6, 0.6 and 1.3, respectively, compared to an average fungicide TFI of 12.7 for the 2016 national reference (Agreste, 2019) ( Figure 1). Thus, the reduction in fungicide TFI on plots planted with resistant varieties range, depending on the year, from 87 to 95%. Field monitoring over three cropping seasons.
Since 2017, all the pests and disease surveyed have been satisfactorily controlled on the plots monitored. For DM, symptoms appeared on the plots from 2018 to 2020 and especially during the cropping season 2018 which suffered an high epidemic pressure. However, although disease incidence may have been high in 2018, severities remained low and did not lead to significant crop losses. For PM, no symptoms were detected in the field on varieties carrying the Run1 gene conferring total resistance. Finally, neither significant impact of the diseases usually controlled by fungicides (anthracnose, black rot), nor of pests such as erineum mites and phylloxera, were observed on the plots, except for black rot, which led to significant losses for one plot in 2018 and 2 plots in 2020 (Table 3).
Laboratory monitoring of resistance efficacy. The resistance factors Rpv1 and Rpv3.1, alone or cumulated, significantly reduced the sporulation of isolates collected on susceptible varieties (sporulation reduced by 64% for Rpv1; 78% for Rpv3.1; 89% for Rpv1/Rpv3.1 pyramids). They also reduced the sporulation of isolates collected on Bouquet varieties (carrying the Rpv1 factor) by 65% for Rpv1, 74% for Rpv3.1 and 88% for the Rpv1/Rpv3.1 pyramid). This is also the case for the resistance factors Rpv1 and Rpv1/Rpv3.1 pyramided which reduced the sporulation of isolates collected from Resdur1 varieties by 69% for Rpv1 and 78% for Rpv1/Rpv3.1 (Table 2). However, we evidenced a lower efficacy of Rpv3.1 when inoculated with isolates collected from Resdur1 varieties. The decrease of efficacy to 47% can be explained by isolates overcoming the resistance conferred by Rpv3.1 (Figure 3).     The field monitoring carried out within OSCAR observatory during these first four years did not reveal any resistance erosion or resistance breaking of the varieties deployed. These field observations were supplemented by a laboratory monitoring of the aggressiveness of DM populations collected from the network of plots. No breakdown of Rpv1 factor nor of the pyramid Rpv1/Rpv3.1 were observed in this monitoring. However, the results confirmed the presence of DM isolates breaking down the Rpv3.1 factor. Importantly, these first four years of monitoring also confirmed the high potential of grapevine resistant varieties to reduce phytosanitary treatments. In the plots planted with these varieties, all the pests remained well controlled in most cases despite drastic reductions of around 90% in the fungicide TFI compared to the national reference. Accordingly, the deployment of resistant varieties in the vineyard should be increasingly important in forthcoming years, confirming its role as a key player toward sustainable viticulture. In this context, it will be essential to monitor the durability of the resistance factors deployed. In this respect, the OSCAR observatory is a warning system allowing to estimate the effectiveness of the resistance genes deployed in the vineyard and to rapidly identify the emergence of adapted DM populations. Table 3 : Incidence and severity of downy mildew, powdery mildew and black rot on leaves and clusters in 40 plots of the OSCAR network sampled at harvest stage in 2020. The plots are classified by variety. The varieties represented here are those with the highest number of data collected at the harvest in 2020. Disease incidence on leaves: frequency of vines with at least one infected leaf. Disease incidence on clusters: frequency of vines displaying at least one infected cluster. Disease severity : percentage of tissue area covered by lesions. Incidence and severity are classified into five classes. The table shows the distribution of plots in each rating class.