As mentioned, disease control can be achieved with the repeated use of fungicides, with the risk of emergence of resistant strains of P. viticola . Recently, with the rise of sustainable viticulture, the elicitation or priming of endogenous defence mechanisms in grapevines has gained significant attention in the scientific community.
Elicitation and enhancement of plant defence mechanisms have been described for grapevines and for other plants . The generic description of plant immune responses has generalised the use of the term PAMP (Pathogen Associated Molecular Patterns), which precisely defines an elicitor to be a microbe- or plant-derived molecule generated by the enzymatic degradation of plant components by the pathogen itself (e.g., galacturonic acid and glucans) during the first step of infection . Plant elicitors were first defined as compounds able to induce phytoalexin production but are also commonly recognised to induce oxidative processes such as the production of ROS (reactive oxygen species), papillae formation, lignification processes and expression of pathogenesis-related (PR) proteins .
In the grape, some common plant defence mechanisms against fungal infections have been described, including stilbenic phytoalexins production, which can be induced by biotic or abiotic stresses, such as UV irradiation . Various molecules, such as laminarin , BABA , BcPG1 , cyclodextrin , PS3 (sulfated laminarin) , botrycin and cinerein extracted from B. cinerea , chitosan and fosetyl aluminium, have been shown to induce chitinase and glucanase activity and stilbenic phytoalexins synthesis and have provided a better understanding of these defence responses. Organic viticulture requires products to protect grapevines without the use of synthetic fungicides. Scientific research is thus necessary to evaluate the activities of new natural products as fungitoxic compounds or as elicitors to confer crop protection [99, 100]. A better knowledge of elicitation processes could contribute to a reduction in the application of fungicides. The synergistic application of priming molecules and fungicides under field conditions may allow for a reduction in the doses of fungicides required to control diseases .
Some plant extracts have been shown to possess direct antifungal properties against phytopathogenic fungi, while others could indirectly inhibit fungal development by eliciting endogenous mechanisms of defence against P. viticola . Direct application of these extracts in the field is not suitable for an efficient evaluation of their efficacy. Recently, the efficacies and the modes of action of various fungicides and elicitors were evaluated on the basis of various markers of resistance to downy mildew developed, as previously described. These markers (rate of infection, quantification of sporulation, quantification of stilbenic phytoalexins and stomatal callose) were used on both single leaves and whole plants of susceptible Vitis vinifera varieties such as cvs. Chasselas, Pinot noir, Cabernet sauvignon or Gamay, which were pre-treated with aqueous suspensions of these products at different concentrations. The preliminary results showed that at least two natural plant extracts out of the 55 tested (root extract of the rhubarb Rheum palmatum and bark extract of the glossy buckthorn Frangula alnus) were effective. They induce, simultaneously, prolonged activation of defence mechanisms, particularly stimulation of the 8-viniferin synthesis, and inhibition of pathogen development to a significant degree. Other treatments (e.g., application of extracts of Galla chinensis and pure gallic acid) induced a massive production of stilbenes for a short period time, but this was insufficient to inhibit the development of downy mildew at 48 h after treatment. Nevertheless, plant protection was ensured by the fungitoxic properties of the applied products. In contrast, copper and tannic acid treatments had fungitoxic effects but did not induce plant defence mechanisms. The ability of R. palmatum and F alnus extracts to induce stilbenic phytoalexins production and peroxidase activity was clearly due to their anthraquinone content. Indeed, application of aqueous solutions of pure anthraquinones such as rhein, frangulin A, emodin, aloe-emodin, chrysophanol and physcion  had effects similar to those of the extracts. However, these plant extracts may also contain other compounds (such as glucans, pectins, lignins or tannins) that could elicit host defences.
Stilbene production provides a reliable metric for the efficacy of elicitors and plant protection products against downy mildew. Since 2005, it has been applied in Agroscope-ACW collaborative projects in integrated pest management and organic and biodynamic viticulture. These experiments provide decisive information in advance of field-testing. Applications of crude plant extracts have potential for grapevine protection. However, a careful characterisation of the chemical composition of these natural extracts as well as the bioactivity of their individual constituents is necessary. Indeed, it is important to identify the molecules responsible for the induction of host defences, assess the safety of their use for wine production and verify their neutral ecological impact when used in large amounts in the field.
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