The French walnut improvement program: preliminary investigations
With 36,000 t of in-shell walnuts produced in 2016, France is in 9th world position among major producers. Walnut orchards increased by almost 20% between 2000 and 2010, and walnut is the most important crop other than apple. Nevertheless, the varietal choice does not appear to be adapted to the new future constraints and the new French walnut improvement program, led by the Ctifl, has to take into account the global warming context and the reduced use of plant protection products. In this way, two preliminary studies have been conducted regarding the effect of climate change on walnut phenology under French climate conditions, and concerning an emerging disease in French walnut orchards, a form of anthracnose, due to Colletotrichum acutatum. On the one hand, as regards phenology aspect, chronological data have been collected for the two most produced cultivars ('Franquette' and 'Lara') in France. A significant advance in bud break date and male and female flowering dates was found. On the other hand, as concerns the emerging disease, a qualitative detached leaf assay (absence or presence of necrosis) have been carried out in order to quickly find out which accessions of the French germplasm (including 220 accessions of J. regia, the cultivated walnut tree, and 39 accessions of related species) are tolerant. Results show that, after 7 days of spores inoculation on detached leaflets, all J. regia accessions have specific symptoms of anthracnose due to C. acutatum (black to brown spots with orange acervules) while two related species, J. cinerea and J. sieboldiana, do not present any symptom, revealing a potential tolerance to C. acutatum.
Bernard, A., Dirlewanger, E. and Lheureux, F. (2020). The French walnut improvement program: preliminary investigations. Acta Hortic. 1280, 77-84
Juglans regia L., French walnut improvement program, chronological data, phenology, Colletotrichum acutatum, detached leaflet assay