[PhD defence] 11/04/2025 - Marianne Avignon: "Understanding the mechanisms of decline in kiwifruit orchards in France and finding remedial solutions" (UMR EMMAH, DISCOVE team)

Marianne AVIGNON will submit her thesis on 11 April 2025 on the following topic: "Understanding the mechanisms of decline in kiwifruit orchards in France and finding remedial solutions"..

Laboratory

UMR 1144 - EMMAH - Mediterranean Environment and Modelling of Agro-Hydrosystems

Composition of the jury

MR YVAN CAPOWIEZ	UMR 1114 - INRAE EMMAH	Thesis supervisor
Ms Safya MENASSERI-AUBRY	Rennes-Angers Agro Institute	Rapporteur
Jay-Ram LAMICHHANE	INRAE Occitanie - UMR AGIR	Rapporteur
Sylvaine SIMON	INRAE UERI	Examiner
Mr Philippe TIXIER	CIRAD - UPR GECO	Examiner
Mrs Joséphine PEIGNé	ISARA	Examiner

Summary

Kiwifruit (Actinidia deliciosa) is a plant that requires a great deal of water during its vegetative growth phase. However, it is also extremely sensitive to anaerobic conditions, rapidly stopping any transfer of sap between the root and aerial systems if the soil is saturated with water. This saturation can lead to rapid deterioration of the root system. In France, a worrying decline has been observed in orchards in the south-west of the country since 2019. The symptoms of this decline are similar to those observed in New Zealand after Cyclone Bola in the late 1980s, and to those seen in Italy over the last ten years or so. It would appear that climate change, with episodes of intense rainfall, more frequent flooding, a lack of cold hours in winter and several heat waves in summer, is contributing to an increase in saturation events in autumn, winter and spring, and more evapotranspiration in summer.

This evapotranspiration can rapidly exceed the absorption capacity of an already weakened root system. The aim of this thesis work was to understand the mechanisms of decline in French orchards and to propose short- and medium-term action levers to limit the phenomenon. A survey was first carried out among around fifty growers in order to study the links between the various production situations and the decline observed in their orchards. A total of 56 plots/areas were studied in the Adour valley and 38 in the Garonne valley. The results showed that dieback is strongly influenced by soil structure, being almost 3 times greater on compacted soils than on aerated soils. Saturating events, such as floods, also lead to greater dieback, especially when they occur repeatedly on compacted soils. The trees in around thirty orchards were then classified according to their state of health, and the field observations were compared with data provided by satellite images from the Sentinel-2 constellation. These images are made up of medium-resolution pixels (10 x 10 m), with several trees grouped together in each pixel. Areas containing healthy trees and those with dead or absent trees were correctly identified, especially when they were larger than 500 m². On the other hand, pixels containing mixed information, with both healthy and unhealthy trees, were more difficult to detect.

However, the addition of agronomic data, such as soil texture, has improved detection. The use of remote sensing could help growers to adapt their cropping practices, particularly in terms of irrigation, thanks to near real-time monitoring of their plots. Finally, we tested three levers for action in four orchards with different soil and climate conditions. These levers - the introduction of plant cover, the massive addition of organic matter and decompaction with a tine tool - were designed to reduce compaction, improve water circulation in the soil and increase the number of fibrous roots. The plant cover had difficulty getting established because of the lack of light and competition from the grasses already present. On the other hand, the addition of compost reduced the apparent density of the soil by 5% and improved the penetrability of the soil by 95% over the first 10 centimetres. It also increased the useful reservoir by 20% over the top 5 centimetres, water content by 8% and earthworm abundance by 500%. Decompaction reduced conductivity by 5% to a depth of 140 cm. These short-term effects improved the physical and biological quality of the soil, but were not enough to halt the decline.

Keywords : Orchards, Soil quality, Agroecology, Remote sensing, Disease mapping