Aspects épidémiologique, physiologique et biochimique de l’interaction Coffea arabica L. et Colletotrichum kahawae, agent causal de l’anthracnose des baies

Abstract

The coffee tree (Coffea arabica L.) is cultivated for its seeds which constitute the most traded raw material worldwide alter oil. Arabica coffee is the most popular for its low caffeine content and aroma. Despite its very high production (65 % of world production), the Arabica coffee tree still faces several constraints, including the anthracnose of the berries, which represents the most important in the production areas in Africa. The disease is manifested by the presence of black necrotic spots appearing on the surface of the berries. Nowadays, the biochemical molecules and the likely interactions between these biomolecules involved in the establishment of the disease are not very well understood. The objective of this thesis is to evaluate the effect of different cultural situations of coffee trees and development stages of berries on the evolution of coffee berry disease and on the accumulation of primary and secondary metabolites in developing berries. The experiments were carried out in the experimental plantations, at 1100 altitude meters (Foumbot), 1800 altitude meters (Santa) and in a peasant farm at 1200 altitude meters (Kouoptamo) for two years (2016 and 2017). The Foumbot experimental plantation consisted of the Java (tolerant) and Caturra (susceptible) varieties, while those of Santa and Kouoptamo consisted only of the Java variety. Thirty-two (32) coffee trees of each of the two varieties were used in this study. Half of the coffee trees (16) were under artificial shade tree by tree and the other half was located in full sun. Weekly observations were carried out between the 6th and the 27th week after flowering (WAF) on three branches coffee tree arranged in the upper, middle and lower part of each of the coffee trees studied. The amino acids and secondary metabolites of the berries collected each after two weeks between the 8th and 16th WAF of each variety by crop area were analyzed by liquid chromatography. This same method was used for the analysis of soluble sugars in inoculated berries in vitro collected at the 22nd and 25th WAF under different lighting modes in situ. Moreover, the effect of the fungicides named propiconazol, difeconazol, chlorotalonil/carbendazim and azoxystrobin on the spread of anthracnose in berries was evaluated in a peasant farm in natural conditions in Kouoptamo on 640 coffee trees distributed in an experimental set-up in five completely Randomized blocks. The results showed an infection rate of 0.72 % and 1.44 % for the Java variety; 8.28 and 16.78 for the Caturra variety at 1100 altitude meters under artificial shade and full sunligth, respectively. At 1800 altitude meters where there was only the Java variety, the infection rate was 8.28 % and 36.78 % on trees under artificial shade and those exposed to the full sunlight, respectively. Shading significantly reduce coffee berry disease incidence. Chromatographic analysis identified 14 free amino acids and a non-proteinogenic amino acid, ỿ-aminobutyric acid (GABA) in berries of both varieties. The high content of acidic amino acids was obtained in the Caturra variety from berries taken under artificial shading, asparagine being the dominant amino acid (19.32 ± 2.98 g/kg fat free dry material /ffdm). The lowest content was obtained in the Java variety of berries taken in full sunlight and aspartate as a weakly represented amino acid (0.36 ± 0.04 g/kg ffdm). As regards hydrophobic amino acids, high tyrosine content was observed in the Java variety regardless of the type of shading. In addition, among the other identified amino acids, a high content of GABA has been observed in the Java variety. Unlike all the other amino acids identified in berries, lysine and tyrosine showed high levels between the 8th and 10th WAF. This is the start of the berry expansion phase where the first symptoms of the disease are seen on the berries. At 1800 altitude meters, hydrophobic amino acids such as valine, leucine, isoleucine and phenylalanine showed high levels in the berries of the Java variety. In short, asparagine, aspartate and glutamate are believed to be involved in the metabolism and development of coffee berries. However, phenylalanine, tyrosine, GABA, lysine, arginine and tryptophan are thought to be involved in the berry's response to stress. The same, valine, leucine isoleucine and glycine are thought to be signaling molecules in response to stress. Analysis of secondary metabolites identified chlorogenic acid, epicatechin and caffeine as the major secondary comprounds of developing green berries. High levels were observed in berries of the Caturra variety, from coffee trees exposed to full sunlight, 7.66 ± 2.11 g/kg ffdm of chlorogenic acid and 0.68 ± 0.38 g/kg ffdm of epicatechin. While the lowest levels were obtained in the berries of the Java variety located under artificial shade, 5.56 ± 0.89 g/kg ffdm of chlorogenic acid and 0.12 ± 0.08 g/kg ffdm of epicatechin. Chlorogenic acid and caffeine could be constitutive determinants for the protection of berries against biotic and abiotic stress. Analysis of the sugar contents revealed that glucose is the soluble sugar accumulated in berries after inoculation with the purified isolate of Colletotrichum kahawae. The berries of the full sunlight Java variety exhibited a 4 times higher glucose content than that observed on the Caturra variety under shade. The evaluation of the effect of fungicides on coffee berry disease showed that azoxystrobin has a highly significant inhibitory effect on coffee berry disease. This is the first report of the use of azoxystrobin against coffee berry disease and can open a new perspective in to optimize effective control of disease in Arabica coffee plantations