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Réguler les maladies d’origine tellurique par une culture intermédiaire de Brassicacées : mécanismes d’action et conditions d’expression dans une rotation betterave-blé

Abstract : Biofumigation, a technique aiming at controlling soil-borne plant pathogens, relies on the toxicity of crop residues (notably Brassicaceous crop residues that release isothiocyanates). The insertion of a biofumigant cover crop during the intercrop period is considered to be a contribution to possible alternatives of the use of pesticides. However, field studies have generated conflicting assessments of the efficiency of biofumigation at the field scale. We hypothesized that the factors responsible for the variable efficacy may take place at two scales: (i) the biofumigant crop scale (during the period of biofumigant crop growth and the period when crop residues are pulverised and incorporated into the soil); and, (ii) the pathogen scale, considering its ecological and epidemiological characteristics (survival of primary inoculum and transmission of primary and secondary infections). Within this framework, the general objective of this thesis is to analyse the mode of action of a brown mustard (B. juncea) cover crop according to the different phases of crop management (residue incorporation and/or crop growth) on the different components of the epidemic (primary inoculum and apparent transmission rate of the epidemic) of Rhizoctonia solani on sugar beet, and to investigate the possible factors responsible for the variable efficacy of a biofumigant crop at the field scale to infer the effectiveness of biofumigation efficacy. With an epidemiological approach, this work combines: (i) controlled condition experiments to better understand the underlying biological mechanisms; (ii) field experiments to investigate the factors which affect the biofumigation efficacy and the impact this has on the epidemic of R. solani on sugar beet; and, (iii) modelling to analyse the field disease progress curves in order to deduce the epidemiological parameters that are affected by the biofumigant crop. The field study showed that growing the biofumigant crop significantly reduced disease incidence compared to the control field (without mustard). Incorporating crop residues further reduced the disease incidence and additionally reduced the conditional disease severity (severity calculated for diseased beets only). Results from modelling showed that the mustard growth phase and incorporation of the residues acted primarily on the transmission of primary infections. It was concluded that the epidemiological mechanisms were affected by the biofumigant crop in different ways during the two phases of interest (the growth phase and incorporation of the residues); the density of primary inoculum can be reduced in both phases but the residues bring further control by reducing pathogen activity. Results from controlled experiments showed that the mustard residues have a more longterm impact on the pathogen than the initial release of toxic compounds. It was further concluded that biological regulation of pathogens may be very important in the longer term control of disease. In another way, efficacy of growing mustard with respect to the control showed more variability over the 3 years of the study than mustard efficacy after incorporation of the residues which in this study showed little variability from one year to the next. The length of the mustard growth period and mustard biomass seemed to be the most relevant factors that can explain variability of efficacy of growing mustard. However, incorporation of the residues may regulate this variability, regardless of the factors commonly cited in literature (crop biomass, size of crushed residues and timing of irrigation linked to residue maceration). Until now, little effort has been directed towards understanding the differential mode of action of a biofumigant crop on soil-borne pathogens, according to the phases of crop management and the epidemiological mechanisms which are affected by biofumigation. Targeting these two areas offers an innovate tool to reduce the variability of biofumigation in cropping systems.
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Natacha Motisi. Réguler les maladies d’origine tellurique par une culture intermédiaire de Brassicacées : mécanismes d’action et conditions d’expression dans une rotation betterave-blé. Sciences du Vivant [q-bio]. AGROCAMPUS OUEST; Université européenne de Bretagne, 2009. Français. ⟨tel-02466694⟩

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