Effects and modes of action of COS-OGA based elicitors against late and early blight on Solanaceae

The development of alternatives to the use of chemical pesticides constitutes the current challenge of the control of plant diseases. The stimulation of plant immune system using plant defense elicitor constitutes an interesting approach. COS-OGA is a novel plant defense elicitor combining fungal-derived chitooligosaccharides (COS) and plant cell wall-derived oligogalacturonides (OGA). A first formulation of this product, FytoSave®, is now registered in Europe to control powdery mildew in grapevine and cucurbit. Another composition of COS-OGA, FytoSol, is under development by Fytofend SA to enlarge the action range of this elicitor. This thesis aims at studying the potential use of the elicitor COS-OGA to protect Solanaceae against Phytophthora infestans and Alternaria solani. Bioassays were performed to evaluate the efficacy of two formulations of the elicitor, FytoSave® and FytoSol, and to understand their modes of action. Preventive sprayings of both formulations were shown to reduce early and late blight of potato and tomato. However, the levels of protection differed according to the formulation of the elicitor and the plant-pathogen interactions, in particular the types (species and strain) of pathogens and their lifestyle, the genotypes of plant species and the environmental conditions. FytoSol is more efficient against the hemibiotrophic and necrotrophic pathogens than FytoSave®. The modes of action of both formulations were investigated by quantification of defense gene expression and enzymatic activities. Our work confirms that FytoSave® activate a defense response through the salicylic pathway in tomato in interaction with A. solani and in potato in interaction with P. infestans. The protective effect of FytoSol was higher but, despite a significant augmentation of endogenous SA in tomato treated leaves, the molecular pathways induced by this elicitor is yet not so clear. The results of this thesis demonstrated that plant defense elicitors interact differently with plant depending on the plant-pathogen interactions. Further studies using whole transcriptome shotgun sequencing and proteomics are required to better understand the modes of action of plant defense elicitors. These information are important to optimize protective effect of elicitors in order to reinforce the interest to their use for the integrated control and encourage their practical use in agriculture.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201

Effect of organic amendments on the health of rice and peanut crops in coastal sandy soils of Thua Thien Hue province, Central Vietnam.

The purpose of this research was to assess the impact of vermicompost on the growth of rice and peanut crops and on the occurrence of pathogens in sandy soils of the Thua Thien Hue province. The field experiments showed that locally produced vermicompost can improve rice and peanut production on sandy soils with low fertility. THe rice yield significantly increased with an organic amendment of 15 tons/ha. The height and the number of flowers on peanut plants greatly improved with 12 tons/ha. The application of 8 tons/ha improved the number of stems per plant, but reduced their respective heights. THe incidence of major diseases caused by Cercospora arachidicola, Sclerotium rolfsii and Aspergillus niger on peanut and by Pyricularia oryzae, Helminthosporium oryzae and Rhizoctonia solani on rice was assessed as a function of organic fertilization rate. Some relationship between the rate of fertilizer and the incidence of some diseases was observed, but no significant difference was obtained. The cold weather at the beginning of 2008 delayed plant growth and probably also reduced disease incidence, which seemed to be low that year. Other pathogens such as Leptosphaerulina crassica, Cercosporidium personatum and Fusarium ssp. on peanut and Rhynchosporium oryzae , Curvularia luntana and Cercospora oryzae on rice were also identified in the experimental fields. A long-term analysis conducted over several years would enable an assessment to be made of the real impact of organic amendments on root and foliar fungal diseases

The novel elicitor COS-OGA enhances potato resistance to late blight

Phytophthora infestans is the causal agent of potato late blight. This pathogen is usually controlled by fungicides, but new European regulations have imposed changes in crop protection management that have led to a search for alternative control measures. The induction of plant defence responses by elicitors is a promising new strategy compatible with sustainable agriculture. This study investigated the effect of eliciting a defence response in potato against P. infestans using a formulation of the COS-OGA elicitor that combines cationic chitosan oligomers (COS) and anionic pectin oligomers (OGA). Trials were conducted under greenhouse conditions to assess the ability of COS-OGA to control P. infestans. The results showed that three foliar applications with this elicitor significantly increased potato protection against late blight in controlled conditions. The activation of potato defence genes was also evaluated by RT-qPCR during these trials. Two pathogenesis-related proteins, basic PR-1 and acidic PR-2, were rapidly and significantly up-regulated by the elicitor treatment. Therefore, these results suggest that the COS-OGA elicitor increases the protection of potato against P. infestans and that this protection could be explained by an increase in the expression of potato defence genes rather than by biocide activity

Multiplex qPCR assay for simultaneous quantification of CYP51‐S524T and SdhC‐H152R substitutions in European populations of Zymoseptoria tritici

Demethylation inhibitor (DMI) and succinate dehydrogenase inhibitor (SDHI) fungicides are currently relied upon for the control of septoria tritici blotch (STB) in European wheat fields. However, multiple mutations have occurred over time in the genes encoding the targeted proteins that have led to a practical loss of fungicide efficacies. Among the different amino acid substitutions in Zymoseptoria tritici associated with resistance to these fungicides, S524T in CYP51 (DMI target) and H152R in SdhC (SDHI target) are regarded as conferring the highest resistance factors to DMI and SDHI, respectively. To facilitate further studies on the monitoring and selection of these substitutions in Z. tritici populations, a multiplex allele-specific quantitative PCR (qPCR) assay allowing for estimation of both allele frequencies in bulk DNA matrices was developed. The assay was then used on complex DNA samples originating from a spore trap network set up in Belgium, Denmark, Sweden, and Ireland in 2017 and 2018, as well as on leaf samples with symptoms. The S524T allele was present in all field samples and its proportion was significantly higher in Ireland than in Belgium, whereas the proportion of H152R was only sporadically present in both countries. The frequency of S524T varied greatly in the airborne inoculum of all four countries; however, the H152R allele was never detected in the airborne inoculum. The method developed in this study can be readily adopted by other laboratories and used for multiple applications including resistance monitoring in field populations of Z. tritici

Spatio-temporal distribution of DMI and SDHI fungicide resistance of Zymoseptoria tritici throughout Europe based on frequencies of key target-site alterations

BACKGROUND: Over the past decade, demethylation inhibitor (DMI) and succinate dehydrogenase inhibitor (SDHI) fungicides have been extensively used to control to septoria tritici blotch, caused by Zymoseptoria tritici on wheat. This has led to the development and selection of alterations in the target-site enzymes (CYP51 and SDH, respectively). RESULTS: Taking advantage of newly and previously developed qPCR assays, the frequency of key alterations associated with DMI (CYP51-S524T) and SDHI (SDHC-T79N/I, C-N86S and C-H152R) resistance was assessed in Z. tritici-infected wheat leaf samples collected from commercial crops (n = 140) across 14 European countries prior to fungicide application in the spring of 2019. This revealed the presence of a West to East gradient in the frequencies of the most common key alterations conferring azole (S524T) and SDHI resistance (T79N and N86S), with the highest frequencies measured in Ireland and Great Britain. These observations were corroborated by sequencing (CYP51 and SDH subunits) and sensitivity phenotyping (prothioconazole-desthio and fluxapyroxad) of Z. tritici isolates collected from a selection of field samples. Additional sampling made at the end of the 2019 season confirmed the continued increase in frequency of the targeted alterations. Investigations on historical leaf DNA samples originating from different European countries revealed that the frequency of all key alterations (except C-T79I) has been gradually increasing over the past decade