Etiologie du syndrome de dessèchement précoce du tournesol : implication de Phoma macdonaldii et interaction avec la conduite de culture

Le dessèchement précoce du tournesol résulte d'une infection de bas de tige par Phoma macdonaldii qui provoque une nécrose encerclante et un rétrécissement du diamètre du collet. Cette attaque entraine une sénescence foliaire brutale occasionnant des pertes dommageables pour la culture. Des contaminations artificielles par des spores ou du mycélium de P. macdonaldii ont permis de reproduire artificiellement des pieds secs et confortent l'implication de ce champignon dans ce syndrome. Dans un contexte où la protection chimique et la résistance variétale sont limitées, la maîtrise de P. macdonaldii par le contrôle cultural pourrait constituer une alternative efficace. Des essais au champ et en serre menés de 2006 à 2009 à l'INRA et au Cetiom ont permis de mettre en évidence l'importance de la conduite de culture du tournesol sur la fréquence et la gravité du dessèchement précoce et de mieux comprendre l'étiologie de ce syndrome. Une alimentation azotée importante, un peuplement dense et à un régime hydrique limitant en post floraison forment trois facteurs particulièrement favorables au développement du syndrome de desséchement précoce attribué à P. macdonaldii. L'alimentation azotée, au travers de la fertilisation, est le principal facteur favorisant la maladie ; le régime hydrique limitant et la densité de peuplement amplifient la maladie. Le faible diamètre de bas de tige (sous de fortes densités) associé à la prolifération du champignon dans le xylème (azote), occasionnerait une réduction de la conductance hydraulique par obstruction des vaisseaux. Amplifiée lorsque la disponibilité en eau du sol est limitante, cet arrêt de la transpiration se traduit par une sénescence anticipée de 40 jours par rapport à la maturité physiologique de peuplement sains. ABSTRACT : Sunflower premature ripening results from an infection by Phoma macdonaldii causing lesions girdling the stem base and narrowing the diameter of the collar. This attack induces a sudden leaf senescence damaging for the culture. Artificial inoculations with spores or mycelium of P. macdonaldii aimed to reproduce the disease and confirm the involvement of this fungus in this syndrome. In a context where chemical protection and plant resistance are limited, control of P. macdonaldii by cultural control could be an effective alternative. Field and greenhouse trials conducted from 2006 to 2009 at INRA and Cetiom showed the importance of sunflower crop management on the frequency and severity of early drying, and better understand the etiology of this syndrome. A significant nitrogen supply, a dense population and a limited water regime postflowering are particularly conducive to premature ripening. Nitrogen supply is the main factor favoring the disease; limited water regime and density amplify the disease. The small stem base diameter (under high densities) associated with the proliferation of the fungus in the xylem (nitrogen), may reduce hydraulic conductance by vessel blockage. Amplified when soil water availability is limiting, this stop of the transpiration leads to early senescence 40 days before physiological maturity of healthy plants

Low plant density can reduce sunflower premature ripening caused by Phoma macdonaldii

In France, premature ripening (PR) is a widespread damage of a fungal disease of sunflower caused by Phoma macdonaldii. Previous results indicated that girdling canker at the stem base, caused by P. macdonaldii, was its primary cause. Previous studies have reported the influence of nitrogen and water supply on the incidence and severity of PR but an additional study was required to analyze the effect of plant density on the level of attack for a more comprehensive cultural control of PR. In a 2-year field study (2008 and 2009) in Toulouse (France), a susceptible cultivar (cv. Heliasol) artificially inoculated at star bud stage with P. macdonaldii was grown at three plant densities (4, 6.5 and 9 plants m−2) factorially combined with three N fertilization rates (0, 50/75 and 150 kg N ha−1) and two water regimes (irrigated and rainfed). P. macdonaldii symptoms were scored weekly to calculate the area under disease progress curve (AUDPC) and percentage of PR plants. Microclimatic conditions were monitored using thermo-hygrometers within the crop. The fraction of photosynthetically active radiation intercepted by the canopy (fPARi) and leaf area index (LAI) were measured at anthesis. Plant water status during the disease progression was characterized by crop simulation (SUNFLO) and N status at anthesis was assessed from shoot N content (Nm) analysis and N Nutrition Index (NNI) calculation. Increasing plant density resulted in a greater proportion of PR plants, and this proportion increased further when N was applied at 150 kg ha−1, the highest rate, and the crop was not irrigated. Despite differing canopy development, differences in microclimatic conditions between density levels were too small to explain the PR differences. However plant N concentration and diameter at stem base were closely related to PR incidence. Thin plants (grown at high density) with non-limiting N supply were the most susceptible to premature ripening. This study opens new avenues for the control of PR through crop management and emphasizes the key role of plant morphology in the development of the disease. Stem base diameter is a morphological trait that could be manipulated through crop management (plant density, N fertilization) and probably breeding in the future when developing integrated disease management systems in sunflower

Nitrogen and water management can limit premature ripening of sunflower induced by Phoma macdonaldii

Premature ripening (PR) is one of the most important diseases of sunflower in France since the 90s. Previous results indicated that girdling canker of the stem base, caused by Phoma macdonaldii was its primary cause but elucidation of critical environmental factors involved is crucial for better control of the disease. A field study was conducted in three contrasting cropping seasons (2006–2008) and investigated the effect of N fertilization (0, 75 and 150 kg N ha−1) and water regime (rainfed, irrigated) on two cultivars with artificial inoculation (AI) and natural infection (NI). Disease assessment was recorded weekly to calculate the area under disease progress curve (AUDPC) and the final percentage of PR plants. Data showed that high levels of N fertilization led to significantly (P < 0.05) more PR than non-fertilization. Water deficit conditions were significantly (P < 0.05) involved in disease severity, and AUDPC and PR were increased when dry conditions were associated with high N supply. This was true for two cultivars which differed in their susceptibility to the disease but cv. Heliasol RM was significantly (P < 0.05) more affected than cv. Melody, partially resistant to PR. Despite contrasting weather patterns, these results demonstrated a clear role of crop management and environmental conditions on the incidence and severity of stem base attacks responsible for the PR syndrome. These findings suggest that sunflower crop husbandry should be adapted to minimize premature ripening induced by P. macdonaldii

PLANT BASED-DIVERSITY PRACTICES IN CONVENTIONAL AND ORGANIC FARMING: A FARMERS’ SURVEY IN FRANCE

Enhancing plant-based diversity within agroecosystems may be an efficient way to achieve the objective of designing more resilient and low-input agricultural systems. The objectives of this study were (i) to describe the plant-based diversity practices implemented in organic or conventional agriculture in France and (ii) to identify the motivations of the farmers to apply such practices. It was also to compare the integration of such practices in organic or conventional farming systems. The survey was carried out in five French regions with 196 farmers that apply at least one plant-based diversification practice. Five diversification practices were studied: diversified crop rotations, use of catch crops, simultaneous intercropping, agroforestry and semi-natural landscape elements. Simultaneous intercropping and agroforestry are less applied compared to catch crops and semi-natural landscape elements because they imply a deeper change in the cropping system. Nevertheless, organic farmers apply more often such practices (simultaneous intercropping and agroforestry) and 40% of them applied more than 4 of these practices at the same time (compared to less than 20% of the conventional farmers). Organic farmers seem thus to be a step further than conventional farmers with regard to the transition towards agroecological systems

Potentiels de régulation biotique par allélopathie et biofumigation ; services et dis-services produits par les cultures intermédiaires multiservices de crucifères

Ce numéro est constitué d’articles issus du colloque CIMS : Des Cultures Intermédiaires Multi-Services pour une production agroécologique performante, organisé le 4 octobre 2017 à Auzeville.Crucifers cover crops have a strong potential for pathogen control (fungi, bacterias, nematodes,weeds, …) thanks to the production of secondary metabolites called glucosinolates. These allelopathiceffects can occur during the crop growing period and can be increased at cover crop termination when alarge quantity of glucosinolates is degraded into the soil (biofumigation principle). Beyond theirpathogen control services these molecules could engender negative effects on beneficials fungi andbacterias. It corresponds to disservices that have to be taken into account according to the specificitiesof the considered agroecosystem. A high variability of allelopathic effects efficiency is sometimesobserved among different studies and can be explained by the influence of both environment andagronomic practices on the glucosinolate production and fate in the soil. Furthermore other factorsinherent of the cover crop use as organic matter incorporation, or host/non-host cover crops effects areinvolved on pest management. Causes of pathogen control (allelopathy or not) are still to be determinefor numerous pathosystems in order to consequently adapt species choices and agronomic practices.Les cultures intermédiaires de crucifères présentent un fort potentiel de gestion des bioagresseurs(champignons, bactéries, nématodes, adventices, …) via la production de métabolites secondaires àeffet biocide, les glucosinolates. Ces effets allélopathiques peuvent avoir lieu lors de la période deculture mais ils sont accentués lors de la destruction des couverts lorsqu’une grande quantité deglucosinolates est dégradée dans le sol (principe de biofumigation). Au-delà des services de régulationdes bioagresseurs ces molécules peuvent engendrer des effets négatifs sur les champignons etbactéries antagonistes, ce qui correspond à des dis-services qu’il convient de prendre en compte enfonction des problématiques du système de culture considéré. Une forte variabilité d’efficacité desprocessus allélopathiques est parfois observée entre les études et s’explique par l’influence nonnégligeable de l’environnement et de la conduite de culture sur la production de glucosinolates et surleur devenir dans le sol. De plus d’autres facteurs inhérents à l’utilisation de cultures intermédiaires sontimpliqués dans la gestion des bioagresseurs comme notamment l’incorporation de matière organique etles effets hôte/non hôte des couverts. Les causes du contrôle des bioagresseurs (allélopathique ounon) restent à déterminer pour de nombreux patho-systèmes dans le but d’adapter en conséquence lechoix des espèces de couverts et les pratiques culturales

Immaturity of the Oculomotor Saccade and Vergence Interaction in Dyslexic Children: Evidence from a Reading and Visual Search Study

Studies comparing binocular eye movements during reading and visual search in dyslexic children are, at our knowledge, inexistent. In the present study we examined ocular motor characteristics in dyslexic children versus two groups of non dyslexic children with chronological/reading age-matched. Binocular eye movements were recorded by an infrared system (mobileEBT®, e(ye)BRAIN) in twelve dyslexic children (mean age 11 years old) and a group of chronological age-matched (N = 9) and reading age-matched (N = 10) non dyslexic children. Two visual tasks were used: text reading and visual search. Independently of the task, the ocular motor behavior in dyslexic children is similar to those reported in reading age-matched non dyslexic children: many and longer fixations as well as poor quality of binocular coordination during and after the saccades. In contrast, chronological age-matched non dyslexic children showed a small number of fixations and short duration of fixations in reading task with respect to visual search task; furthermore their saccades were well yoked in both tasks. The atypical eye movement's patterns observed in dyslexic children suggest a deficiency in the visual attentional processing as well as an immaturity of the ocular motor saccade and vergence systems interaction