Stratégie de prospection hydrogéologique du socle de la bordure orientale tchadienne par optimisation du nombre et de la profondeur des sondages de reconnaissance

Sous le climat à faible pluviosité du Tchad, les altérites sont dénoyées et seul le socle fracturé est aquifère. Le taux d'échec des forages atteint 60% car les fractures ont une répartition très discontinue comme le montre leur organisation fractale. Cela entraîne la coexistence de secteurs productifs et stériles. A l'échelle kilométrique, on peut ajuster le nombre de sondages de reconnaissance par village en fonction des caractéristiques climatiques, topographiques et géologiques. On définit ainsi des zones de productivité forte, où le taux de succès atteint 79%, moyenne et faible. On peut alors affecter chaque village d'un " potentiel d'investigation " qui est le produit du nombre de sondages par leur profondeur prévisionnelle.A l'échelle locale, une analyse en composantes principales des paramètres de forage montre que la présence d'eau souterraine est liée aux caractéristiques du socle fracturé et non altéré. Une analyse discriminante fournit une " équation de productivité " qui permet de prévoir 90% des résultats en cours de foration: dès que le forage a traversé une dizaine de mètres de socle non altéré, elle permet de définir une profondeur limite d'investigation dépendant des caractéristiques intrinsèques de chaque site. Elle est surtout applicable dans les zones les moins productives où l'on observe systématiquement un surcreusement inutile des forages négatifs.On dispose ainsi d'une stratégie de prospection alliant le nombre et la profondeur des forages. Elle permet de limiter la profondeur des forages implantés sur des sites peu productifs et de reporter le métré ainsi récupéré sur des sites plus prometteurs.The aim of this study is to define a new strategy for groundwater prospection in sahelian basement aquifers. At present, the number and the depth of boreholes are fixed a priori in the project document: these parameters are the same for all the villages, regardless of their environmental context. In fact, during the drilling campaign, we systematically observe a useless overdrilling of negative boreholes that affects the cumulative drilled length of the project (Table 1). This is particularly important in granitic basement areas under the low rainfall sahelian climate: water is difficult to find because of low success rates, and the driller needs to ensure no groundwater indication appears a few meters under the fatal 60 m depth. An illustration of this methodology is proposed for the Guéra, Ouaddaï and Biltine provinces of eastern Chad (Figure 1). This 150 000 km2 area is situated at the border of the Chadian basin from 10 to 15 ° north latitude at elevations of 400-700 m, with an annual rainfall between 200 and 600 mm. The geology is represented by precambrian granitoïds. Tectonics are well developed with many fractures, faults and photolineations from metric to multi-kilometric scales.In Chad, weak recharge rates imply that the weathered rock reservoir is unsaturated and the aquifer is constituted by the fractured granitic basement. Thus, the overall success rate of 500 boreholes is only 42%. The unequal distribution of fractures leads to the presence of productive and barren adjacent areas with significantly different success rates. A statistical analysis of photolineations shows their fractal distribution with a dimension around 1.57, similar to the 1.59 dimension obtained in fractal fracture models (Figure 2). Fracturation is a main component of hydrogeological knowledge in basement areas and its variations between the villages can explain the different potentials of basement productivity: we must consequently adjust an "investigation potential" depending of the characteristics of each area. The proposed strategy of prospection determines the number of boreholes to drill and their specific depth.At the kilometer scale, the total number of boreholes can be adjusted according to climatic, topographical and geological characteristics (Table 2). We show that only four parameters can explain a range of success rate from 0 to 79% in different villages (Table 3): altitude, average rainfall, petrography and fracturation intensity (measured in situ). Thus we can define the investigation potential which is the previous depth divided by the theoretical success rate of the area including the village. It is interesting to notice that the success rate in the high productivity class is similar to the average value obtained in more rainy basement countries of West Africa: for example 79% in south-west of Burkina Faso or 73% in Togo.At the local scale, a principal components analysis on 12 drilling parameters was performed. It shows that the appearance of groundwater is mainly correlated to parameters describing the unweathered fractured rocks (Figure 3). A discriminant analysis was then performed on four of these parameters: thickness of unweathered drilled basement, depth of first water arrival, number of water arrivals and hammer velocity in the unweathered basement. This yields a "productivity equation" which allows one to anticipate 90% of the borehole results (Table 4). According to this equation, we can define a maximum investigation depth based on the geological characteristics of each borehole site. The last section presents the complete strategy of groundwater basement prospection and two examples applied to Chad. For an average aquifer depth of 60 m, the investigation potential of each village depends on its productivity class: it varies theoretically from 10 boreholes (i.e. 600 m) in low productivity area to 1.3 boreholes (i.e. 76 m) in high productivity zones (Table 5). This potential must then be distributed among the different sites according to the result of their productivity equations.The village of Getgéré is situated in a particularly unproductive zone (see Table 3) where about ten boreholes are statistically needed to obtain a positive result: its investigation potential is supposed to be 300 m. Four negative boreholes were drilled from 62 to 75 m with a total depth of 261 m. In fact, the productivity equation showed all these sites were unproductive from drilled depths of 28 to 40 m deep (Table 6): the same result could have been obtained with only 130 m drilled; 131 m were uselessly consumed. With this excess drilled length, we could have drilled eight extra shallower boreholes and increased the probability of success in obtaining a productive well.The village of Eroua is situated in a productive area where the success rate is 79%: its investigation potential is 60 / 0.79=76 m. The first borehole was negative at depth of 74 m, but the productivity equation already indicated this result after only 38 m of drilling. At the second site, a positive borehole was obtained at 41 m depth where the equation foresaw 43 m. Finally, the cumulative drilled length was 115 m and the investigation strategy would have permitted the transfer of 34 m to another more promising site

The Amateur Sky Survey Mark III Project

The Amateur Sky Survey (TASS) is a loose confederation of amateur and professional astronomers. We describe the design and construction of our Mark III system, a set of wide-field drift-scan CCD cameras which monitor the celestial equator down to thirteenth magnitude in several passbands. We explain the methods by which images are gathered, processed, and reduced into lists of stellar positions and magnitudes. Over the period October, 1996, to November, 1998, we compiled a large database of photometric measurements. One of our results is the "tenxcat" catalog, which contains measurements on the standard Johnson-Cousins system for 367,241 stars; it contains links to the light curves of these stars as well.Comment: 20 pages, including 4 figures; additional JPEG files for Figures 1, 2. Submitted to PAS

Contrôle du Phomopsis, maladie émergente, en culture de carotte porte-graîne

The first triangular browning umbels caused by Diaporthe angelicae on carrot seed crops were observed in France in 2007. This fungal phytopathogen negatively impacted carrot seed production. Little is known about this fungus. In order to control it in carrot seed production, a better understanding of its life cycle and its epidemiology is required. So, this is the reason why a three-years research project named DIAPOCAR was initiated in 2012. It appears that flower is the main sensitive organ and flowering the most sensitive stage of the umbel. In the field, contaminations are particularly observed during flowering of the umbels of order I and II if the weather is favourable to disease. Field fungicide trials show significant efficacy differences between fungicides and only three of them are efficient against carrot phomopsis. Finally, model simulations with the sunflower phomopsis model are promising and this tool to aid decision-making can be used to apply fungicides only when the weather is favourable to the disease. It should help carrot seed growers to determine the best timing for fungicide applications. Thus, a greater understanding of the biology of this fungus has led to a better control of phomopsis on carrot seed crops

Imaging features of retroperitoneal extra-adrenal paragangliomas in 10 dogs

Retroperitoneal paragangliomas are rare tumors of the neuroendocrine system. Only a few canine case reports are available with rare descriptions of their imaging features. The objectives of this multi-center, retrospective case series study were to describe the diagnostic imaging features of confirmed retroperitoneal paragangliomas and specify their location. Medical records and imaging studies of 10 affected dogs with cytological or histopathologic results concordant with retroperitoneal paragangliomas were evaluated. Dogs had a median age of 9 years. Four of them had clinical signs and laboratory reports compatible with excessive production of catecholamines. Six ultrasound, four CT, four radiographic, and one MRI studies were included. The paragangliomas did not have a specific location along the aorta. They were of various sizes (median 33 mm, range: 9–85 mm of length). Masses had heterogeneous parenchyma in six of 10 dogs, regardless of the imaging modality. Strong contrast enhancement was found in all CT studies. Encircling of at least one vessel was detected in six of 10 masses, clear invasion of a vessel was identified in one of 10 masses. In five of 10 cases, the masses were initially misconstrued as lymph nodes by the on-site radiologist. Retroperitoneal paragangliomas appear along the abdominal aorta, often presenting heterogeneous parenchyma, possibly affecting the local vasculature, and displaying strong contrast enhancement on CT. Clinical signs can be secondary to mass effects or excessive catecholamine production. Underdiagnosis and misdiagnosis of this tumor are suspected as they can be silent, of small size, or confused with other structures

First Report of Root and Collar Rot Caused by Fusarium tricinctum and Fusarium avenaceum on Carrot in France

In 2017, carrot (Daucus carota L.) seed production represented around 22% of the area devoted to the production of vegetable fine seeds. Since 2015, symptoms of root and collar rot have been observed in carrot seed parcels located in the Central Region, one of the most important production zone in France. Diseased plants became dried prematurely, compromising seed development. Depending on the year and the climatic conditions, the disease in a same field can be considered as epidemic (rate losses between 30 to 100% of plants in 2016) or can impact plants more sporadically (less than 10% in 2017 and 2018). Sixteen diseased carrot samples (Nantaise type) were collected from five fields of seed production in the Central Region: two fields in 2016 and 2017, one field in 2018. Seven fungal isolates, obtained from lesions, were grown on Potato Dextrose Agar (PDA) medium and incubated for one week at 20°C in darkness. From the colony top, fluffy mycelium pigmented in pink, red, purple or orange was observed, with a red color at the reverse. To induce sporulation, isolates were grown on Synthetischer Nährstoffarmer Agar (SNA) medium during three weeks at 24°C in near-UV radiations under a 12h-photoperiod. Four isolates (FT001, FT003, FT007, FT017) developed orange sporodochia with lunar or crescent-shaped macroconidia (40.3 ± 0.8 × 5.9 ± 0.1 µm; n=90) and lime or pear-shaped microconidia (10.7 ± 0.2 × 7.7 ± 0.2 µm; n=60), as described in Fusarium tricinctum (Leslie and Summerell 2006). Three isolates (FA001, FA002, FA006) developed orange sporodochia with sickle-shaped macroconidia (50.5 ± 1.1 × 5.0 ± 0.1 µm; n= 60), but no microconidia, as observed in Fusarium avenaceum (Leslie and Summerell 2006). To confirm the identification, DNA was extracted from the mycelium of the seven isolates and molecular markers (ATP citrate lyase, ACL1; RNA polymerase II, RPB2) were used for PCR amplification (Gräfenhan et al. 2011; O’Donnell et al. 2013). The ACL1 sequences from the seven field isolates (GenBank Accession numbers MK183788-MK183791; MK181528-MK181530) were 99-100% identical with the ACL1 sequence of a reference F. tricinctum isolate (query coverages 99-100%; E-values of 0.0) and a reference F. avenaceum isolate (query coverages 98-99%; E-values of 0.0) [respectively DAOM 235630 isolate, GenBank Acc. No. JX397813 and BBA64135 isolate, GenBank Acc. No. JX397768, Niessen et al. 2012]. Using RPB2, sequences from field isolates (GenBank Acc. No. MK183109-MK183115) were 98.5-99.9% identical with the RPB2 sequence of a reference F. tricinctum isolate (query coverages 96-100%; E-values of 0.0) and a reference F. avenaceum isolate (query coverages 95-100%; E-values of 0.0) [respectively MRC 1895 isolate, GenBank Acc. No. MH582113 and MRC 1413 isolate, GenBank Acc. No. MH582082, O’Donnell et al. 2018]. To confirm pathogenicity, FT001 and FA002 were inoculated on collars of 10-weeks old carrot plants in the greenhouse. Forty plants per isolate and 40 control plants were used. Ten microliters of a conidial suspension (105 conidia.mL-1) - or sterile water for the controls - were deposited at the collar, previously wounded using a scalpel blade. Necrotic lesions developed at 20 dpi (FT001) and at 30 dpi (FA002). Fusarium tricinctum and F. avenaceum were re-isolated from the lesions and identified by sequencing using ACL1 and RPB2 markers. No isolation of Fusarium was obtained from the controls. To our knowledge, this is the first report of F. tricinctum and F. avenaceum in carrot in France

Umbel browning and stem necrosis caused by Diaporthe angelicae / Phomopsis dauci on carrot in France

International audienc

Umbel browning and stem necrosis on carrot in France: isolation and characterization of the fungal pathogen

International audienc

Umbel browning and stem necrosis on carrot seed crop in France: characterization of the fungal pathogen, seed testing and control methods

International audienc

Characterization of fungal pathogens (Diaporthe angelicae and D. eres) responsible for umbel browning and stem necrosis on carrot in France

A collection of 102 Diaporthe isolates was compiled from lesions on carrot, parsley and wild Apiaceae species in France from 2010 to 2014. Molecular typing based on ITS rDNA sequences resulted in the identification of 85 D. angelicae and 17 D. eres isolates. Based on sequences of the 30 part of the IGS rDNA, intraspecific variability was analysed for 17 D. angelicae and 13 D. eres isolates from diverse plant species, locations in France, and plant tissues. The genetic diversity was greater for D. angelicae isolates than D. eres isolates. In vitro sensitivity of five D. angelicae and four D. eres isolates to each of nine fungicides was similar for isolates of both species, with a marked variation in fungicide sensitivity depending on the active ingredient. To assess the pathogenicity of D. angelicae and D. eres isolates on carrot, one isolate of each species was inoculated onto umbels in a controlled environment. Typical lesions were observed for both isolates. Carrot crop debris collected from a seed production field in France and placed in controlled conditions produced perithecia and ascospores typical of Diaporthe, that were further characterized molecularly as belonging to D. angelicae. Detection of Diaporthe species on seed lots from three carrot production fields in France was investigated. Both species were detected on seeds by conventional PCR assay, with a greater frequency for D. angelicae than D. eres (67% vs 33%, respectively). Overall, the results highlighted that umbel browning in carrot seed crops in France was mainly caused by D. angelicae