Evaluating field‐scale sampling methods for the estimation of mean plant densities of weeds

The weed flora (comprising seven species) of a field continuously grown with soyabean was simulated for 4 years, using semivariograms established from previous field observations. Various sampling methods were applied and compared for accurately estimating mean plant densities, for differing weed species and years. The tested methods were based on (a) random selection wherein samples were chosen either entirely randomly, randomly with at least 10 or 20 m between samples, or randomly after stratifying the field; (b) systematic selection where samples were placed along diagonals or along zig‐zagged lines across the field; (c) predicted Setaria viridis (L.) P. Beauv seedling maps which were used to divide the field into low‐ and high‐density areas and to choose the largest sample proportion in the high‐density area. For each method, sampling was performed with 5–40 samples. Systematic methods generally resulted in the lowest estimation error, followed by the random methods and finally by the predicted‐map methods. In case of species over‐ or under‐represented along the diagonals or the zig‐zag sampling line, the systematic methods performed badly, especially with low sample numbers. In those instances, random methods were best, especially those imposing a minimal distance between samples. Even for S. viridis, the methods based on predicted S. viridis maps were not satisfactory, except with low sample numbers. The relationships between sampling error and species characteristics (mean density, variability, spatial structures) were also studied.     

Immunity to schistosomes: progress toward vaccine

Among the major parasitic infections, schistosomiasis may be the most promising candidate for human vaccination. Information about mechanisms of immunity, gained mainly from experimental models but likely to be relevant to human infection, indicates a dynamic balance between protective and regulatory (blocking) mechanisms. Besides cell-mediated responses leading to macrophage activation, antibody-dependent cell-mediated cytotoxicity systems involving precise antibody isotypes and nonlymphoid cells (mononuclear phagocytes, eosinophils, and platelets) appear to be essential effectors of immune attack. The slow development of immunity in humans seems related to the production of antibodies that cross-react with schistosomulum surface antigen and block the binding of antibodies of the effector isotype. Schistosomes that survive in the bloodstream and produce chronic infections may evade the immune system as a result of intrinsic changes in membrane susceptibility and of transient expression of target antigens; at other stages of the parasite life cycle, cross-reactive molecules may be secreted that play an essential role in the induction of immunity. Several schistosome proteins have been characterized as candidates for vaccination. Among these, an antigen of 28 kilodaltons has been cloned and shown to be immunogenic in humans and protective in mice, rats, and baboons.     

Assessment of phylogenetic signal in the germination ability of Phelipanche ramosa on Brassicaceae hosts

Phelipanche ramosa is a generalist parasitic weed known to cause yield losses in various crops, especially in winter oilseed rape, its new preferred host in France. This parasitic plant is also able to complete its life cycle on many Brassicaceae weeds, which are thus important alternative hosts. We studied a set of 14 common Brassicaceae weeds and characterised their ability to induce the germination of three genetically distinct pathovars of P. ramosa, based on in vitro experiments. We then investigated whether phylogenetic relatedness among Brassicaceae weeds could inform on their ability to induce germination of P. ramosa by testing for a phylogenetic signal in the germination rate of the parasite. In the presence of some phylogenetic signal, phylogenetic distances among species might be used as a surrogate to predict the ability to induce germination of potential additional Brassicaceae hosts. The three pathovars studied showed different germination patterns. Moreover, we found substantial variation in the germination rate of P. ramosa among the different Brassicaceae species, with a significant effect of the clustering of species into two ancient phylogenetic lineages. However, no significant phylogenetic signal was detected overall, that is we could not exclude that germination rates were randomly distributed over the phylogeny. We suggest that further analyses should be conducted across wider sets of potential hosts to better characterise the existence of a phylogenetic signal of the ability of weeds to induce the germination of P. ramosa.     

Abundance of Rhamphicarpa fistulosa in rainfed lowland rice fields in the Savannah region of Togo: Moderate influence of physico-chemical characteristics of soils

Rainfed lowland rice fields in northern Togo are increasingly infested by the facultative hemiparasitic weed Rhamphicarpa fistulosa that is widely reported throughout sub-Saharan Africa to be one of the most damaging weeds in rice fields. In this geographical area, some studies have shown that soil characteristics may influence the presence and/or the level of infestation of some parasitic weeds, but none have been conducted on R. fistulosa. Based on surveys conducted in northern Togo over two years in 66 lowland rice fields, we characterised the level of infestation of R. fistulosa as well as physico-chemical characteristics of soils. We highlighted that fields with medium-to-high levels of infestation of R. fistulosa (more than 5 plants per m²) were mainly characterised by a coarser soil texture, a higher potassium content and a higher pH, while fields without R. fistulosa were characterised by a lower proportion of silt content. In contrast, the level of infestation of R. fistulosa appeared only marginally influenced by soil organic carbon and nitrogen in our study. Considering these results, options to modify the main physico-chemical characteristics of soils that influence the level of infestation of R. fistulosa are likely difficult to implement. In order to continue rice production in these lowland fields, we suggest several measures to better manage R. fistulosa seedbank and development.     

Breeding strategy in foxtail millet,Setaria italica (L.P. Beauv.), following interspecific hybridization

Summary An interspecific cross between foxtail millet Setaria italica and its wild relative Setaria viridis was undertaken in order to introduce from the latter triazine resistance in the cultivated plant. Four backcross generations obtained with S. italica as recurrent parent were studied. Results show that only two backcross generations associated with selection are enough to eliminate weedy characters and to return to the cultivated type. S. viridis could thus be an interesting source for improvement of foxtail millet without a complex and time consuming breeding strategy.