Rivers and landscape ecology of a plant virus, Rice yellow mottle virus along the Niger Valley.

To investigate the spread of Rice yellow mottle virus (RYMV) along the Niger River, regular sampling of virus isolates was conducted along 500 km of the Niger Valley in the Republic of Niger and was complemented by additional sampling in neighbouring countries in West Africa and Central Africa. The spread of RYMV into and within the Republic of Niger was inferred as a continuous process using a Bayesian statistical framework applied previously to reconstruct its dispersal history in West Africa, East Africa, and Madagascar. The spatial resolution along this section of the Niger River was the highest implemented for RYMV and possibly for any plant virus. We benefited from the results of early field surveys of the disease for the validation of the phylogeographic reconstruction and from the well-documented history of rice cultivation changes along the Niger River for their interpretation. As a prerequisite, the temporal signal of the RYMV data sets was revisited in the light of recent methodological advances. The role of the hydrographic network of the Niger Basin in RYMV spread was examined, and the link between virus population dynamics and the extent of irrigated rice was assessed. RYMV was introduced along the Niger River in the Republic of Niger in the early 1980s from areas to the southwest of the country where rice was increasingly grown. Viral spread was triggered by a major irrigation scheme made of a set of rice perimeters along the river valley. The subsequent spatial and temporal host continuity and the inoculum build-up allowed for a rapid spread of RYMV along the Niger River, upstream and downstream, over hundreds of kilometres, and led to the development of severe epidemics. There was no evidence of long-distance dissemination of the virus through natural water. Floating rice in the main meanders of the Middle Niger did not contribute to virus dispersal from West Africa to Central Africa. RYMV along the Niger River is an insightful example of how agricultural intensification favours pathogen emergence and spread.

The Challenge of Improving Soil Fertility in Yam Cropping Systems of West Africa.

Yam (Dioscorea spp.) is a tuber crop grown for food security, income generation, and traditional medicine. This crop has a high cultural value for some of the groups growing it. Most of the production comes from West Africa where the increased demand has been covered by enlarging cultivated surfaces while the mean yield remained around 10 t tuber ha-1. In West Africa, yam is traditionally cultivated without input as the first crop after a long-term fallow as it is considered to require a high soil fertility. African soils, however, are being more and more degraded. The aims of this review were to show the importance of soil fertility for yam, discuss barriers that might limit the adoption of integrated soil fertility management (ISFM) in yam-based systems in West Africa, present the concept of innovation platforms (IPs) as a tool to foster collaboration between actors for designing innovations in yam-based systems and provide recommendations for future research. This review shows that the development of sustainable, feasible, and acceptable soil management innovations for yam requires research to be conducted in interdisciplinary teams including natural and social sciences and in a transdisciplinary manner involving relevant actors from the problem definition, to the co-design of soil management innovations, the evaluation of research results, their communication and their implementation. Finally, this research should be conducted in diverse biophysical and socio-economic settings to develop generic rules on soil/plant relationships in yam as affected by soil management and on how to adjust the innovation supply to specific contexts.

The adaptation of Rice yellow mottle virus to the eIF(iso)4G-mediated rice resistance.

The rymv1-3 allele of the eIF(iso)4G-mediated resistance to Rice yellow mottle virus (RYMV) is found in a few Oryza glaberrima cultivars. The same resistance-breaking (RB) mutations emerged in the central domain of the VPg after inoculation of isolates of different strains. The RB mutations were fixed, often sequentially, at codons 41 and 52 which paralleled an increase in virus accumulation. RB mutations also emerged after inoculation of an avirulent infectious clone, indicating that they were generated de novo in resistant plants. Only virus isolates with a threonine at codon 49 of the VPg broke rymv1-3 resistance, those with a glutamic acid did not. A small subset of these isolates overcame rymv1-2 resistance, but following a specific pathway. Comparison with the RB process of rymv1-2, a resistance allele found in a few Oryza sativa cultivars, showed similarities in the mode of adaptation but revealed converse virulence specificity of the isolates.

Identification of a second major resistance gene to Rice yellow mottle virus, RYMV2, in the African cultivated rice species, O. glaberrima.

Rice yellow mottle virus (RYMV) is the most damaging rice-infecting virus in Africa. However, few sources of high resistance and only a single major resistance gene, RYMV1, are known to date. We screened a large representative collection of African cultivated rice (Oryza glaberrima) for RYMV resistance. Whereas high resistance is known to be very rare in Asian cultivated rice (Oryza sativa), we identified 29 (8%) highly resistant accessions in O. glaberrima. The MIF4G domain of RYMV1 was sequenced in these accessions. Some accessions possessed the rymv1-3 or rymv1-4 recessive resistance alleles previously described in O. glaberrima Tog5681 and Tog5672, respectively, and a new allele, rymv1-5, was identified, thereby increasing the number of resistance alleles in O. glaberrima to three. In contrast, only a single allele has been reported in O. sativa. Markers specific to the different alleles of the RYMV1 gene were developed for marker-assisted selection of resistant genotypes for disease management. In addition, the presence of the dominant susceptibility allele (Rymv1-1) in 15 resistant accessions suggests that their resistance is under different genetic control. An allelism test involving one of those accessions revealed a second major resistance gene, i.e., RYMV2. The diversity of resistance genes against RYMV in O. glaberrima species is discussed in relation to the diversification of the virus in Africa.

[In vitro activity of different fungicides on the growth in Mycosphaerella fijiensis var. difformis Stover and Dickson, Cladosporium musae Morelet and Deightoniella torulosa (Syd.) Ellis, isolated parasites of the banana phyllosphere in the Ivory Coast]. / Activités in vitro de différents fongicides sur la croissance chez Mycosphaerella fijiensis var. difformis Stover et Dickson, Cladosporium musae Morelet et Deightoniella torulosa (Syd.) Ellis, parasites isolés de la phyllosphère des bananiers en Côte-d'Ivoire.

In Côte-d'Ivoire, banana leaf surfaces are attacked by Mycosphaerella fijiensis var. difformis, Cladosporium musae, and Deigthonielle torulosa. Control is based on fungicides recommanded for Mycosphaerella fijiensis. Fungicides belonging to triazoles and strobilurines types were added, at different concentrations, to the PDA medium, using this PDA medium containing no fungicide as the control. Mycelium disc and spores of Cladosporium musae, Mycosphaerella fijiensis and Deightoniella torulosa were put on the different media. Total inhibition of mycelium growth of every fungus on the PDA amended with propiconazole was observed. The fungicides used show different activities according to their concentration and their mode of action. The application of a fungicide should talke into account the pathogenic fungus involved in the leaf attacked. Against Mycosphaerella fijiensis var. difformis, the Ci(50) are different according to the concentrations (0.44 to 1.06 ppm). Correlations ranked from 0.71 to 0.91 are also different according to fungicide used. The Ci(50) of Cladosporium musae (0.10 to 2.44 ppm) and Deightoniella torulosa (0.26 to 0.52 ppm) are different and their correlations are respectively 0.78 to 0.99 and 0.86 to 0.95. An assessment of the sensitivity of parasitic fungi of banana can be made by mycelium growth measurement of fungi isolated from resistance zones.