A survey involving more than 129 farmers was carried out in 1998 and 1999 in Ouémé, Benin, to investigate the importance of pests and diseases as constraints to cowpea (Vigna unguiculata (L.) Walp.) production. Results indicated that in the Ouémé valley Sclerotium rolfsii Sacc. and Aphis craccivora Koch are specific concerns. Weeds such as Justicia anselliana (Nees) T. Anders, Ipomoea aquatica Forsskal and Commelina erecta L. were also reported as being troublesome. On the Ouémé plateau Imperata cylindrica L. Beauv. was reported to be a major weed. Callosobruchus maculatus Fab. and Bruchidius atrolineatus Fab. may cause up to 100% loss within a few months in storage. Birds and rodents were also reported. In certain areas on the plateau, farmers have developed pest control methods based on indigenous knowledge. In the valley the population density of Eichhorniacrassipes (Mart) Solms. during flooding is used by farmers to predict aphid infestation. A number of plant species used to protect cowpea were reported. In Gbékandji village, natural enemies such as Rhabdepyris sp., Evania sp., and Chelonus sp., were observed. They were rare in the valley, where farmers rely on chemical control.

Thirty-seven cassava genotypes from Benin, including advanced breeding lines, were tested for their reaction to bacterial blight in the forest-savanna transition, wet savanna and dry savanna zones of Benin. Sixteen genotypes were repeated in 12 environments. In year 1998, genotypes RB92164, RB92022, TMS30572, BEN86004, RB92033 and Dangbo2, and in year 2000, genotypes RB92202, RB92151, RB92132 and TMS30572 were resistant in one ecozone. Among the more resistant genotypes, CAP94030, BEN86040, RB89509, RB92132 and TMS30572 showed low interaction across environments and were most stable in disease reaction. Ten genotypes were classified as high yielding across environments. Among the more resistant group of genotypes, only TMS30572 and RB89509 were high yielding, with RB89509 being unstable in yield across environments. Selection of genotypes proved reliable only after artificial inoculation. Comparing environments, artificially inoculated treatments in the wet savanna zone and in the forest-savanna transition zone with stable high symptom severity proved most suitable for screening of genotypes, while the wet savanna zone with low natural infection in year 1998 was suitable for production of propagation material, and the site in the dry savanna zone with natural infection in year 1998 was the best environment for cassava production. The correlation between disease severity and root yield was significant only for the non-inoculated treatment in the dry savanna zone in year 2000 (R ¼ )0.58), but not in any other environment. Among the 37 genotypes tested, several genotypes can be recommended to farmers in specific ecozones, and genotype TMS30572 revealed as relatively stable in disease resistance and in high yield across ecozones.

No abstract

To clarify the diversity of blast (Pyricularia oryzae Cavara) races in West Africa, we investigated the pathogenicity of 96 blast isolates collected from different ecosystems in six countries, Bénin, Burkina Faso, Côte-d'Ivoire, Ghana, Mali, and Nigeria, and characterized using rice (Oryza sativa L.) differential varieties for 23 resistance genes and a susceptible control variety Lijangxintuanheigu (LTH). Virulent blast isolates occurred with high frequencies against LTH or differential varieties carrying Pia, Pi19(t), Pi12(t), Pit, Pii, Pi3 and Pi5(t). Conversely, they occurred at low frequencies against differential varieties carrying Pish, Pi9(t), Piz, Piz-5, Piz-t and Pita-2 and at intermediate frequencies against those carrying Pib, Pi1, Pik, Pita and Pi20(t). The isolates were recharacterized as reaction types of five groups, U, i, k, z and ta; accordingly, LTH and the differential varieties were categorized based on the chromosome locations of the resistance genes harbored in each genetic background. Twelve, seven, thirteen, eight and seventeen reaction types were found in groups U, i, k, z and ta, respectively. Thirteen of these, namely U43, U63, i7, k100, k106, k177, z00, z03, z04, ta003, ta031, ta403 and ta431, showed high frequencies of blast isolates and were considered dominant reaction types. We used the infection types of the differential varieties against these blast isolates to conduct a cluster analysis, and the isolates were classified into two clusters, I and II. Substantial differences in frequencies between both clusters were found in the reactions of differential varieties carrying these genes in the Pii, Pik and Pita chromosome regions. Both clusters I and II were distributed in an upland ecosystem with high and similar frequencies, whereas group II was mainly distributed in the irrigated lowland. These results suggested that many types of blast races were distributed in the upland ecosystem, while the limited ones were in lowland in West Africa. Finally, these blast isolates were categorized into 79 races in accordance with a new designation system based on the five differential variety groups used for reaction typing.

The severity of cassava bacterial blight at two sites in the forest-savanna transition (FST) and dry savanna (DS) zones of West Africa were studied by assessing the effects of: (i) shift of planting date; (ii) potassium fertilizer application and mulching; (iii) intercropping cassava with sorghum or cowpea vs. cassava monoculture; and (iv) the combination of these measures. Disease severity of bacterial blight in two genotypes was generally reduced by 20 -60% by late planting, without a negative effect on cassava root yield, in monocropping systems in most treatments in the FST zone (reduction in four treatments, and increase in two treatments, out of 19) and the DS zone (two of eight treatments in 1 year). Late planting led to crop failure in the DS zone in the second year. Intercropping cassava with sorghum reduced bacterial blight severity significantly, up to 80% at normal (all treatments) and late planting time (three out of six treatments) in the FST zone, and in some treatments (four out of seven) at normal planting in the DS zone. Intercropping of cassava with cowpea in the DS zone also reduced disease severity. Cassava-sorghum intercropping generally had no effect on root yield compared with cassava monocropping at both planting times in the FST zone and provided an additional harvest of the intercrop, while yield was affected by intercropping in the DS zone at late, and in some treatments (three out of seven) at normal, planting time. Mulching and potassium treatment had no effect on disease severity, but increased or decreased root yield in some treatments in both sites. Analysis of combined data showed that cropping system, year, site, and site combined with planting date were the highest significant determinants of variation in bacterial blight development.