Arbuscular mycorrhizal fungi in the Sub-Saharan Savannas of Benin an their association with yam (Dioscorea spp.) : potential of yam growth promotion and reduction of nematode infestation

The arbuscular mycorrhiza is the most widely occurring and important microbial symbiosis for agricultural crops and well known to facilitate plant mineral nutrient uptake, particularly under conditions of P-limitation - as it is common in tropical soils due to leaching or/and severe immobilization - and, moreover, it is understood to improve plant water relations and provide resistance against pests and pathogens. Yam (Dioscorea spp.) is the most important tuber crop in terms of coverage area in West Africa, particularly in Benin and Togo. Alarmingly, the annual yam production per hectare has recently decreased considerably due to a loss of soil fertility and pest and disease (especially nematode) damage. Under field conditions, yam and arbuscular mycorrhizal fungi (AMF) are naturally associated with each other. Currently, however, data on the ecology of AMF in West Africa are lacking with very limited information on the mycorrhizal status of yam. There may be potential to improve growth and to protect plants against nematodes by AMF but this is possibly dependent on specific AMF-nematode-host combinations. The present project aimed at studying AMF indigenous to West Africa, with respect to yam growth promotion and yam nematode suppression. The specific objectives focused on assessing the (1) diversity of AMF, including their distribution, abundance and relation to agronomic practices and ecological conditions; (2) the mycorrhizal status of yam; and (3) the specific associations between (a) yam-AMF, and (b) yam-AMF -nematode in vivo under greenhouse conditions. In the first part of our study, we determined the influence of three ecological zones (from wettest to driest) and of land use intensity on the diversity of AMF in the yam growing area of Benin, West Africa. In each zone, four ‘natural’ and four ‘cultivated’ sites were selected. ‘Natural’ sites included three natural forest savannas (at least 25-30 years old) and a long-term fallow (6-7 years old). ‘Cultivated’ sites comprised yam fields established immediately following forest clearance, mixed cropping maize (Zea mays) and peanut (Arachis hypogaea) fields, peanut fields, and fields under cotton (Gossypium hirsutum), which was the most intensively managed crop. Soil samples were collected towards the end of the wet season in each zone. AMF spores were extracted and identified morphologically. A total of 60 AMF species was detected, with only seven species sporulating in AMF trap cultures that were set up with various AMF host plants. Higher species richness was observed in the northern most, driest ecological zone Sudan Savanna (SU) than in the adjacent zones to the south with increasing humidity, namely the Northern Guinea Savanna (NG) and the Southern Guinea Savanna (SG), mainly due to a high proportion of species in the Gigasporaceae, Acaulosporaceae and Glomeraceae. Within each ecological zone, spore density and species richness were generally higher in the natural savannas and in association with yam than in the other cultivated sites. These parameters were lowest under the intensively managed cotton, and intermediate in the fallows, indicating that the high richness of the natural savannas is not necessarily restored during fallowing. Assuming that yam is an arbuscular mycorrhizal crop, we addressed the question of which AMF species are associated with yam. Our aim was to propagate the AMF communities from three natural forests and three adjacent yam fields of the SG in Benin in trap cultures and to assess the AMF richness, identifying those associated with yam. Soil samples were collected in the dry season (February 2005) and used to identify AMF spores directly and also to establish AMF trap cultures on yam (tissue culture plantlets of D. rotundata and D. cayenensis) and, for comparison, on Sorghum bicolor. In the trap cultures, AMF root colonization was particularly high in yam (70-95%), compared with S. bicolor (11-20%). Based on spore morphotyping, 37 AMF species were detected in the ‘trap’ rhizosphere of S. bicolor, while 28 and 29 species were identified as fungal symbionts of D. cayenensis and D. rotundata, respectively. Following eight months cultivation in trap cultures, yam tuber dry weight was generally higher in mycorrhizal than in non-mycorrhizal control pots. We also hypothesized that indigenous AMF species and strains isolated from yam plantlets in trap cultures may be more beneficial for yam plant growth compared to non-indigenous isolates. We screened indigenous AMF species and strains that have been isolated from the trap cultures and compared their effects on micro-propagated white yam plantlets (D. rotundata) (cv. TDr89- 02461) against exotic AMF isolates in pot experiments over seven months. First, we tested several indigenous and non-indigenous (South America and Asia) G. etunicatum strains with regard to their effect on yam growth promotion and mineral accumulation in the tissues. Secondly, three isolates each of nine indigenous AMF species and three additional non-tropical AMF species were screened on the same yam cultivars. We found that most tropical AMF isolates of G. etunicatum increased yam tuber dry weight, while the non-tropical AMF isolates had a lower or no effect, but instead increased tuber P concentrations, when compared to nonmycorrhizal controls. Glomus mosseae, G. hoi, G. etunicatum, Acaulospora scrobiculata and A. spinosa generally had a positive effect on tuber growth, while isolates of G. sinuosum and Kuklospora kentinensis generally did not. Finally, we assessed the interaction between yam and AMF in the presence or absence of plant parasitic nematodes. Yam vplants cultivated in vitro were used, which were inoculated with commercial inocula of G. mosseae and G. dussii (Biorize, Dijon France). In the presence of nematodes (Scutellonema bradys and Meloidogyne spp.), inoculation of G. mosseae generally increased growth of micropropagated yam plantlets and yam tuber weight production, especially cultivars from D. alata. Tubers were, in general, less infected with S. bradys, but not necessarily with Meloidogyne spp. However, application of G. mosseae and G. dussii to micropropagated plantlets resulted in improved quality of yam tubers, when challenged with nematodes, compared to nematode inoculation without AMF, indicating a positive effect of AMF on yam productivity. Our results indicate that the AMF richness is high in the ‘yam belt’ of Benin, but that it is strongly influenced by the ecological zone and by the intensity of land cultivation after forest clearance. Our results also indicate that in controlled pot studies, AMF can suppress nematode damage and additionally lead to improved quality and weight of yam tubers. The present results remain preliminary, however, while results from ongoing studies currently in the field will help to determine further their potential in the longer term. These results provide exciting prospects for African crop production, in addition to illuminating the wide and diverse species richness of West African AMF and their potential benefits.