Heavy Metal Concentrations in Water Samples from Lake Kivu, Rwanda

Heavy metals are among the pollutants threatening the living organisms including human beings. The presence of heavy metals in water is of great concern due to their toxicity to aquatic organisms, humans and ability to accumulate in food chains. Thus, there is a need to regularly monitor heavy metal levels in aquatic medium. The present study was conducted to assess heavy metal concentrations in Lake Kivu water. Water samples from Lake Kivu were taken from three sites, namely: Rusizi, Karongi and Rubavu. Heavy metals in the samples were analyzed using Atomic Absorption spectro-photometer (SHIMADZU AAS-6800) and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).Copper, lead, cadmium, chromium, manganese, mercury and arsenic concentrations were determined. Copper ranged from 3.24 to 10.01 μg/l, lead varied from 8.81 to 37.44 μg/l, cadmium ranged from 5.01 to 14.01 μg/l,chromium was between 139.5 and 226.6 μg/l, manganese varied from 598.3 to 795.7 μg/l, mercury ranged from 0 to 0.047 μg/l, while arsenic was not detected in any of the analyzed water samples. Except for arsenic, all the other heavy metals exceeded the EPA (US Environment Protection Agency) maximum permissible limit for class III surface water intended for fish consumption and recreation. Thus, there is a need to establish the sources of lake water pollution by heavy metals in order to design further strategies limiting the amounts of heavy metals entering Lake Kivu. Keywords: Cadmium, chromium, copper, lead, manganese, mercur

Comparing effectiveness, cost- and time-efficiency of control options for Xanthomonas wilt of banana under Rwandan agro-ecological conditions

Xanthomonas wilt of banana (XW) is a major disease affecting banana throughout East and Central Africa (ECA). Initial control was through complete diseased mat uprooting (CDMU), which had limited adoption due to high labour demands and cost. Further research demonstrated single diseased stem removal (SDSR) was an effective, less labour intensive and less costly alternative to CDMU. A comparative assessment of the two control practices was needed to foster uptake by policy makers, especially in countries like Rwanda where SDSR had not been tested or scaled up. To test SDSR in Rwanda, a study was conducted in farmers’ fields comparing the effectiveness, labour cost and time demands of CDMU and SDSR. SDSR was equally effective as CDMU for XW control, with initial XW incidences of 3.0 to 9.4% being reduced to <0.5% within 3 months of using either method. The major benefit of SDSR lies in its significantly lower resource requirements. The total time needed for applying SDSR on a single plant was 88% less, averaging 4.3 min (standard deviation, sd = 0.3) compared to 36.5 min (sd = 4.5) for CDMU on a complete mat. Accordingly, the cost of labour was significantly lower for SDSR. The main cost of CDMU (78% of total cost) arose from the need to replant with healthy banana plantlets. The total cost of SDSR (26 Rwandan Francs [Frw], sd = 2) was 96% lower than that for CDMU (619 Frw, sd = 27). Hence, the incentive to use SDSR for managing XW is very high. These findings will boost SDSR adoption by both policy makers and farmers in ECA

Effect of plant parasitic nematodes and arbuscular mycorrhizal fungi on banana (Musa spp.) in the East African Highland cropping systems

Banana (Musa spp.) is an important crop in the East African Highlands (EAH). Although root health and interaction with soil microorganisms are amongst its major yield determinants, they remain poorly investigated in this region. Understanding these relationships may help to improve yield. In this PhD thesis, we investigated the effect of plant parasitic nematodes, in particular, Praty-lenchus goodeyi, and arbuscular mycorrhizal (AM) fungi on the EAH banana. We demonstrated that P. goodeyi was the main species contributing to root necrosis. However, its abundance was positively correlated with high plant density and mulch. Pratylenchus goodeyi had limited impact on yields in flatter fields (crest and valley bottom) where soil conditions were more optimal for root growth, whereas on steep slopes root death was increased and yield was reduced even under moderate pressure from P. goodeyi. A field experiment showed that P. goodeyi had low impact on yield of highland banana (AAA-EA). Hence, the studies conducted at country, regional and field level suggested that root necrosis caused by P. goodeyi has very little impact on banana plant vigour. These results challenge the general perceptions and previous findings on the negative impact of root lesion nematodes on banana performance. Our studies showed that AM fungi were present in all the sites investigated, but their population density, diversity and root colonization highly varied as a function of rainfall, soil texture, P content, and tillage. The AM fungi colonized banana roots up to 120 cm depth, and plants with poor vigour were more dependent on this symbiotic association than the plants with good vigour. Root colonization decreased with soil depth and reduction in soil water content, and higher colonization lead to more vigorous root systems. Inoculation with AM fungi resulted in greatest plant growth in poor soils and varieties with least developed root system. Our findings suggest that AM fungi play an important role in the EAH. Further research should focus on development of appropriate, AM-fungi friendly, crop and soil management practices at farm and watershed scale.(AGRO 3) -- UCL, 200

Relationship between soil properties, crop management, plant growth and vigour, nematode occurrence and root damage in East African Highland banana-cropping systems: a case study in Rwanda

Parasitic nematodes are widespread in Musa cropping systems in African lowlands where they are known to limit crop production. However, their distribution is very poorly known in the large parts of the East African Highland banana ecology. We carried out a survey in 188 fields in Rwanda to assess and understand nematode occurrence and damage under a wide range of agro-ecological conditions. Altitude varied from 900 to 1800 m above sea level and soil types were distinctly different in the five eco-regions sampled and derived from diverse parent materials; i.e., Ruhengeri (Andosol), Gitarama-Butare (Acrisol), Kibungo (Nitisol), Gashonga (Ferralsol) and Bugarama (Fluvisol and Vertisol). Crop management practices, root health parameters and nematode infection in roots were recorded for a single East African highland banana cultivar (Intuntu, AAA-EA). Plant-parasitic nematodes from five genera were identified: Pratylenchus goodeyi, Helicotylenchus multicinctus, Meloidogyne spp., Radopholus similis and Hoplolaimus pararobustus. Pratylenchus goodeyi was the dominant species in all eco-regions except in Bugarama (lowland). Only the presence of P. goodeyi significantly correlated with root necroses. Altitude was strongly correlated with root densities of P. goodeyi and R. similis. A possible negative impact by P. goodeyi on banana yields was masked by the fact that nematode populations were positively correlated with high plant density and/or mulching practices, which led to relatively high plant vigour irrespective of soil type. Therefore, controlled field experiments will be needed to assess whether root necrosis caused by P. goodeyi at high altitude (>1400 m) actually has a detrimental impact on banana yields, similar to that observed for root-lesion nematodes at lower altitudes

Mineral and organic contribution to nutrient pool in black anthropogenic soils from the African Great Lakes Region

In the African Great Lakes Region, low crop yields due to widespread soil infertility involve difficulties to cope with demographic pressure. Improving the productivity and sustainability of the dominant banana-based cropping systems requires a better understanding of nutrient pools. Four sites differing in parent rock (granite, shale and other pelitic rocks) were selected from Rwanda and Burundi. In two or three homesteads per site, soils were sampled as a function of distance to farmer’s house. Two major types of soil properties discriminated the soils: (i) contents of organic matter and major plant nutrients (Ca, Mg, P), both linked to past and current soil management practices, (ii) nature of parent rock, linked to the abundance of iron and the occurrence of muscovite as residual mineral. All soils were deeply and strongly weathered. Typical anthropogenic soils similar to the Amazonian Dark Earths occurred under ancient banana cropping where frequent supply of manure, ash and kitchen residues was done. Close to the house, the thickness of man-made dark Ah horizons could exceed 160 cm. Accumulation of organic matter promoted root exploration, thereby enhancing uptake of water and nutrients, and banana plant biomass. Yet, the soil gradient was strong in some sites and smooth to absent in ancient banana cropping areas. Gradients were revealed by contrasted stocks of a.o. carbon (19-12; 29-15; 30-22 kg m-3), and total content of Ca, Mg and K (420-370; 570-290; 370-280 molc m-3). The organic contribution to the nutrient stock of Ca, K and Mg ranged between 33 and 12%, and was highest around farmer’s house in contrasted sites, but also in ancient banana cropping areas whatever the distance to the house. High population pressure and soil infertility require sound management practices, based on better insight into soil formation and soil management processes

Mycorrhizal colonization of major banana genotypes in six East African environments

In the densely populated East African highlands (EAH), banana is a major staple food. In the last decade, serious yield declines were observed caused by soil fertility depletion and increased pressure of pests and diseases. Arbuscular mycorrhizal (AM) fungi form symbiosis with plant roots improving nutrients and water absorption, and protecting plants against a number of biotic (e.g. root pathogens) and abiotic (e.g. drought) stresses. Use of crop genetic diversity could be an appropriate option to improve production by using highly mycotrophic genotypes. This study assessed AM fungal root colonization of 18 banana genotypes across 6 locations in East Africa (Rwanda, Burundi, Kenya, DR Congo, Uganda and Tanzania). Soil and root sampling was carried out on each genotype. Data were collected on root colonization frequency and intensity, soil pH, cation exchange capacity and mineral nutrients. The results suggested that (i) high receptivity but low banana genotype specificity to the AM fungi exist among the five genotypes present in all the study sites; (ii) high variability in root colonization exists within the site among a wide range of Musa genotypes: from AA, AAA to AAB, ABB and tetraploids. © 2012 Elsevier B.V

Arbuscular mycorrhizal fungi in the East African Highland banana cropping systems as related to edapho-climatic conditions and management practices: case study of Rwanda

Root colonization, soil population density and diversity of arbuscular mycorrhizal (AM) fungi were assessed in 188 banana fields in contrasting soil types across five eco-regions in Rwanda (Butare-Gitarama, Gashonga, Bugarama, Kibungo, Ruhengeri). Root colonization was observed in all banana plants, whatever the soil type, field site and farm. The population density was higher on the wetter (1 300-1 500 mm yr(-1)) volcanic soils (Gashonga 59.8 and Ruhengeri 48.5 propagules 100 g(-1) soil, respectively) as compared to the dryer (900-1200 mm yr(-1)) soils derived from schist and granite (Butare-Gitarama 2.0, Kibungo 8.5, Bugarama 14.7 propagules 100 g(-1) soil). The diversity was highest in Kibungo and lowest in Butare-Gitarama (10 and 2 spore morphotypes, respectively). These results suggested that AM fungi were widespread in banana cropping systems in Rwanda, but that root colonization, population density and diversity varied considerably depending on edapho-climatic conditions (i.e. rainfall, soil texture and P content) and soil management practices (tillage). (C) 2009 Elsevier Ltd and The British Mycological Society. All rights reserved

The influence of the topographic position within highlands of Western Rwanda on the interactions between banana (Musa spp. AAA-EA), parasitic nematodes and soil factors

Soil properties vary according to the topography. They affect water uptake and root exploration in the soil. Consequently, they may also influence the spread of plant-parasitic nematodes. This study reports on the effect of toposequence-related variations in soil on banana yields, foliar nutrient status, and nematode impact. Twenty banana plots were visited within 6 hills/valleys at each of the three toposequence positions: valley bottom, mid-slope and crest. Important variability in plant growth, nutrition and soil properties was observed within the toposequence. Significantly better plant growth (height and girth) was observed in the valley bottoms, where banana bunch weight was 1.7-3.4 kg higher (although not significant) than at upper toposequence positions. Best plant growth was observed in valley bottoms in contrast to the highest N and K foliar deficiencies in this position. Plants in the valley bottoms had higher foliar Ca and Mg, and K compared to those in the crest. Plants in the mid-slope had greater percentage of dead roots (19.1%), compared to the plants in the valley bottoms (12.3%) and the crest (14.2%). Soils in the valley bottoms were deeper, sandier, with lower organic matter, lower N. and K compared to the soils at higher toposequence. Nematodes likely play a key role in banana root damage, however, their effect appear to be in relation to various soil factors at each position. The abundance of Pratylenchus goodeyi had generally limited impact on banana yields in fields having less than 5% slope (crest and valley bottom) where soil conditions were more optimal for root growth. However, in the presence of increased run-off on steeper middle slopes, root death was increased even under moderate pressure from P. goodeyi. (C) 2010 Elsevier B.V. All rights reserved

Agro-climatic Characterization of Potato Production Areas in Rwanda: Meteorological Data Analysis and Farmer Perceptions

From years ago, climate change effects are happening in different areas of the world, including Rwanda. Potato as one of main commodities grown mainly in the cooler and wetter highlands of Rwanda, its production could be facing the global warming. The purpose of this study is to carry out weather (temperature and rainfall) change dynamics in the potato growing zones of Rwanda over the last thirty years. Weather daily data from 1987 to 2016 were collected from three weather stations, at Kawangire for low elevation, at Gikongoro mid elevation, and at Kora high elevation, respectively. Farmers were interviewed to know their perceptions on weather conditions during past thirty years. Weather data were analyzed by ombrothermic diagram, Nickolson Index and graphing with generated trend equation methods. Interview data were analyzed with Chi-square test at P =.05.&nbsp; Results have shown as dry June, July and August months in low elevation; June, July and August in medium elevation and June and July in high elevation depending on decade. Rainfall was increasing in low and medium elevations, while it was decreasing in high elevation. Temperature was decreasing in low elevation area and it was increasing in medium and high elevation areas. All interviewed farmers confirmed long term-shift in temperature and precipitations. A total of 95%, 54% and 43% of farmers from respectively low, medium and high elevations reported that weather was becoming wetter during last years. As conclusion, potato growing cooler and wetter conditions of high elevation are shifting to low elevation in Rwanda. For climate change mitigation, Rwanda needs to promote potato in its Eastern part, the low elevation region, in order to increase the crop production in the future