Scielo RSS <![CDATA[Tropical and subtropical agroecosystems]]> vol. 13 num. 1 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Special issue</b>: <b>Conservation and Sustainable Management of Belowground Biodiversity in Kenyan Agroecosystems</b>]]> <![CDATA[<b>Below-ground biodiversity in Kenya</b>]]> <![CDATA[<b>Conservation and sustainable management of belowground biodiversity in Kenyan agroecosystems</b>]]> <![CDATA[<b>Abundance and diversity of legume nodulating rhizobia in soils of Embu District, Kenya</b>]]> A major strategy towards addressing soil fertility depletion is the conservation and sustainable use of rhizobia that are able to fix nitrogen in the soil in association with legumes. The study assessed abundance and diversity of legume nodulating rhizobia (LNB) in soils collected from six different land use systems in Embu District, Kenya. The populations were estimated by the most-probable-number (MPN) plant infection technique using Macroptilium atropurpureum (DC.) Urban (Siratro) as the trap host species. Symbiotic effectiveness was measured for the isolates in association with Siratro. Isolated rhizobia were characterized morphologically and genetically by PCR-RFLP and partial sequencing of 16S rRNA genes. The LNB populations in soils collected from the different land uses in Embu ranged from 0 to 2.3 x 10² cells g-1 soil. There was apparent land use effect on abundance of LNB with fallow system giving high abundance. A total of 250 pure isolates were obtained from the root nodules of Siratro trap plants. The isolates were characterized on yeast extract mannitol mineral salts agar (YEMA) media containing bromothymol blue and grouped into fast growers (acid-producing) and slow growers (alkali-producing) (70% and 30 % of isolates respectively). PCR-RFLP analysis categorised the rhizobia into five species in the genera Rhizobium, Bradyrhizobium, Mesorhizobium and Agrobacterium. Land use system under tea had four of the five species found in the area whereas natural forests had two species. Land use significantly impacted on the diversity of rhizobia (PO.05) with soils under tea having the highest diversity with a mean Shannon diversity index of 1.304 compared to the lowest (0.297) recorded in natural forest. Isolated rhizobia strains formed effective nodules on Siratro. However, the level of nitrogen fixation varied among isolates while symbiotic efficiency ranged from 27-112%. The findings indicate that abundance and diversity of rhizobia does not necessarily decrease with agricultural intensification as hypothesized but recommends further studies to obtain a clearer understanding of the relationship between soil rhizobia diversity and land use and management. <![CDATA[<b>Abundance and diversity of soil mites (acari) along a gradient of land use types in Taita Taveta, Kenya</b>]]> The abundance and diversity of soil mites was monitored along a gradient of land use types (LUTs) during the wet seasons in soils of Taita Taveta, Kenya. Sampling of mites from soils was carried out in eight LUTs which included maize-based system (Zea mays), coffee (Coffea Arábica), horticulture, napier grass (Pennisetum purpureum), fallow, pine (Pinus patula), cypress (Cypressus lusitanica), natural forest. LUT significantly influenced abundance, richness and diversity of the soil mites. During the short rains the diversity of soil mites increased in the order napier ,maize-based system, horticulture, coffee, fallow, natural forest, pine forest, cypress plantation while the long rains season abundance increased in the order maize-based system, coffee, horticulture, napier, natural forest, fallow, pine forest, cypress forest. Higher abundance, richness and diversity of the mites was observed in the less disturbed forest ecosystems unlike the agro-ecosystems, which are often disturbed with intensive cultivation A total of 37 families were recorded with 20 oribatid families, 10 mesostigmatid families and 7 prostigmatid families. The families that ranked highest in abundance across the LUT were Scheloribatidae, Oppidae (Oribatida) and Rhodacaridae (Mesostigmata). Land use type influenced significantly (P<0.05) the abundance and diversity of soil mites where intensification lowered the diversity and abundances resulting in less complex mites community structures. <![CDATA[<b>Effect of soil fertility management practices and <i>Bacillus subtilis</i> on plant parasitic nematodes associated with common bean, <i>Phaseolus vulgaris</i></b>]]> On-farm and on-station field experiments were carried out to determine the potential of combining Bacillus subtilis with soil fertility management practices for controlling plant parasitic nematodes associated with common bean, (Phaseolus vulgaris). The treatments were Bacillus subtilis (isolate K194), B. subtilis plus cow manure, B. subtilis plus mavuno, Bacillus subtilis plus calcium ammonium nitrate + tripple super phosphate, manure alone, mavuno alone with calcium ammonium nitrate + tripple super phosphate as the control. The recommended farmers' practice entailed application of tripple super phosphate and calcium ammoniun nitrate at the rate of 1000 and 890 kg/ha, respectively. Manure and mavuno were applied at the rate of 10 tons and 890 kg/ha, respectively. The on-farm trial was carried out in 12 different farms. The combination of Bacillus subtilis inoculum and cow manure led to a 54% reduction in numbers of plant parasitic nematodes, compared to the untreated control. Consequently, damage by root-knot nematodes produced galls with galling indices 1.6 and 4.5 respectively in plots treated with the combination (B. subtilis and cow manure) and the untreated control, respectively. Compared to the other treatments, combining B. subtilis and organic amendments resulted in the highest nematode diversity. It can therefore be concluded that the plant parasitic nematodes associated with common bean can be maintained at levels below economic threshold using B. subtilis combined with cow manure, an integration which also demonstrated conservation of the nematode diversity. <![CDATA[<b>Effect of integrated soil fertility management interventions on the abundance and diversity of soil Collembola in Embu and Taita Districts, Kenya</b>]]> The study aimed at identifying soil fertility management practices that promote the Collembola population, diversity and survival in the soil. Soil samples were randomly collected from on farm plots amended with: 1-Mavuno ((Ma)-is a compound fertilizer containing 26% Potassium, 10% Nitrogen, 10% Calcium, 4% Sulphur, 4% Magnesium and trace elements like Zinc, Copper, Boron, Molybdenum and Manganese)), 2-Manure (Mn), 3-Trichoderna (Tr) inoculant (is a soil and compost-borne antagonistic fungus used as biological control agent against plant fungal diseases), 4-Farmers practice ((FP) where Tripple Super Phosphate (T.S.P.) and Calcium Ammonium Nitrate (C.A.N.) fertilizers are applied in the soil in mixed form), 5-Tripple Super Phosphate (T.S.P.), 6-Calcium Ammonium Nitrate (C.A.N.). These treatments were compared with 7-Control (Co) (where soil fertility management interventions where not applied). Soil Collembola were extracted using dynamic behavioural modified Berlese funnel and identified to the genus level. Occurrence of Collembola was significantly affected by soil fertility amendments in both Taita and Embu study sites (P<0.05). Twenty two genera of soil dwelling Collembola were recorded, with control and organic manure treated plots recording high diversity with a Shannon 1.86 in Embu and a Shannon 2.09 in Taita, respectively. There was significant difference (P<0.05) of seasonality on soil Collembola occurrence at both Embu and Taita. Application of cow manure and addition of Trichoderma inoculants promoted the soil Collembola. The study has demonstrated that application of organic amendments encouraged the soil Collembola while inorganic fertilizers negatively impacted on these soil organisms. <![CDATA[<b>Effect of soil fertility management practices on nematode destroying fungi in Taita, Kenya</b>]]> The study aimed at identifying soil fertility practices that promoted nematode destroying fungi in the soil and the treatments comprised of Mavuno fertilizer, Triple super- phosphate and calcium ammonium nitrate (TSP+CAN), cow manure and a control where no amendments were applied. This experiment was replicated in ten farms for three planting seasons. There were significant difference (P= 1.705 x 10-06) in occurrence of the nematode destroying fungi between soil fertility treatments. The highest mean occurrence of nematode destroying fungi was 1.6 which was recorded in soils amended with cow manure and the least was in soils from the control plots. A mean of 0.78 was recorded in soils from both TSP+CAN and Mavuno fertilizers. Plots amended with cow manure gave the highest diversity of nematodes followed by the control, then TSP+CAN and least in Mavuno with shannon indices of 0.34, 0.15, 0.13 and 0.11 respectively. Sixty percent of all the isolated nematode destroying fungi genera were from plots treated with cow manure and only twenty percent were from plots amended with the inorganic fertilizer. <![CDATA[<b>Farmers knowledge, attitudes and practices (KAP) in Embu and Taita benchmark sites before and after below-ground biodiversity project interventions</b>]]> In Kenya the belowground biodiversity project was implemented in Embu and Taita which are biodiversity hotspots. The objective of the BGBD project was to enhance awareness, knowledge and understanding of below ground biological diversity important for sustainable agricultural production by demonstration of methods for conservation and sustainable man power. The objective of the paper is to present and analyze farmers' knowledge, attitudes and practices at different timelines of the project, as well as identify factors limiting adoption of the best bet technologies of enhanced biodiversity conservation and increased agricultural production. In early 1960's when natural forest was cleared for farming activities no farm inputs were introduced in food crops until after 10 years. Ever since, to the present similar farm imputs and crops have been propagated continuously leading to declined food production hence the BGBD project interventions. A triangulation of methods, both quantitative and qualitative participatory approaches were used to obtain data from farmers in both benchmark sites. The results showed that most farmers were more aware of the diversity of organisms in the soil at the end of the project compared to the beginning. As much as the best bet technologies were identified, their adoption was constrained by the unavailability of the technologies in local agro shops. This forced farmers to continue using the farming techniques that had deleterious effects on the soil. Hence farmers adopt what works, but which must be locally available. In this, cost considerations did not matter, but simply inaccessibility. <![CDATA[<b>Impact of land use on the distribution and diversity of entomopathogenic nematodes in Embu and Taita Districts, Kenia</b>]]> Natural entomopathogenic nematodes (EPNs) are considered as potential biological control agents against soil-borne insect pests. This study was conducted to determine the impact of land use on the distribution, occurrence and diversity of entomopathogenic nematode community. Isolation of EPNs was done using the baiting technique and application of morphological identification methods revealed presence of the genus Steinernema. Land use intensification negatively affected the occurrence and recovery frequency in soils of Embu and Taita districts. The occurrence of EPNs was high in soils from coffee than maize and beans which had more nematodes than planted forest and napier grass followed by natural forest and tea respectively. PCR-RFLP of the internal transcribed spacer region on the ribosomal(r) DNA of the EPN isolates and digestion of the products by Alu I enzyme showed molecular variations among the isolates. The study has demonstrated that the frequency of occurrence and species variation of EPNs is different in various land uses. <![CDATA[<b>Improved seedling emergence and growth of maize and beans by <i>Trichoderma harziunum</i></b>]]> An indigenous strain of Trichoderma spp. was tested for its ability to promote seed germination and growth of maize and bean seedlings grown in the field at Embu District, Kenya. The trial was carried out for three seasons with the following treatments; two types of fertilizers, cow manure, and Trichoderma seed coat. Seedlings were counted 14 days after emergence from soil and a sample gently uprooted using a spade. Shoot height, root length, stem and root diameter measurements were taken. Trichoderma inoculation significantly increased rate of maize seed germination but had no effect on emergence of bean seedlings. Maize seeds coated with Trichoderma inoculum and planted on soils without fertilizer addition recorded the highest germination rate of 82.7% followed by seeds coated with the inoculum and planted in soils treated with manure (82.2%). Combination of the inoculum and fertilizer performed better at improving maize seed germination compared with fertilizers applied singly. This was the case for shoot and root growth. Seeds coated with the inoculum and planted in soils ammended with Triple Superphosphate and Calcium Ammonium Nitrate recorded the greatest shoot and root growth in both maize and beans. Increased growth of shoot and root caused by Trichoderma implied that there was beneficial effect of inoculation on plant growth and development since root collar and stem diameters were a measure of survivability of seedlings. <![CDATA[<b>Evaluation of selected soil fertility management interventions for suppression of <i>Fusarium</i> spp. in a maize and beans intercrop</b>]]> Fusarium root rot of maize and beans is a common problem in Taita District, Kenya causing economic losses to the small scale farmers. The pathogen attacks maize and beans at all growth stages causing rot at the seedling stage, yellowing of the leaves, stunted growth, and death if severe. Potentially effective crop rotations to maintain the pathogen at low levels are not currently practical due to the small size of farms while fungicides are out of reach to the small scale farmer due to high prices. This study aimed at assessing alternatives to fungicides in controlling root infection by Fusarium sp. in maize and beans cropping systems. Field trials were done in Taita District where agriculture contributes to 95% of household income with limited use of any soil fertility amelioration by farmers. The following were tested in the trials; three types of inorganic fertilizers, cow manure, and Trichoderma inoculan! Planting was done during the long and short rains. Soil and roots were collected from the rhizosphere during harvesting and assessed for inoculum density while the roots were evaluated for incidence of infection by Fusarium spp. The most common species in both soil and roots were F. oxysporum (Schlecht) Hans, and F. sporotrichoides Sherb. Addition of soil amendments had a positive effect of reduced root infection and in some cases lowering inoculum density in the soil. Of the four integrated soil fertility interventions, Mavuno fertilizer had the highest yield and was the most effective in suppressing root colonisation by Fusarium spp. <![CDATA[<b>Molecular characterization and identification of biocontrol isolates of <i>Trichoderma harzianum</i> from Embu District, Kenia</b>]]> Species in the genus Trichoderma are important commercial source of several enzymes, biofungicides and growth promoters. The most common biological control agents of the genus are strains of T. harzianum, T .viride and T. viriens. In this study, sixteen selected isolates of T. harzianum from different land use types in Embu, Kenya were tested for antagonistic action against five soil borne phytopathogenic fungi (Rhizoctonia solani, Pythium sp, Fusarium graminearum, F. oxysporum f sp phaseoli and F. oxysporum f sp Lycopersici) using dual culture assay and through production of non-volatile inhibitors. Seven isolates were further characterized using RAPD-PCR procedure to determine genetic variability. All T. harzianum isolates had considerable antagonistic effect on mycelial growth of the pathogens in dual cultures compared to the control. Maximum inhibitions occurred in Pythium sp-055E interactions (73%).The culture filtrates obtained from Czapek's liquid medium reduced the dry weight (mg) of the mycelia significantly while those from the potato dextrose broth showed minimum inhibition growth. Pythium sp. was most sensitive compared to other pathogens. Genetic similarities generated using Jaccard's coefficient of similarity ranged from 0.231 to 0.857 for isolates 055E, 011E, 010E and 015E. Since all T. harzianum isolates evaluated were effective in controlling colony growth of the soil borne pathogens both in dual cultures and in culture filtrates they could be tried as a broad spectrum biological control agent in the green house and under field conditions. <![CDATA[<b>Pest status of bean stem maggot (<i>Ophiomyia</i> spp.) and black bean aphid <i>(Aphis fabae)</i> in Taita District, Kenya</b>]]> On-station and on-farm studies were carried out in Taita district, Kenya, to assess the effect of various soil fertility regimens on bean stem maggot (BSM) (Ophiomyia spp.) and black bean aphid (BBA) (Aphis fabae) incidence. These studies aimed at gathering information useful in the development of an IPM programme for the project on conservation and sustainable management of below-ground biodiversity (CSM-BGBD). Field survey of bean stem maggot and black bean aphid was conducted in both the on-station and on-farm plots four weeks after bean emergence. From each plot forty bean plants were sampled. The parameter assessed included stems, leaves and flowers and the percent numbers of plants having bean stem maggot and black bean aphid were expressed as the percentage incidence for that particular plot. The incidence of bean stem maggot and black bean aphid were significantly higher in the on-farm plots than in the on-station plots. On the other hand, influences of integrated soil fertility management (ISFM) interventions and location on bean stem maggot and black bean aphid incidence were minimal. These findings point to the fact that, soil fertility management interventions alone are not effective in mitigating the challenge of insect pest. Instead, there is a strong need to adopt a holistic management approach that incorporates both pest management (IPM) and soil fertility management (ISFM). <![CDATA[<b>Assessment of <i>Trichoderma</i> isolates for virulence efficacy on <i>Fusarium oxysporum F. sp. </i></b><b><i>Phaseoli</i></b>]]> Trichoderma has been widely studied for their biocontrol ability, but their use as biocontrol agents in agriculture is limited due to the unpredictable efficiency which is affected by biotic and abiotic factors in soil. Isolates of Trichoderma from Embu soils were evaluated for their ability to control Fusarium oxysporum f. sp. phaseoli., in vitro and promote seedling growth in the greenhouse. Bioassays were run using dual cultures and diffusible compound production analysis. The Trichoderma isolates significantly (p ≤ 0.01) reduced the mycelial growth of the pathogen. The principle mechanisms of niche competition, mycoparasitism, and antibiosis were observed in growth of the pathogen mycelium in the presence of Trichoderma spp., through development of inhibition zones. There was coiling of hyphae around the pathogen mycelium coupled by lysising of cell wall Trichoderma spp., where T. reesei and T. koningii were the most effective isolates. Studies were indicative of the synergistic ability of Trichoderma spp., being an effective biocontrol of bean seedlings against Fusarium wilt while also promoting plant growth. <![CDATA[<b>Genetic diversity and symbiotic efficiency of legume nodulating bacteria from different land use systems in Taita Taveta, Kenya</b>]]> Populations of Legume Nodulating Bacteria (LNB) were assessed under glasshouse conditions from soils collected in Taita Taveta district, Kenya from various landuse systems. The populations were estimated by the most-probable-number (MPN) plant infection technique using Macroptilium atropurpureum (DC.) Urban (siratro) as the trap plant. The LNB populations varied from 1.1 x 10 to 6.1 x 10(6) cells g-1 of soil. There was apparent landuse effect on abundance of LNB with maize-bean cropping system and Shrubland giving high population estimates. Two thousand isolates of LNB were obtained from the nodules of siratro trap plant. These isolates were characterized on yeast extract mannitol mineral salts agar (YEMA) media containing bromothymol blue and two distinct rhizobia growth rate types were identified: fast growers (acid-producing) at 78.6% while slow growers (alkali-producing) comprised 21.4%. Symbiotic effectiveness of a selected number of the isolates ranged from 6.7% to 96.4% and no clear influence of landuse was observed. RFLP of amplified 16S rRNA genes of isolates with HaeIII and TaqI grouped the isolates into seven ribotypes and partial sequencing of 16S rRNA genes of isolates representative of the ribotypes further grouped the isolates into six genera namely; Sinorhizobium, Bradyrhizobium, Herbaspirillum, Agrobacterium, Rhizobium and Burkholderia. Landuse type was found to significantly influence the diversity of LNB (PO.05). The highest LNB richness of five genera was found in indigenous forest soils. While fallow/shrubland and maize based system had a total richness of four genera. Each of the remaining landuses had LNB total richness of two.