Bio

Publications


2013

Musieba, F, Sheila O, Mibey RK, Wanjiku S, Moraa K.  2013.  Proximate composition, amino acids and vitamin profile of Pleurotus citrinopileatus: An indigenous mushroom in Kenya. American Journal of Food and Technology. 8:1-7.

2012

Wachira, PM, Okoth SA, Kimenju JW, Mibey RK, Kiarie JW.  2012.  Restoration of natural regulatory mechanisms of plant parasitic nematodes in agricultural systems.
Otieno, PK, Ochanda JO, Okoth S.  2012.  Evaluation of potency of native Bacillus thuringiensis against maize insect pests and aflatoxin producing fungi in Kenya. Journal of Tropical Microbiology and Biotechnology. 1(7):9-15.
Otieno, PK, Ochanda JO, Okoth S.  2012.  Isolation and characterization of Bacillus thuringiensis from coastal intertidal brackish sediments and from farmlands in Machakos District Kenya. . Journal of Tropical Microbiology and Biotechnology. 1(7):16-20.
Mnyazi, JJ, Okoth S, Wachira P, Karanja N, Kahindi J, Njuguini S, Ichami S, Mung'atu J, Okoth P, Huising J.  2012.  Impact of land use types on occurrence of arbuscular mycorrhizal fungi in Taita-Taveta district Kenya. Journal of Agriculture, Ecosystem and Environment. (157):32–39.
Musieba, F, Okoth S, Mibey RK, Wanjiku S, Moraa K.  2012.  Suitability of locally available substrates for cultivation of the Kenyan Pleurotus citrinopileatus Singer. American Journal of Food Technology. 7(10):650-655.
Okoth, S, Nyongesa B, Ayugi V, Kang’ethe E, Korhonen H, Joutsjoki V.  2012.  Toxigenic Potential of Aspergillus Species Occurring on Maize Kernels from Two Agro-Ecological Zones in Kenya. Toxin. (4):991-1007. Abstract

Two agro-ecological zones in Kenya were selected to compare the distribution in maize of Aspergillus spp. and their toxigenicity. These were Nandi County, which is the main maize growing region in the country but where no human aflatoxicoses have been reported, and Makueni County where most of the aflatoxicosis cases have occurred. Two hundred and fifty-five households were sampled in Nandi and 258 in Makueni, and Aspergillus was isolated from maize. Aspergillus flavus and A. parasiticus isolates were tested for the presence of aflD and aflQ genes. Positive strains were induced to produce aflatoxins on yeast extract sucrose and quantified using liquid chromatography-tandem mass spectrometry (LCMSMS). Aspergillus flavus was the most common contaminant, and the incidence of occurrence in Nandi and Makueni was not significantly different (82.33% and 73.26%, respectively). Toxigenic strains were more prevalent than non-toxigenic strains. All the toxigenic strains from Makueni were of the S-type while those from Nandi belonged to the L-type. Quantitative differences in aflatoxin production in vitro between isolates and between strains were detected with S strains producing relatively larger amounts of total aflatoxins, B toxins and lower values for G toxins. This was in accord with the frequent aflatoxicosis outbreaks in Makueni. However some L strains produced considerable amounts of B toxins. Given the widespread distribution of toxigenic strains in both regions, the risk of aflatoxin poisoning is high when favorable conditions for toxin production occur.

Okoth, SA, Kola MA.  2012.  Market samples as a source of chronic aflatoxin exposure in Kenya. African Journal of Health Sciences. (20):56-61. Abstract

Surveillance of food and feed quality in Kenya has not reached effective level due to the expensive procedures of mycotoxin analysis and poor structures in quality control. Most foodstuffs and feeds sold in local markets do not go through any quality control measures. The outbreaks of aflatoxicoses every year since the major outbreak that occurred in 2004 (CDC, 2004; Muture and Ogana, 2005, Azziz-Baumgartner et al., 2005) suggests that the population is exposed to aflatoxins in their diet. Chronic exposure could be a more serious problem than the outbreaks of aflatoxicosis that attract attention at the time they occur. This paper analyses the extent to which market food and feed samples expose the residents of urban Nairobi Province to aflatoxins. Using TLC method of aflatoxins analysis, maize for food and feed samples collected randomly as part of routine surveillance between the years 2006-2009 were tested. Only 17% of the total maize sampled and 5% of feed were fit for human and animal consumption respectively. Maize Grain Grade 11 and maize milled products were significantly highly contaminated compared with Maize Grain Grade 1 throughout the period of sampling. There was no significant difference in level of contamination among the feeds tested

2011

Musieba, F, Okoth S, Mibey RK.  2011.  First record of the occurrence of Pleurotus citrinopileatus Singer on new hosts in Kenya. AbstractWebsite

The occurrence of the edible basidiomycete Pleurotus citrinopileatus is reported for the first time in Kenya. The mushroom was collected from the dead logs and branches of Antiaris toxicaria (Pers.) Lesch., Polyscias fulva(Hiern) Harms, and Ficus thoningii Bl. in Kakamega forest. These trees are indigenous and are new hosts for this species. This mushroom is used as food by local communities in Kenya but it has not been documented nor studied. It is used solely as a product of the wild. People collect it and prepare it traditionally with other foods for consumption.

Okoth, SA, Siameto E.  2011.  Evaluation of selected soil fertility management interventions for suppression of fusarium spp. in a maize and beans intercrop. AbstractWebsite

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 inoculant. 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) Snyd.et 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

Okoth, SA, Otadoh J, Ochanda J.  2011.  Improved seedling emergence and growth of maize and beans by Trichoderma harziunum. Journal of Tropical and Subtropical Agroecosystems. (13):65-71.
Otadoh, JA, Okoth SA, Ochanda J, Kahindi JP.  2011.  Assessment of Trichoderma isolates for virulence efficacy on Fusarium oxysporum F. sp. Phaseoli. (13):99-107. Abstract

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.

Wachira, PM, Okoth S, Kimenju J, Mibey RK, Kiarie J.  2011.  Effect of soil fertility management practices on nematode destroying fungi in Taita, Kenya. Tropical and Subtropical Agroecosystems. (13):43-49. Abstract

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

Kawaka, JF, Kimenju JW, Ayodo G, Mwaniki SW, Muoma JO, Okoth SA, Orinda GO.  2011.  Impact of land use on the distribution and diversity of entomopathogenic nematodes in Embu and Taita districts, Kenya. Tropical and Subtropical Agroecosystems. (13):59–63. Abstract

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

Siameto, EN, Okoth S, Amugune NO, Chege NC.  2011.  Molecular characterization and identification of biocontrol isolates of Trichoderma harzianum from Embu district, Kenya. Tropical and Subtropical Agroecosystems. (13):81–90. Abstract

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.

2010

Okoth, SA, Siameto E.  2010.  Suppression of Fusarium spp. in maize and beans by soil fertility management. . Journal of Yeast and Fungal Research. 1(2):35-43.
Siameto, EN, Okoth S, Amugune NO, Chege NC.  2010.  Antagonism of Trichoderma farzianum isolates on soil borne plant pathogenic fungi from Embu District, Kenya. Journal of Yeast and Fungal Research . 1(3):47-54. Abstract

Species in the genus Trichoderma are important as commercial source of several enzymes and as biofungicides/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 anatognism 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 nonvolatile 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 controls. 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 inhibited the most compared to other pathogens. Genetic similarities generated using Jacquard’s coefficient of similarity ranged from 0.231 between isolates 055E and 011E to 0.857 between isolates 010E and 015E. The technique of RAPD was efficient in demonstrating the DNA polymorphism in the isolates of T. harzianum tested showing intraspecific genetic variability. 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.

2009

Wachira, PM, Kimenju JW, Okoth S, Mibey RK.  2009.  Stimulation of nematode destroying fungi by organic amendments applied in management of plant parasitic nematodes . Asian Journal of Plant Sciences. 8(2):153-159.
Mukundi, ND, Okoth SA, Mibey RK.  2009.  Influence of land use on the distribution and diversity of Pythium spp. . Journal of Tropical and Subtropical Agroecosystems. 11:347–351.
Okoth, SA, Okoth P, Wachira P, Roimen H.  2009.  Spatial distribution of Trichoderma sp. in Embu and Taita regions, Kenya. Journal of Tropical and Subtropical Agroecosystems. (11):291-302.
Okoth, SA, Okoth P, Muya E.  2009.  Influence of soil chemical and physical properties on occurrence of Trichoderma spp. in Embu, Kenya. . Journal of Tropical and Subtropical Agroecosystems. (11):303-312.
Okoth, SA, Odhiambo J.  2009.  Influence of soil chemical and physical properties on Trichoderma spp. occurrence in Taita Region, Kenya. . Journal of Tropical and Subtropical Agroecosystems. (11):403-413.
Maina, PK, Okoth S, Monda E.  2009.  Impact of land use on distribution and diversity of Fusarium species in Taita Taveta, Kenya. (11):323-335. Abstract

The effect of Land Use Types (LUTs) on distribution and diversity of Fusarium species in soil were evaluated in Taita Taveta district, Kenya. Soil samples were collected from sixty points across a land use gradient covering six different LUTs, at 0 to 10 and 10 to 20 cm soil depths. Using Fusarium-selective media, a total of 1865 Fusarium isolates were recovered from the soil samples which resulted into 26 Fusarium species with Fusarium oxysporum and Fusarium solani being the dominant species in this area. Difference in Fusarium abundance, diversity and richness across the LUTs was significant (P<0.001) with horticulture being the richest and the most diverse LUT. The top soil layer had significantly higher Fusarium abundance and richness (P<0.05). A Principal Component Analysis (PCA) based on the relative Fusarium species abundance differentiated the LUTs with 79.69 %. There were significant positive correlation between P and pH levels with Fusaria abundance, richness and diversity (P<0.001). Abundance and diversity of Fusarium was also positively correlated with soil Mg and K (P<0.05). However, a significant negative correlation between exchangeable acidity and abundance (r=-0.605), richness (r=-1.317) and diversity (r=-0.16) was observed (P<0.05). Negative correlation was also observed between Nitrogen and richness (r=-2.94) and diversity (r=-0.67) of Fusarium species.

Maina, PK, Okoth S, Njoroge CN, Monda E.  2009.  Genetic relatedness among Fusarium species isolated from Taita Taveta region, Kenya. (11):337-345. Abstract

Genetic relatedness among twenty six Fusarium species isolated from soils in six different Land Use Types (LUTs) across a land use gradient was evaluated using Random Amplified Polymorphic DNA (RAPD) assay. The six LUTs were horticulture, maize, napier grass, fallow/shrub land, planted forests as well as indigenous forests. Six primers were used in this assay. Amplification products were examined and presence or absence of each size class of bands was scored as 1 and 0, respectively. The resulting matrix was used to compute Jaccard’s similarity coefficients and Unweighted Pair Group Method with Arthmetic mean (UPGMA) cluster analysis using computer package NTSYS-pc. Jaccard’s similarity coefficients ranged from 0.257 to 0.583 among the Fusarium species studied indicating high genetic diversity. The UPGMA cluster analysis grouped the 26 Fusarium species into two main clusters with the first cluster comprising of 17 species with genetic similarity values ranging from 26.2 to 58.3 %. The second cluster comprised of 9 species with genetic similarity values ranging from 29.5 to 56.34 %. Fusarium avenaceum and F. nygamai depicted the highest genetic similarity of 58.3 %.

Wachira, P, Mibey R, Okoth S, Kimenju J, Kiarie J.  2009.  Diversity of nematode destroying fungi in Taita Taveta, Kenya. (2):60-69. Abstract

The diversity of nematode destroying fungi in Taita Taveta, Wundanyi division, Coast Province, Kenya, was investigated between May 2006 and December 2007 aiming at harnessing their potential in the biological control of plant parasitic nematodes in the area. Given that the intensity of land cultivation is continually increasing in the study area, it is prudent to document the status of the nematode destroying fungi before the remaining forest habitats are ultimately disrupted. Soil samples were collected from forest, maize/ bean, napier grass, shrub and vegetable fields, which represented the main land use types in the study area. The soil sprinkle technique method was used to isolate the nematode destroying fungi from the soil. The fungi were identified to species level. Eighty-five isolates, distributed in eight genera and 14 taxa were identified as nematode destroying fungi. The species identified were Arthrobotrys dactyloides, Arthrobotrys oligospora, Arthrobotrys superba, Acrostalagamus obovatus, Dactyllela lobata, Harposporium aungulilae, Harposporium liltiputanum, Harposporium spp, Haptoglosa heterospora, Monacrosporium asterospernum, Monacrosporium cianopagum, Myzocytium, spp, Nematoctonus georgenious and Nematoctonus leptosporus. Vegetable land use had the highest diversity of nematode destroying fungi. The results show that the study area is rich in nematode destroying fungi with A. oligospora being widespread and a possible candidate for biological control of plant parasitic nematodes

Wachira, PM, Okoth S, Kimenju J, Mibey RK.  2009.  Influence of land use and soil management practices on the occurrence of nematode destroying fungi in Taita-Taveta, Kenya. Tropical and Subtropical Agroecosystems. (10):213-223. Abstract

Due to the increased concerns about the effect of agrochemicals on soil health and soil biodiversity, use of biological methods has become most acceptable alternative methods for farmers to control soil pathogens during crop production. A study was therefore undertaken to determine the occurrence of nematode destroying fungi in Taita Taveta with the aim of isolating and characterizing them for biological control of plant parasitic nematodes. Twenty eight fungal isolates, distributed in three genera, were identified as nematode destroying fungi from all the positive soil samples. Out of the isolates that were identified, 71, 25 and 4 % were in the genera Arthrobotrys, Monacrosporium and Nematoctonus respectively. Arthrobotrys oligospora had an occurrence frequency of 42.9% which was the highest followed by A. dactyloides, M.cionopagum, Monacrosporium sp and Nematoctonus sp with frequencies of 28.6, 17.9 and 7.1and 3.6% respectively. The occurrence of nematode destroying fungi was affected by land use and organic inputs (P ≤ 0.05) while it was not affected by crop rotation (P ≥ 0.05). Napier land use was more diverse than the other land uses with a mean shannon diversity index of 0.717 followed by horticulture (index 0.497). Maize /bean, coffee/beans, fallow and shrub land uses had a mean shannon index of 0. The same trend was observed on richness where napier had a mean richness of 2.2, horticulture 1.8, maize bean 1 while shrub, fallow and coffee/ beans all had mean richness of 0.2. A.oligospora was the most frequently isolated fungi (42.9 %) and showed high potential in biocontrol of plant-parasitic nematodes and was recommended for further studies and development as a biological control agent.

Mwangi, MW, Monda EO, Okoth SA, Jefwa JM.  2009.  Effect of Trichoderma harzianum and arbuscular mycorrhizal fungi on growth in tomato (Lycopersicum esculentum Mill) seedlings, napier (Pennisetum purpureum L) and tea (Camellia sinensis L) cuttings. Tropical and Subtropical Agroecosystems. (11):423-429. Abstract

A green-house study was conducted to investigate the ability of an isolate of Trichoderma harzianum (P52) and arbuscular mycorrhizal fungi (AMF) in enhancing growth in tomato seedlings, tea and napier grass cuttings. The effect of these bio-inoculants on growth was compared with the influence of Diammonium phosphate (DAP) fertilizer and the interactions of these three factors (P52, AMF and DAP). The plants were grown in plastic pots filled with sterilized soils. A completely randomized design was used and growth measurements taken on height shoot and root dry weights. It was observed that isolate P52 and DAP fertilizer individually enhanced growth in tomatoes, tea and napier while AMF only enhanced growth in tomatoes. Combinations of P52 and DAP; P52, AMF and DAP enhanced growth significantly (P<0.05). Trichoderma harzianum and AMF showed potential for use as biofertilizers to reduce on chemical inputs in the perspective of sustainable agriculture and conservation of natural resources.

2008

Wasonga, C, Okoth S, Mukuria JC, Omwandho CA.  2008.  Mushroom polysaccharide extracts delay progression of carcinogenesis in mice. Journal of Experimental Therapeutics and Oncology. (7):147-152.
Wachira, PM, Kimenju JW, Okoth S, Mibey RK.  2008.  Effect of land use on occurrence and diversity of nematode destroying fungi in Taita Taveta, Kenya. Asian Journal of Plant Sciences. Asian Journal of Plant Sciences. 7(5):447-453.
Wachira, PM, Okoth S, Kimenju JW, Mibey RK.  2008.  Influence of land use and soil management practices on the occurrence of nematode destroying fungi in Taita Taveta, Kenya. Tropical and Subtropical Agroecosystems. (10):213–223.
Wachira, P, Mibey R, Okoth SA, Kimenju JW.  2008.  Diversity of nematode destroying fungi in Taita Taveta, Kenya. Fungal ecology. 2(2):60–65.

2007

Okoth, S, Jefwa MJ, Karanja N, Kahindi J, Muya E, Okoth P.  2007.  Exploring the potential of beneficial soil fungi to enhance productivity in agricultural systems.
Jefwa, JM, Khayota B, Ngugi G, Otieno V, Musila W, Okoth S.  2007.  Biodiversity and utilization of edible mushrooms by forest margin communities of the lowland coastal forests of Kenya. 1st World Conference on the conservation and sustainable use of wild fungi. :33-35.
Okoth, SA, Roimen H, Mutsotso B, Muya E, Okoth P.  2007.  Land use systems and distribution of Trichoderma species in Embu region, Kenya. Journal of Tropical and Subtropical Agroecosystems. (7):105-122.

2004

Okoth, SA.  2004.  An overview of the diversity of microorganisms involved in decomposition in soils. Journal of Tropical Microbiology. (3):3-13.
Okoth, SA, Ohingo M.  2004.  Dietary aflatoxin exposure and impaired growth in young children from Kisumu, Kenya: Cross sectional study. African Journal of Health Sciences. (11):43-54.

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