Distribution of Cassava Bacterial Blight and Reaction of Selected Cassava Genotypes to the Disease in Kenya

Cassava bacterial blight (CBB) disease is an important bacterial disease of cassava. A study was carried out to determine the distribution of CBB in Kenya and to evaluate selected cassava genotypes for reaction to the disease. A survey was conducted in all the cassava growing regions within the country where cassava leaves showing symptoms of CBB were collected and isolated for biochemical characterization and PCR detection of the causal agent. The isolates were then used to determine the reaction of seven cassava genotypes to the disease. The disease was present in 17 out of the 21 counties surveyed. The bacteria extracted from the leaf samples conformed to all the biochemical and physiological tests specific to Xanthomonas axonopodis pv manihotis and to xanthomonads in general.  Polymerase chain reaction amplified the expected 500 base pairs fragment. Disease prevalence was highest in Kwale County at 100% Kilifi County recorded the highest incidence at 64%. All the genotypes evaluated in the greenhouse had area under disease progress curve (AUDPC) values higher than 52 which grouped them as susceptible. The study confirms the wide distribution of CBB in Kenya and the presence of the disease in the coast region, which was previously considered CBB free. The study also shows that some of the cassava genotypes being targeted for improvement by other projects are susceptible to the disease, and therefore the need consider resistance to CBB in developing improved cassava genotypes. Keywords: Cassava bacterial blight, Distribution, Resistance DOI: 10.7176/JNSR/9-4-0

Pathogenicity of Selected Kenyan Entomopathogenic Nematodes of Genus Steinernema against Banana Weevil (Cosmopolites Sordidus)

Banana weevil is a pest with a great impact on banana production in the world. The objective of this study was to evaluate the pathogenicity of the Kenyan entomopathogenic nematodes (EPNs) of genus Steinernema against the weevil Cosmopolites sordidus (Germar) under laboratory conditions. Adult weevils were trapped from banana fields in Maragua, Central Kenya and larvae extracted from the infested banana rhizomes. The adults were treated with three Kenyan EPNs Steinernema weiseri, Steinernema yirgalemense and the new Steinernema spp and with Steinernema carpocapsae ALL strain as a standard and plain distilled water as a control in five replicates. The treatments were 500ijs, 750ijs and 1000ijs per adult on petri dishes and 1000ijs, 3000ijs and 5000ijs per adult on pseudo-stems. The treatments for larvae in petri dishes were 300ijs, 400ijs and 500ijs. The adults were not susceptible to all the nematodes at all doses and preparations while larvae were highly susceptible to the four test EPNs at all concentrations. There was a significant difference at P<0.05 in the mortality of larvae between nematode Steinernema carpocapsae and Steinernema weiseri. A significant difference was observed between nematode Steinernema carpocapsae and Steinernema yirgalemense. There was also a significant difference between the new Steinernema spp and Steinernema weiseri and finally in the two nematodes the new Steinernema spp and Steinernema yirgalemense. All the test nematodes caused over 90 % larval mortality within 48 hours. The mean percent mortality of larva increased with nematode concentration for all the test nematodes. The results indicated that the four test EPNs significantly (P<0.05) caused high mortality within 24-48 hrs. The findings indicate that the banana weevil larvae are susceptible to the local entomopathogenic nematodes and potentially useful in the management of the banana weevil. It is however recommended that more research be conducted on the formulation and application technology to enhance their effectiveness in the field

Agricultural Nematology in East and Southern Africa : Problems, Management Strategies and Stakeholder Linkages

This is the peer reviewed version of the following article: Herbert Talwana, et al, ‘Agricultural nematology in East and Southern Africa: problems, management strategies and stakeholder linkages’, Pest Management Science Vol. 72 (2): 226-245, February 2016, which has been published in final form at http://dx.doi.org/10.1002/ps.4104. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. © 2015 Society of Chemical Industry.By 2050, Africa’s population is projected to exceed two billion. Africa will have to increase food production more than 50% in the coming 50 years to meet the nutritional requirements of its growing population. Nowhere is the need to increase agricultural productivity more pertinent than in much of sub-Saharan Africa where it is currently static or declining. Optimal pest management will be essential, because intensification of any system creates heightened selection pressures for pests. Plant-parasitic nematodes and their damage potential are intertwined with intensified systems and can be an indicator of unsustainable practices. As soil pests, nematodes are commonly overlooked or misdiagnosed, particularly where appropriate expertise and knowledge transfer systems are meager or inadequately funded. Nematode damage to roots results in less efficient root systems that are less able to access nutrients and water, which can produce symptoms typical of water or nutrient deficiency, leading to misdiagnosis of the underlying cause. Damage in subsistence agriculture is exacerbated by growing crops on degraded soils and in areas of low water retention where strong root growth is vital. This review focuses on the current knowledge of economically important nematode pests affecting key crops, nematode control methods, and the research and development needs for sustainable management, stakeholder involvement and capacity building in the context of crop security in East and Southern Africa, especially Kenya, Tanzania, Uganda and Zimbabwe.Peer reviewe

Biocontrol-based strategies for improving soil health and managing plant-parasitic nematodes in coffee production

Coffee is an important commodity for Kenya, where production is steadily declining, despite a global rise in demand. Of the various constraints affecting production, plant-parasitic nematodes are a significant, but often overlooked, threat. As a perennial crop, treating plantations once infected with nematodes becomes difficult. The current study evaluated the drenching application of two biocontrol agents, Trichoderma asperellum and Purpureocillium lilacinum, for their nematode control efficacy, as well as their impact on the soil nematode community structure on mature, established coffee trees in Kenya. Seven Arabica coffee field trials were conducted over two years on trees of various ages. All the fields were heavily infested with Meloidogyne hapla, the first report of the species on coffee in Kenya. Both fungal biocontrol agents were detected endophytically infecting roots and recovered from soil but not until six months after initial applications. The population densities of M. hapla had significantly declined in roots of treated trees 12 months after the initial application, although soil nematode density data were similar across treatments. Based upon the maturity index and the Shannon index, treatment with T. asperellum led to improved soil health conditions and enrichment of diversity in the microbial community. Application of P. lilacinum, in particular, led to an increased abundance of fungivorous nematodes, especially Aphelenchus spp., for which P. lilacinum would appear to be a preferred food source. The soils in the trials were all stressed and denuded, however, which likely delayed the impact of such treatments or detection of any differences between treatments using indices, such as the functional metabolic footprint, over the period of study. A longer period of study would therefore likely provide a better indication of treatment benefits. The current study positively demonstrates, however, the potential for using biologically based options for the environmentally and climate-smart management of nematode threats in a sustainable manner on established, mature coffee plantations

The Pochonia chlamydosporia Serine Protease Gene vcp1 Is Subject to Regulation by Carbon, Nitrogen and pH: Implications for Nematode Biocontrol

The alkaline serine protease VCP1 of the fungus Pochonia chlamydosporia belongs to a family of subtilisin-like enzymes that are involved in infection of nematode and insect hosts. It is involved early in the infection process, removing the outer proteinaceous vitelline membrane of nematode eggs. Little is known about the regulation of this gene, even though an understanding of how nutrients and other factors affect its expression is critical for ensuring its efficacy as a biocontrol agent. This paper provides new information on the regulation of vcp1 expression. Sequence analysis of the upstream regulatory region of this gene in 30 isolates revealed that it was highly conserved and contained sequence motifs characteristic of genes that are subject to carbon, nitrogen and pH-regulation. Expression studies, monitoring enzyme activity and mRNA, confirmed that these factors affect VCP1 production. As expected, glucose reduced VCP1 expression and for a few hours so did ammonium chloride. Surprisingly, however, by 24 h VCP1 levels were increased in the presence of ammonium chloride for most isolates. Ambient pH also regulated VCP1 expression, with most isolates producing more VCP1 under alkaline conditions. There were some differences in the response of one isolate with a distinctive upstream sequence including a variant regulatory-motif profile. Cryo-scanning electron microscopy studies indicated that the presence of nematode eggs stimulates VCP1 production by P. chlamydosporia, but only where the two are in close contact. Overall, the results indicate that readily-metabolisable carbon sources and unfavourable pH in the rhizosphere/egg-mass environment may compromise nematode parasitism by P. chlamydosporia. However, contrary to previous indications using other nematophagous and entomopathogenic fungi, ammonium nitrate (e.g. from fertilizers) may enhance biocontrol potential in some circumstances

EVALUATION OF THE QUALITY STATUS OF AFRICAN NIGHTSHADE SEED PRODUCED BY FARMERS IN KENYA

Seed as an important input in crop production should be of high quality, pathogen free, have desired germination and emergence. This study was carried out to evaluate the quality status of African nightshade (Solanum scabrum and S. villosum Miller) seed produced by farmers in Kenya. A household survey was carried out in 240 farms using a structured questionnaire and a total of 164 samples of farm saved, market and certified seeds were obtained during this survey. Seed samples were tested in the laboratory at the University of Nairobi and Pearson’s correlation between seed quality and germination parameters was done. The analysis showed that majority of farmers use farm saved (50%) or seed purchased from the local markets (28%) which have low purity and germination rates. The farm, market and certified seed differed (p<0.05) significantly with regard to seed purity, moisture content, seedling vigor index and germination percentage. Farm saved and market seed had low seed purity 68.6% and 74% respectively, compared to certified seed 94.4%. In addition, only certified seed met the recommended moisture and germination percentage as per the International Seed Testing Association (ISTA) standards. There were significant (p≤ 0.05 and p≤0.01) positive correlation comparing seed quality and germination parameters, for example seed purity had significant positive correlation (r=0.76**) with germination percentage. This study affirms that the use of clean seed increases germination percentage of crops

INFLUENCE OF LAND USE AND SOIL MANAGEMENT PRACTICES ON THE OCCURRENCE OF NEMATODE DESTROYING FUNGI IN TAITA TAVETA, KENYA

Due to the increased concerns about the effect of agro-chemicals 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

EFFECT OF SOIL FERTILITY MANAGEMENT PRACTICES AND BACILLUS SUBTILIS ON PLANT PARASITIC NEMATODES ASSOCIATED WITH COMMON BEAN

On-farm and on-station field experiments were carried out to determine the potential of combining Bacillus subtilis with soil fertility management practices on plant parasitic nematodes and growth of common bean (Phaseolus vulgaris). The treatments were Bacillus subtilis (isolate K194), B. subtilis plus cow manure, B. subtilis plus mavuno, Bacillus subtilis plus farmer practice, manure alone ,mavuno alone with farmer practice as the control. The 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 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 was 1.6 and 4.5 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 evenness diversity. It can be 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

IMPACT OF LAND USE ON THE DISTRIBUTION AND DIVERSITY OF ENTOMOPATHOGENIC NEMATODES IN EMBU AND TAITA DISTRICTS, KENYA

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

NEMATODE COMMUNITY STRUCTURE AS INFLUENCED BY LAND USE AND INTENSITY OF CULTIVATION

This study was conducted to determine the effect of land use and intensity of land cultivation on the nematode community structure. The land use types represented in the study sites were natural forest, plantation forest, tea, coffee, napier grass, agroforestry, fallow and annual crop cultivation dominated by maize intercropped with beans. Nematode diversity and abundance decreased with intensity of land cultivation, with the natural forest being regarded as the benchmark. The decrease in nematode diversity was assessed using Shannon, Simpson and species richness indices and was used to reflect the underlying changes in physical, chemical and biological properties of soil environment. The highest maturity indices for free-living and plant parasitic index were recorded in the natural forest and intensively cultivated land under annual crops (maize/beans), respectively. Plant parasitic nematodes were predominant in soils that were under agricultural production while saprofagic nematodes dominated the forested land as exemplified by the ratios of free-living to plant parasitic which were, 5.18 and 0.54 in the natural forest and annual crop production systems respectively. Changes in the nematode community structure, as exhibited by diversity indices, may be a reflection of real differences in the soil characteristics and changes in ecosystem functions