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

Tomato Management Practices and Diseases Occurrence in Mwea West Sub County

Tomato is an important crop in Mwea West Sub County, Kirinyaga County, Kenya. A survey was carried out in the area to investigate tomato management practices, diseases and pests that hinder tomato production. The study endeavoured to establish farmers’ knowledge on fusarium wilt disease and root-knot nematodes and the methods used to control them. Data was collected from two hundred and eighteen randomly selected small holder producers who were equally distributed in the study area. Data collected included tomato management practices, diseases and pests that hinder production. Majority (85.3%) of the respondents were males while a few (14.7%) were female. The respondents (71.6%) indicated that tomato was the most important crop grown for income generation in the area. Most important varieties grown were cultivars, Safari, Kilele F1, Prosta F1 and Rio- Grande. Most important diseases affecting tomato crop were; early blight (Alternaria solani) and late blight (Phytophthora infestans), fungal wilts (Fusarium sp. Verticillium sp. Rhizoctonia sp.) and bacterial wilt (Ralstonia solanacearum). Plant parasitic nematodes and pests (thrips, aphids, spider mites) were also reported in the study area. There was a significant(P<0.05) association between the following variables; type of land owner and major use of land, type of land owner and cropping system, source of agricultural information and whether or not to apply pesticides into the soil. Farmers were quite knowledgeable about tomato farming as they had access to information from various sources; however there are still major gaps in knowledge especially on diseases and pests. Keywords: Tomato, diseases, pests, nematodes, managemen

Evaluation of resistance reaction of maize germplasm to common foliar diseases in Kenya

Use of resistance varieties is the most practical method of managing crop diseases. There is a great variation in terms of resistance reaction to diseases among the various maize germplasm and with the liberization of seed market, the sector has witnessed proliferation of massive introduction of new varieties whose reaction to diseases cannot be ascertained. This study was conducted to evaluate the reaction of maize varieties to northern leaf blight (NLB), common maize rust, gray leaf spot (GLS) and maize streak disease (MSD). The experiment was conducted at Kabete Field Station, University of Nairobi for two seasons namely short rains and long rains. The germplasm was bought from the commercial seed stockists and the landraces obtained from KARI Katumani and from farmers. The diseases were assessed by monitoring and scoring for disease incidence and severity. The appropriate scoring keys were used for determination of disease severity. All the varieties showed symptoms of the four diseases in both seasons but the intensity of the diseases differed significantly among the different varieties. Disease incidence was highest for common rust with a mean of 14.29% for the variety DH04, while disease incidence was highest (19.21%) for northern leaf blight in season two for Kinyanya which is a landrace. Gray leaf spot and the common smuts had the lowest mean incidence ranging from 0 to 0.25% for common smut and 0 to 2.6% for gray leaf spot. Season two had comparatively higher disease incidence means compared to season one. Meteorological data showed that season two had more rains and this explains the reasons behind this. Though all the varieties screened were found to be affected by the diseases to various levels, the varieties displayed significant differences in the disease incidence and severity. This shows that use of resistance varieties should be considered in the management of maize diseases. Focus should also be on pyramiding genes for resistance in the breeding programmes to develop varieties with multiple resistance to different diseases. Key words: Disease incidence and severity, disease score, symptoms, varietie

Reaction of Selected Coffee Germplasm to Root-Knot Nematodes in Kenya

Coffee is one of the most important cash crops in Kenya and a leading export earner. Nematodes are among the most important biotic constraint in coffee production in Kenya and crop improvement work has mainly been breeding for resistance to diseases such as coffee berry disease and coffee leaf rust. However resistance has been used successfully in other coffee producing countries and it is one of the most economical and practical nematode management strategies. A greenhouse study was conducted to test the response of local and exotic coffee germplasm to root knot nematodes (RKNs). Ten (10) cultivars provided by Coffee Research Foundation (CRF) were tested for resistance to Meloidogyne incognita under greenhouse conditions (25±2oC). Nematodes were extracted from the roots using Modified Baermann Technique and enumerated using Cobbs slide. After 90 days of plant growth, the disease severity was evaluated and the experiment repeated twice. Galling indices (GI), egg mass indices (EMI) and nematode populations recovered from soil samples indicated a range of responses from resistant to highly susceptible.  Three breeder’s lines including Robusta tree 1, Robusta tree 2 and Robusta tree 3 were rated resistant with galling indices of 1.2-3.0. This study has demonstrated the potential of host resistance as a strategy in the management of nematodes in coffee for increased productivity. Field evaluation needs to be conducted to confirm these findings. The identified resistance sources can be utilized to deploy resistance genes to improve existing varieties that have high commercial value but lack resistance to nematodes.   Key words: Resistance, susceptible, galling indices, nematode population, cultivar

Plant Parasitic Nematodes Associated With Coffee in Kenya and Factors Influencing their Occurrence, Abundance and Diversity

Frequent detection of galls on coffee roots has raised concerns of nematodes in coffee production systems in Kenya. This study aimed at determining the occurrence of nematodes associated with coffee in Kenya and the role of crop management, cultivars, soil properties and agro ecological zones on the abundance and frequency of nematodes. A survey was conducted in the prime coffee growing areas in 10 counties namely; Machakos, Makueni, Kiambu, Embu, Kirinyanga, Nyeri, Meru, Kisii, Nandi and Trans-Nzoia. Nematodes were extracted using a combination of centrifugal floatation and Modified Baermann techniques and identified to genera level.  Nutrient analysis was carried out using the Double Mehlich method. Results showed that nematodes belonging to 30 genera were recovered from coffee agro-ecosystems. Plant parasitic nematodes were the most prevalent with 64% frequency (19 genera) of occurrence followed by bacterial feeders at 24%. The genus Tylenchulus, Meloidogyne and Pratylenchus were the most dominant across all the coffee growing areas. Coffee farms in the coffee-tea zones (Upper Midland 1) harbored the highest numbers of plant parasitic nematodes, followed by Upper Midland 2 and least in the marginal coffee growing zones (Upper Midland 3). Well managed farms had less plant parasitic nematodes compared to neglected farms. K and P significantly contributed to the variation in the nematode community composition. This study demonstrated the prevalence of plant parasitic nematodes, factors that influence their abundance and distribution and justifies need for further management of nematodes in coffee production. Key words: Abundance, agro-ecologial zones, diversity, nematode genera, nutrient

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

A survey of root knot nematodes and resistance to Meloidogyne incognita in sweet potato varieties from Kenyan fields

AbstractThe root knot nematode, Meloidogyne is one of the most economically damaging plant parasitic nematode groups, and are widely distributed in Kenyan agro-ecosystems. The aim of this study was to determine the diversity of Meloidogyne species in Kenyan sweet potato fields and identify sweet potato varieties that exhibit resistance to M. incognita. Meloidogyne species were collected from Nyanza, Western, Eastern and Central Provinces of Kenya. Mitochondrial DNA was used to differentiate Meloidogyne species. The most common species in all sampled regions was M. incognita. Meloidogyne hapla was recorded for the first time in Kenyan sweet potato growing areas (Mosocho, Matayos, Teso South, Manyatta, and Nzaui sub-counties), while M. enterolobii was observed in Kiharu, Matayos and Mosocho sub-counties and a novel Meloidogyne sp. was identified in Kiharu sub-county. Seventy-two sweet potato varieties collected from both agricultural fields and research stations in Kenya were evaluated for resistance to M. incognita under greenhouse conditions in two separate trials. Known susceptible (Beauregard) and resistant (Tanzania) sweet potato varieties were included as controls. Responses of sweet potato varieties to M. incognita infection was assessed by the number of eggs present and level of galling on a scale of 1–5, where 0 = 0 galls and 5 ≥ 100 galls. The reproduction index (RI) was used to classify the varieties as resistant or susceptible. There was a significant difference (P < 0.001) in the number of eggs, GI and RI among the varieties tested. Forty nine sweet potato varieties were considered very resistant and may be used in breeding programs to incorporate resistance against M. incognita into commercial cultivars of sweet potato or to use them in crop rotation programmes for management of RKN. The results on Meloidogyne species diversity in Kenyan sweet potato fields will also be useful in nematode management programs