PHD-Department of Agricultural Sciences and Technology

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Browsing PHD-Department of Agricultural Sciences and Technology by Author "Nyamwamu, Paul Ateng’a"

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    Vector and pollen transmission mechanisms for the maize lethal necrosis disease in Nairobi County, Kenya
    (Kenyatta University, 2024-11) Nyamwamu, Paul Ateng’a
    Maize (Zea mays L.) is a major staple food crop for over 100 million people in sub-Saharan Africa where it is produced on 35 million hectares annually. The maize yields have stagnated at 1.75 t/ha against the potential of 6t/ha due to pests and diseases. Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV) induces the maize lethal necrosis (MLN) disease which threatens maize production. The disease reduces yields by 90-100%. Effective management of the disease is impended by gaps in the knowledge on MCMV and SCMV transmission mechanisms. Frankliniella occidentalis (Pergande) and F. williamsi (Hood) transmission efficiency of MCMV remains unclear. Knowledge on the genetic diversity of F. occidentalis and F. williamsi transmitting the virus has not been conclusive. The role of these thrips on MLN disease spread was determined through vector-virus assay tests. The genetic diversity of thrips infesting maize affected by MLN disease was determined. Colonies of vector thrips; F. occidentalis were reared on Phaseolus vulgaris pods and F. williamsi on maize stalk cuttings. Virus transmission efficiency of vector thrips was compared at 0.5, 1.0, 1.5, and 24-hour virus acquisition access periods (VAAP). Transmission competence of MCMV by vectors was compared at 1, 3, 24, and 48-hour inoculation access periods. Persistence transmission of MCMV by vector thrips was investigated at retention periods of 1, 2, 3, and 4 days. Rates of new plant re-infections (4, daily cycle) was determined. Larvae thrips transmission of MCMV was tested. Virus transmission via pollen was tested through mechanical inoculation and by vector thrips fed on infected pollen. Thrips species and their biotypes involved in MCMV epidemiology were identified by analysis of mitochondrial cytochrome c oxidase COI-5ʹ (DNA barcodes). Sixty-four plants were each inoculated, placed in 16 wooden cages covered with nylon-mesh (114 μm) in a complete randomized design arrangement with four replicates. Virus transmission was tested by ELISA and PCR. Maize lethal necrosis induced disease severity (5.00 ± 0.1) than MCMV and SCMV (4.2 ± 0.4, 4.1 ± 0.3). Sugarcane mosaic virus induced leaf symptom severity similar to that of MCMV. Transmission efficiency of MCMV by F. occidentalis was highest at 24 hour AAP (81.2 %) while 0.5 AAP had the least (6.2 %); an indication of transmission without a latent period. Frankliniella occidentalis had higher (51.6 %) MCMV transmission rates than F. williamsi (26.6 %). Maize chlorotic mottle virus transmission rate of F. occidentalis rose while that of F. williamsi decreased after 24hour IAP. Means of infected plants by both thrips were significantly different (t = 2.77, DF = 362.97, P = 0.006). Plants inoculated by F. occidentalis had higher symptom severity than those inoculated by F. williamsi (0.27, 0.19), with 1.17 mean difference (higher by 17.6 %). Plants re-infected by thrips were significantly different (F = 10.27, DF = 2, 17, P = 0.001). Frankliniella occidentalis re-infected more plants than F. williamsi (0.32 ± 0.04, 0.27 ± 0.04) at 0.12 LSD. Plants inoculated by F. williamsi after day 1 post acquisition period were significantly less from those inoculated by F. occidentalis (F = 5.66, DF = 2, 17, P = 0.013). Both thrips transmitted MCMV in a non-persistent pattern and thus should be targeted in IPM strategies for MCMV control. Maize chlorotic mottle virus was mechanically transmitted and detected in 3.12 % of the total plants tested while SCMV was not detected in any of the leaf samples. None of the thrips transmitted the viruses from infected pollen. About 95% of the sampled thrips were of Frankliniella genus with a similarity range of between 80.04 - 81.98%. Frankliniella genus is a major vector for MCMV spread and therefore IPM control strategies should target the thrips for effective MLN management