Screening Cassava Cultivars for Resistance to Cassava Viruses and Cassava Green Mites under Advanced Yield Trials in Kenya
Loading...
Date
2024-10
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Kenyatta University
Abstract
Cassava is a perennial shrub with an edible carbohydrate-rich root growing in the
tropical and subtropical areas of the world including Kenya. Africa accounts for over
half of the world’s cassava production. Despite its economic significance, cassava is
affected by a myriad of biotic and abiotic constraints among which are viral diseases
and pests. The objective of the study was to identify high yielding cultivars resistant to
Cassava mosaic disease (CMD), Cassava Brown Streak Disease (CBSD) and cassava
green mites (CGM). Fifteen cassava cultivars (92/00061, 990005, Kileleshwa, TC 14,
TC17, TC19, TC2, TC20, TC4, Thika2, Thika5, Thika6, TME419, Wakahiu3 and
Wakahiu4) were selected from KALRO breeding site in Thika and local farmers, and
planted at KALRO, Kiboko, a high pressure zone for CMD, CBSD and CGM. The
cultivars were replicated in three blocks and data collected from 30 plants per block.
Symptomatic descriptors were used to score plants that were affected by CMD, CBSD
and CGM. Incidence and severity of both diseases and CGM was scored at 3 and 6
months after planting (MAP) while root tubers were harvested at 9 MAP. Disease
severity (DSV) was assessed using a scale of 1 to 5 where 1 denoted no symptoms and
5 corresponded to severe disease symptoms while the disease incidence (DSI) was the
percentage of the number of leaves infected. Disease development over time was
monitored using area under disease progress curve (AUDPC) and area under severity
index progress curve (AUSiPC). DNA and RNA were extracted from leaf samples for
molecular detection of CMBs and CBSVs through Polymerase chain reaction (PCR)
and Reverse transcriptase PCR (RT-PCR) respectively. Data was imported into
Statistical Analysis Software (SAS) and variance analyzed using ANOVA. In this
study, the varieties were classified as tolerant and susceptible. Genotypic differences
for either diseases incidence (INC) or severity (SVY) and marketable root yield (MRY)
was significant (P≤0.05) indicating that cassava yield and response to CMD and CBSD
was genotype-dependent. Based on this, the genotypes were grouped into disease
tolerant (DT) and disease susceptible (DS) classes with significantly higher MRY of
23.8 t/ha bulked by a DT genotype TME-419 and least MRY of 2.1 t/ha recorded in a
DS cultivar Thika2. Incidences and severity of CMD, CBSD and CGM showed a
negative correlation with yield and this confirmed the negative impact of both diseases
and CGM to cassava yield. Molecular diagnostics using PCR and RT-PCR further
confirmed the presence of CMBs and CBSVs in both tolerant and susceptible cultivars
while the viruses were not detected in resistant cultivars in the present study. The
variants of CMBs and CBSVs that were identified were EACMV & ACMV and CBSV
& UCBSV respectively. Four (990005, TC4, TC14 & TME419) high yielding and DT
cassava genotypes identified in the current study could potentially be used as parents in
future breeding programs for introgression of tolerance traits in farmer preferred but
susceptible local land races.
Description
A Thesis Submitted in Partial Fulfilment of the Requirements for the Award of the Degree of Master of Science (Biotechnology) in the School of Pure and Applied Sciences of Kenyatta University, October 2024
Supervisors:
1.Steven M. Runo
2.. Charles Orek