Paul Shanahan

Combining Ability Analysis of Storage Root Yield and Related Traits in Cassava at the Seedling Evaluation Stage of Breeding

Selection of genotypes at the seedling evaluation stage of cassava breeding for advancement is largely based on high heritability traits. Moreover, combining ability and family data at this stage are rarely analyzed, leading to a loss of potential genetic information. This study was conducted to estimate the general combining ability (GCA) of nine cassava parents and specific combining ability (SCA) of their crosses for storage root yield (SRY) and related traits, as well as to determine the gene action controlling these traits. Thirty-six full-sib cassava families were developed from a 9 x 9 half-diallel mating design and evaluated in a 6 x 6 triple lattice design. The family mean squares (MS) were significantly different for all traits assessed, suggesting significant differences among families for all traits. The GCA MS were significant for all traits, whereas SCA MS were significant for only storage root number (SRN). Percentage sum of squares attributable to GCA accounted for >50.0% of variability expressed by families in five of the seven traits studied, indicating predominance of additive gene effects in controlling expression of most traits. Parent CT4 was the best general combiner for SRY, SRN, and resistance to cassava brown streak and mosaic diseases, while crosses NASE3 x CT2, CT5 x CT3, and NASE3 x CT4 had desirable SCA effects for SRY. Correlation analysis indicated a possibility of selecting for SRY and quality traits simultaneously. Simultaneous selection of these traits at the seedling generational stage of cassava breeding could reduce the cassava breeding cycle.

Genetic variation and association among factors influencing storage root bulking in cassava

Cassava ( Manihot esculenta Crantz) is an important storage root crop with largely unexplored and unexplained potentially valuable genetic variability. Genetic variability is important in selecting suitable genotypes for crop improvement. The present study was aimed at assessing the extent of variability in cassava storage root bulking, based on fresh storage root yield accumulated over time. Twelve cassava genotypes were evaluated in a randomized complete block design at three contrasting locations in Uganda. Assessments were done from 5 to 13 months after planting at intervals of 2 months. Genotype, harvest time, location and their interactions were significantly different for fresh storage root yield and most of the other traits assessed. Estimates of variance components revealed that a large portion of the phenotypic variance was accounted for by the genotypic component for all traits assessed indicative of substantial genetic variability among the genotypes evaluated. This genetic variability is important in a hybridization and/or selection programme because it implies that significant genetic gain through phenotypic selection is possible for the traits assessed. Fresh storage root yield was positively and significantly correlated with storage root girth, harvest index, shoot mass and storage root number. The information generated will inform future breeding initiatives to develop early-bulking cassava genotypes with farmer-preferred traits in Uganda.

Farmers' knowledge of cassava brown streak disease and its management in Malawi

ABSTRACT There is little information on farmers’ knowledge about cassava (Manihot esculenta Crantz) brown streak disease (CBSD), despite extensive studies on incidences and severities. The objective of this study was to assess farmers’ knowledge of CBSD diagnosis and management. The study was conducted in three districts of Malawi by administering semi-structured interviews in combination with disease incidence and severity surveys. Farmers’ knowledge of disease diagnosis and management was associated with CBSD incidence and severity. High levels of knowledge about CBSD were observed in areas with high disease incidence. Only 10.1% of the farmers were capable of identifying the foliar symptoms of the disease. On average, 75.0% and 71.7% of the farms had leaf and storage root incidences, respectively. At harvest, 88.3% of the farmers’ fields exhibited storage root necrosis. CBSD leaf and storage root severities differed significantly (P < 0.001) from one district to the other and between varieties. Most farmers were found to lack a source of clean planting material. High needs for extension services on cassava cultivation methods and pest management were identified, but few farmers received such services. The lack of new improved varieties was reported as the most important constraint of cassava production, beyond CBSD. Education of farmers on the efficient management of this viral disease through selection of clean planting material should be provided. Additionally, the development of early root bulking cultivars as a long-term solution in avoiding CBSD impact should be supported.

Genetic variation in cassava for postharvest physiological deterioration

Cassava storage roots start deteriorating 1 to 3 days after harvest due to postharvest physiological deterioration (PPD) which reduces the potential of cassava as a commercial crop. The objective of this study was to examine the extent of genetic variability in cassava for PPD. Genetic variability is important in selecting suitable genotypes for crop improvement. Twelve cassava genotypes were evaluated in a randomized complete block design at three different locations in Uganda for five different harvest times, and PPD assessment was done 7 days after each harvest time. Genotype, location, harvest time, genotype × harvest time and harvest time × location were highly significantly different for PPD. Estimates of phenotypic, genotypic and environmental variance components revealed that a large portion of the phenotypic variance was accounted for by the genotypic component which is an indication of substantial genetic variability among the test genotypes for PPD. This genetic variability is important in a breeding programme because it indicates that significant genetic gain through phenotypic selection is possible for PPD. The results give guidance to breeding efforts for improved shelf life of cassava storage roots.

Cassava mosaic disease and associated viruses in Zambia: occurrence and distribution

Cassava mosaic disease (CMD) is caused by cassava mosaic begomoviruses (CMBs), which are transmitted by the whitefly (Bemisia tabaci) in sub-Saharan Africa. CMD and the pathogenic viruses associated with cassava were assessed in a country-wide survey in seven provinces of Zambia between April and May 2009. CMD incidence was highest in Northwestern (71.2%) and lowest in Western (34.3%) provinces. Disease symptoms were severe in Eastern (3.94) and Lusaka (3.88), moderate in Central (3.54), Luapula (3.48) and Northern (3.31) and mild in Northwestern (3.01) and Western (2.50) provinces. In addition, the numbers of adult whitefly were assessed on cassava fields, and found to be highest in Lusaka (2.12) and lowest in Central (0.02) province. Polymerase chain reaction detected two virus species: African cassava mosaic virus (ACMV) and East African cassava mosaic virus (EACMV), that occurred as single and dual infections in 65.4% (ACMV), 25% (EACMV) and 9.6% (ACMV + EACMV) of the positive reactions. None of the samples were positive for EACMV-Ug. This is the first comprehensive report of CMD and the pathogenic viruses infecting cassava in Zambia.

Analysis of phenotypic variability for yield and quality traits within a collection of cassava (Manihot esculenta) genotypes

Abstract Genetic diversity is essential for crop improvement and knowledge of the genetic variability within a breeding population facilitates its future exploitation. This study examined the phenotypic and genetic variability for specific cassava key traits in a collection of 30 genotypes at five environments in Rwanda. Seventy-three percent of the variation in dry matter content (DMC) and 98% of the variation in total carotene (TC) content was explained by genotypes. The broad-sense heritability (Hbs) of TC was high (99.2%) and the predicted genetic advance (GA; %) from selection was 159.7%. The phenotypic variance coefficients for all traits were higher than the genotypic variance coefficients. The highest Hbs (%) and GA for carotenoids was an indication that conventional breeding could improve carotenoid content in cassava using simple recurrent selection. The postharvest physiological deterioration (PPD) showed significant (p < 0.05) negative correlation and interrelation with TC and DMC, indicating that the high-TC and low-DMC cultivars could have delayed PPD. The higher-TC genotypes could be used to improve TC in the cassava population and consequently could contribute to the development of delayed-PPD cultivars. The results confirm that there is useful variation for key traits within the collection that could be indicative of a broad range of useful variability in natural populations, landraces and cultivars that could be exploited for breeding.

Genetic inheritance of pulp colour and selected traits of cassava (Manihot esculenta Crantz) at early generation selection: Genetic inheritance of pulp colour and selected traits of cassava

BACKGROUND The early generation selection of cassava quantitative and qualitative traits saves breeding resources as it can shorten breeding schemes. Inheritance analysis provides important breeding information for developing new improved varieties. This study aimed at developing an F1 segregating cassava population and determining mode of gene action of pulp colour and selected traits at early generation selection (F1 seedling and clones). RESULTS The 15 families exhibited significant (P < 0.05) phenotypic variation between offspring. The general combining ability (GCA) was significant for all traits except cassava brown streak disease on leaves, whereas specific combining ability (SCA) was significant for all evaluated traits. The Garukansubire and Gitamisi genotypes were the best general combiners for improving fresh storage root yield, while G1 and G2 were the best general combiners for improved carotenoid (yellow/orange pulp colour) and delayed physiological postharvest deterioration. The pulp colour had the highest GCA/SCA ratio and percent sum of squares due to GCA. CONCLUSION The 15 F1 families exhibited essential genetic diversity for cassava improvement. The expression of most cassava traits was controlled by both additive and non-additive gene action. The study elucidated the role of dominance effects over the additive effects for the evaluated traits. However, the pulp colour was predominantly controlled by additive gene action. This implies the possibility of improving cassava through conventional breeding using recurrent selection for most traits.

Gene action controlling cassava brown streak disease resistance and storage root yield in cassava

Cassava (Manihot esculenta Crantz) brown streak disease (CBSD) is currently one of the major constraints to sustained cassava production in Malawi. Its economic impact is mainly manifested in the storage roots where it causes pitting, necrosis and constriction. CBSD can effectively be managed by using resistant varieties as well as early harvesting especially if the varieties are early bulking (high storage root yield). However, development of resistant and high storage root bulking varieties requires an understanding of gene action controlling inheritance of the two traits. Currently, there is very little information in Malawi regarding the inheritance pattern and relative importance of general (GCA) and specific combining ability (SCA) of these two traits. Therefore, a study was conducted to determine mode of gene action as well as importance of GCA and SCA in the genetic control of CBSD resistance and storage root yield. Information generated is essential in the selection of parents and breeding strategies for an effective breeding programme. Thirty-six crosses were generated using a 6 × 6 North Carolina Design II and later evaluated for CBSD resistance and storage root bulking at two locations using a triple square lattice design. Data for the progenies were analysed using REML in Genstat (17th ed.) while family (crosses) data were analysed using a PROC Mixed procedure performed in SAS® 9.3 Software. The GCAf × location, GCAm × location and SCA × location interaction effects were highly significant (P < 0.01) for CBSDS and CBSDI. Pooled analysis across two locations showed highly significant (P < 0.01) variance among GCA due to females (GCAf) and GCA due to males (GCAm) for FSRY. These GCAf, GCAm and female × male interaction (SCA) effects accounted for 19.8, 33.4 and 46.7% of the sum of squares for CBSDS, respectively. The results suggested that additive gene effects predominantly controlled CBSD resistance and high storage root bulking. Four parental genotypes (Silira, Mulola, Phoso and Mkondezi) were identified as the best general combiners for the CBSD, high storage root bulking and other traits. Thirteen progenies exhibiting CBSD resistance and high storage root bulking were identified and selected for advancement.