Rob Melis

Response of Potato Genotypes to Bacterial Wilt Caused by Ralstonia Solanacearum (Smith)(Yabuuchi et al.) In the Tropical Highlands

Potato varietal resistance to bacterial wilt disease caused by Ralstonia solanacearum Yabuuchi et al., (Microbiology and Immunology 39:897–904, 1995) is the best management of the disease. Because the causal pathogen exhibits strong host-pathogen-environment interaction, screening the potential parents for resistance under the target growing environmental conditions is the first important step for effective resistance breeding. The objective of this study was to determine the response to bacterial wilt of selected potato genotypes currently grown by farmers in Kenya and candidate clones from the International Potato Center to identify parents that can be used in the local breeding program to develop resistant cultivars. A study was carried out at Kabete, Kenya for three consecutive seasons between November 2011 and February 2013. Thirty six potato genotypes were planted on an inoculated field at the Kenya Agricultural Research Institute (KARI), Kabete using alpha lattice experimental design with three replications. Data collected were days from planting to onset of wilting (DTOW), bacterial wilt incidence (BWI), total tuber weight (ton ha−1) (TTW), total tuber numbers/hectare (TTN), proportion of ware sized tubers (PWTTW), proportion of symptomatic tubers based on weight (PSTTW), proportion of symptomatic tubers based on tuber numbers (PSTTN) and latent infection (LI) of the tubers. Almost, all the potato genotypes evaluated in this study were susceptible to bacerial wilt. Ranking of genotypes based on resistance differed among the three seasons. On average, the three most resistant genotypes were Kenya Karibu, Kenya Sifa and Ingabire. The study identified eight potato genotypes (Meru, Ingabire, Kenya Karibu, Sherekea, Kihoro, Tigoni, Bishop Gitonga and Cangi) to be used as promising parents for subsequent crosses. The chosen genotypes are prolific in pollen production and popularly grown by Kenyan farmers.ResumenLa Resistencia varietal de la papa a la enfermedad de la marchitez bacteriana, causada por Ralstonia solanacearum Yabuuchi et al., (Microbiology and Immunology 39:897–904, 1995), es el mejor manejo de la enfermedad. Considerando que el agente patógeno causal presenta una interacción fuerte hospedante-patógeno-ambiente, las pruebas de padres potenciales para resistencia bajo condiciones ambientales de crecimiento enfocadas, es el primer paso importante para el mejoramiento efectivo para la resistencia. El objetivo de este estudio fue determinar la respuesta a la marchitez bacteriana de genotipos de papa selectos que actualmente se cultivan por productores en Kenia y clones candidatos del Centro Internacional de la Papa, para identificar padres que pudieran usarse en el programa local de mejoramiento para desarrollar variedades resistentes. Un estudio se efectuó en Kabete, Kenia, durante tres ciclos consecutivos entre noviembre de 2011 y febrero de 2013. Se plantaron 36 genotipos de papa en un campo inoculado en el Instituto de Investigaciones Agrícolas de Kenia (KARI). En Kabete se usó un diseño de látice alfa con tres repeticiones. Los datos tomados fueron los días desde la siembra hasta el establecimiento del marchitamiento (DTOW), incidencia de la marchitez bacteriana (BWI), peso total de tubérculo (ton ha-1) (TTW), número total de tubérculos/ha (TTN), proporción de tubérculos de tamaño comercial (PWTTW), proporción de tubérculos sintomáticos con base en el peso (PSTTW), proporción de tubérculos sintomáticos con base en el número de tubérculos (PSTTN) e infección latente (LI) de los tubérculos. Casi todos los genotipos de papa evaluados en este estudio fueron susceptibles al marchitamiento bacteriano. La clasificación de los genotipos con base en la resistencia varió entre los tres ciclos de cultivo. En promedio, Los tres genotipos más resistentes fueron Kenya Karibu, Kenya Sifa, e Ingabire. El estudio identificó ocho genotipos de papa ((Meru, Ingabire, Kenya Karibu, Sherekea, Kihoro, Tigoni, Bishop Gitonga y Cangi) para usarse como progenitores prometedores para cruzas subsecuentes. Los genotipos seleccionados son prolíficos en producción de polen y se cultivan popularmente por los productores kenianos.

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.

Breeding options for improving common bean for resistance against bean fly (Ophiomyia spp.): a review of research in eastern and southern Africa

Bean fly (Ophiomyia spp.) is a key pest of common bean (Phaseolus vulgaris L.) throughout eastern and southern Africa. It is known to cause total crop loss especially under drought stress and low soil fertility. This review underscores the importance of bean fly to bean production. It discusses the research achievements on genetic improvement of common bean for resistance against bean fly attack and highlights further opportunities available for rapid advance. The paper dwells on conventional breeding approaches and possibilities for utilization of marker-assisted selection. Mechanisms of common bean resistance to bean fly have been considered with a view to understand the genetic control. To maximize the effectiveness of host-plant resistance against bean fly, multiple insect resistances should be incorporated into a single bean genotype in order to ensure stability. However, this should be within the background of integrated pest management strategy.

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.

Phenotypic characterisation of potato (Solanum tuberosum) genotypes in Uganda

Identification of genetic variation and interrelationships among germplasm collections is essential for parental selection and trait identification among parents for use in breeding programmes. The aim of this study was to characterise 48 potato genotypes to identify suitable parents for crop improvement purposes. Genotypes were evaluated in the field using an alpha lattice design with three replications at the Kachwekano and Karengyere research stations in Uganda. Site had significant effects on genotype performance for all measured parameters. Genotypic effects were significant (p < 0.01) for total tuber yield, main stem number and plant height. The mean tuber yield for the two sites was 29.8 t ha−1 and tuber yield was higher in Kachwekano than Karengyere. The highest-yielding genotype in Kachwekano was 396038.105 (54.5 t ha−1) and in Karengyere was NAKPOT5 (50.9 t ha−1). Significant positive correlations (p ≤ 0.001) were observed between tuber yield and plant height; duration of flowering, and days to flowering and plant height. The most stable genotypes with regard to tuber yield were Rutuku, 395112.32, 395017.14 and 393220.54. Cluster analysis revealed three principal clusters with nine subclusters. Variation for the different traits exhibited by genotypes in this study should be exploited in crop improvement programmes.

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.