A study was conducted to determine the extent of genetic diversity among African cassava (Manihot esculenta Crantz) accessions resistant to the cassava mosaic virus disease (CMD), using simple sequence repeat (SSR) markers. The accessions included a breeding stock (clone 58308), five improved lines, 62 CMD resistant and 10 CMD susceptible landraces. Genetic diversity was assessed among accessions in five cluster groups derived from UPGMA analysis on data from 18 SSR primer pairs. Average gene diversity, He, was high in all cluster groups, with an average heterozygosity of 0.591 ± 0.061. The estimator of inbreeding Fis revealed a low level of inbreeding within groups and averaged À0.262 ± 0.142. Gene diversity among all accessions was 51.4% and gene diversity within cluster groups was 46.6%, while 4.8% was due to diversity between the different cluster groups. The amount of genetic differentiation measured by Gst and Fst were 9.6% and 12.1% respectively, indicating a weak genetic structure.

Six cassava cultivars, three yellow and three white-fleshed roots were crossed in a 3 9 3 topcross mating design to generate nine F 1 populations. One thousand, one hundred and ten botanical seeds from the 9 populations were sown in pots and maintained for 42 days in a screenhouse. The emerged seedlings were transplanted to the field in April, 2010 alongside their parents (from stem cuttings), family by family. Four hundred and sixty-four progenies survived and were harvested. Both field and laboratory data were used to evaluate total carotene content (TCC), dry matter content (DMC), storage fresh root yield (SFRY) and other root quality traits. Seed germination for different populations ranged between 15.5 and 80.9 % with a mean of 43.19 %. Phenotypic variation in DMC, TCC, SFRY, biomass, root number, harvest index and cassava mosaic disease (CMD) were recorded in all the families. Average values for the populations were TCC: 4.59 lg g -1 , DMC, 33.58 % and SFRY, 18.42 t ha -1 . Narrow sense heritability by midparent-offspring regression analysis and genetic gains were estimated for TCC, DMC, SFRY and reaction to CMD. TCC, DMC and CMD gave high heritability estimates of 0.73, 0.83 and 0.84, respectively. SFRY, on the other hand, had a low heritability estimate (0.15). TCC was negatively correlated with DMC across all evaluation stages and locations. There were very high levels of variation in the segregating F 1 progenies for all the traits. Also, narrow sense heritability estimate showed that genetic factors played a more important role than environmental factors for TCC, DMC and CMD, suggesting that reliable selection with simple recurrent phenotypic selection would be rewarding.

Twenty-two accessions of okra (Abelmoschus esculentus), maintained at the Plant Genetic Resources Centre, Bunso, Ghana, were assayed for diversity in esterases, and total and storage proteins. A total of 34 reproducible and easily scorable bands were exposed with the number of bands per accession ranging from one to 21. All but nine of the bands were polymorphic. Storage proteins were the most diverse while esterases revealed the least diversity. Similarity matrices were calculated using the Jaccard coefficient, and input into cluster analysis. The phenogram produced by the UPGMA of the Jaccard similarity matrix from the pooled data of the esterases, and total and storage proteins revealed three major clusters at the 55% level of similarity. Accession 5 collected from Nyinguto was relatively distant from the other main clusters and separated at the 42% level of similarity. The second and third clusters comprised 11 and 10 accessions, respectively. It was observed that 18 out of the 22 accessions were distinct accessions. Similarity indices ranged from 29% to 100%. The wide range of similarity indices, coupled with the clustering of accessions, suggests useful variability in the collection for genetic conservationists and plant breeders.

The incidence of alfalfa mosaic virus (AMV) in lucerne seed and pods during maturation, when monitored by sap transmission to Phaseolus (infective virus) and ELISA (AMV antigen), showed that infective virus incidence decreased rapidly with maturation, whereas antigen incidence declined slowly and was always higher than infective virus. Infective virus and antigen incidence were higher in mature seed of cv. Maris Kabul than cv. Europe because virus inactivation/degradation were more rapid in cv. Europe. Seed infection with virus originating from pollen, ovules or both was found in pods and seeds 12–15 days after pollination between healthy or AMV‐infected plants; this was before maturation‐associated virus inactivation. Ovule transmission was more frequent than pollen transmission. AMV antigen was present in embryos and testas of mature seed; infective virus only in embryos. Non‐infective but ELISA‐positive antigen in testa extracts accounted for the higher incidence of ‘seed‐borne AMV’ compared with embryo‐associated seed transmission to seedlings. Tests with dry mature seed either underestimated (infectivity tests) or overestimated (ELISA) eventual seedling infection. Infectivity and ELISA tests gave identical incidence values for 17 to 29‐day‐old seedlings.

Virus species causing cassava mosaic disease have been categorized into three classes based on their reaction with monoclonal antibodies (MAbs) and their distribution (2). These viruses have different, scarcely overlapping distribution: African cassava mosaic begomovirus (ACMV) occurs in Africa west of the Rift Valley and in South Africa; East African cassava mosaic (EACMV) occurs in Africa east of the Rift Valley and in Madagascar; and Indian cassava mosaic virus (ICMV) occurs in India and Sri Lanka (2). During 1998, surveys were conducted in farmers' fields in Ghana to assess the incidence and reaction of local cassava cultivars to cassava mosaic disease. Leaf samples from symptomatic plants were indexed by triple antibody sandwich-enzyme-linked immunosorbent assay with crude extracts and monoclonal antibodies obtained from the International Institute of Tropical Agriculture (IITA). Each sample was assayed with monoclonal antibody SCR 23, which detects ACMV and EACMV, SCR 33, which detects ACMV, and SCR 58, which detects ICMV. None of the samples reacted with SCR 58. Two of the samples collected from the western region of Ghana produced strong reactions with MAb SCR23 but did not react with ACMV-specific MAb SCR 33. This result was consistent in three separate experiments conducted on the samples, confirming that the virus was EACMV and not ACMV. The results extend the work by Ogbe et al. (1) and provide further evidence of the occurrence of EACMV in west Africa. References: (1) F. O. Ogbe et al. Plant Dis 83:398, 1999. (2) M. M. Swanson and B. D. Harrison. Trop. Sci. 34:15, 1994.

Severity of storage rots in different sections of white yam tubers (Dioscorea rotundata Poir.) was investigated. Yam samples with rots were collected from a yam barn and from selected markets in Accra, Ghana, to identify the most predominant pathogens associated with the rots. Nine fungal spoilage microorganisms, including Aspergillus flavus, Aspergillus niger, Botryodiplodia theobromae, Fusarium culmorum, Fusarium oxysporium, Fusarium sp., Penicillium brevi-compactum, Penicillium sp. and Rhizopus stolonifer and a bacterium species Erwinia carotovora were identified. The mean incidence of occurrence of the organisms on rotten tissues ranged from 1.2% to 28.5%. Of the 10 microorganisms isolated, B. theobromae, F. oxysporium and R. stolonifer were the most frequently encountered spoilage microorganisms in the markets. E. carotovora, Fusarium solani and Penicillium sp. were relatively sparse (incidence not exceeding 3%) compared to the other yam spoilage microorganisms. The surface area and weight of necrotic tissue induced by the spoilage fungi in the various zones of the tubers over a 28-day storage period were assessed. All the spoilage microorganisms produced rots in the yams, although to different degrees. The severity of the rots increased in weight and area over the period when the tubers were in store but were normally not significantly different in the zones of tubers. There was, however, a linear progression of rots in the various zones of the yam tubers. Although there was generally no significant (P ! 0.05) difference in the severity of rots induced by the different microorganisms in the tubers, R. stolonifer commonly induced more rot in the zones of the tubers compared to B. theobromae and F. oxysporium.

Phenotypic and seed protein analyses were performed on 31 accessions of Lima bean assembled in Ghana. Data on 16 phenotypic characters consisting of eight quantitative and eight qualitative were analysed. There were significant differences among the accessions based on the eight quantitative characters. Seed protein analysis showed 17 bands with relative mobility of bands, which ranged from 0.01 to 0.86. An ordinal logistic regression analysis showed significant evidence for seed coat, pod beak shape and seed size association. Cluster analysis based on both phenotypic and protein data provided evidence for differences among the accessions. Quantitative characters were associated with some specific clusters.

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