Regression analysis of genotype x environment interaction in sweet potato

Summary Two experiments, each involving a set of 10 sweet potato clones, were conducted for three years at 4 sites (Ekona, Ebolowa, Nkolbisson, and Bambui Plain) in Cameroon. Data obtained were subjected to analysis of variance to determine the presence of genotype x environment (G x E) interactions, and to regression analysis to assess the performance of clones across anvironments. Environments were assessed in two ways: (i) the mean response of all clones (dependent assessment), and (ii) the average performance of a different set of clones (independent assessment).The first experiment (Expt 1) produced higher yields but had fewer stable clones than the second (Expt 2).The analysis of variance revealed that the clones interacted significantly with environments for all traits.The study has identified high yielding and stable sweet potato clones for distribution to growers in the major areas of cultivation in the tountry. Despite slight differences in numbers of clones judged stable by the various regression indices in the two methods of environmental assessments, the rankings of clones on the basis of their linear regression coefficients were similar. In a developing country like Cameroon, with limited resources and where sophisticated equipment for obtaining physical or biological measures of the environment may be lacking, the mean performance of genotypes may still be the most reliable measure of environment in evaluating the stability of performance of crop cultivars.     

Genotype × environment Effects on Severity of Cassava Bacterial Blight Disease caused by Xanthomonas axonopodis pv. manihotis

Nine cassava genotypes were grown for three years at six sites representing three agro-ecological zones in Nigeria to study their reaction to cassava bacterial blight (CBB), investigate genotype × environment (G × E) interaction patterns for their reaction to CBB, and to identify genotypes with stability to the disease, using the additive main effects and multiplicative interaction (AMMI) statistical model. Environments, genotypes and G × E interactions accounted for 71.8%, 12.0% and 16.2% of the treatment sums of squares (SS), and were highly significant (P<0.0001) for the disease, indicating that genotypes responded differentially to CBB infection across environments. Clones 30555, 91934, U/41044, and 4(2)1425 showed the least CBB disease ratings. Other clones showed erratic and fluctuating reactions to CBB from environment to environment and were thus considered unstable to the disease. CBB was most severe in 1989 (with a mean score of 2.46) and least so in 1990 (with a score of 2.06). The sites with the most disease were Ibadan, Ilorin and Ubiaja (1989), Ibadan and Ubiaja (1990) and Mokwa (1991). Because of the favourable conditions for disease development at those sites, they could be appropriate for screening cassava genotypes for CBB resistance. The AMMI model selected AMMI1 as the best predictor for CBB because it had the smallest actual root mean square prediction difference (0.37646), and explained 90.7% of the G × E interaction for CBB. The AMMI model was successful in selecting the genotypes 30555, U/41044 and 4(2)1425 and the environments Ibadan 1989, Ilorin 1989 and Onne 1990 with stability of reaction to the disease.     

Response of sweet potato clones to weevils and environments in Cameroon

Three sites differing in soil type, vegetation and weevil pressure, were used to evaluate the field performance of 18 improved sweet potato clones and two local cultivars, and their reaction to weevils (Cylas spp.) in Cameroon. Higher yields were obtained in the wet season than in the dry season. The highest yields were obtained in Nyombe in both wet and dry seasons; yields were halved in each of the other sites. Clones suffered more weevil damage in the dry season than in the wet season. Clones Tib 1, 1611, and 502 were very tolerant to weevils, whereas the high yielding clones 1112 and 1639 were very susceptible. Root yield was not correlated with root weevil damage, and yield potential and yield stability were found to be unrelated. Stability methods differed in identifying stable clones, and the rankings of clones varied with the trait measured. However, both Eberhart-Russell and Shukla stability methods rated clones 048, Tib 1, 1602, 1639, 002 and Njombe as stable for root yields. Also, there was no apparent relationship between weevil tolerance and cultivar adaptability. This study shows that the main sweet potato crop should be grown during the first cropping season when there is abundant moisture for slip sprouting and establishment, and when there is less soil cracking to expose roots to weevil infestation. The study suggests that although root yields are usually not adversely affected, weevil control measures should be seriously considered in commercial sweet potato production because the market value of the roots is much reduced by high weevil infestation. Lastly, it suggests that since fewer clones were found to carry high levels of tolerance to weevils, further research is necessary in breeding for weevil resistance.     

Variations in the yielding ability of sweet potato clones in grassland and forest ecologies in Cameroon

Twenty sweet potato genotypes were evaluated for four years in seven locations representing three agro-ecological zones in Cameroon, to study their performance across environments, to identify cultivars suitable for the different zones and to study their reaction to weevils and viruses in target sites. The best storage root yields (13.3 t ha^?1) were in the humid forest region and the lowest (7.4 t ha^?1) in the grassland region. Storage root counts were about the same in the humid forest and in the grassland regions. Root yields were not affected by weevil attack, but virus severity was associated with storage root yield depression. Shukla’s stability method was less conservative than the Eberhart-Russell method of assessing stability. The genotype grouping technique of Francis and Kannenberg was considered easier to use in grouping clones into classes of desirability and stability. The study suggests that it may not be necessary to select separately for different agro-ecological zones because high yielding cultivars of sweet potato developed for one zone are most likely to be adaptable to other zones.