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 fields 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.