Alternaria leaf petiole and stem blight is an important disease of sweetpotato (Ipomoea
batatas (L.) Lam.) causing yield losses in both landraces and improved cultivars. The most
important species causing economic yield loss in Uganda are Alternaria bataticola and A.
alternate with A. bataticola the most aggressive and widely distributed. The study was
conducted to: i) establish farmer-preferred sweet potato attributes, production constraints and
Alternaria leaf petiole and stem blight awareness; ii) evaluate Ugandan sweet potato
germplasm for Alternaria leaf petiole and stem blight resistance; iii) determine the mode of
inheritance of resistance to Alternaria leaf petiole and stem blight and storage root yield
components of sweet potato through estimation of the general combining ability (GCA) of the
parents and the specific combining ability (SCA) of the parents for each cross; and iv)
determine the adaptability and farmer acceptability of selected F1 genotypes across
environments. The participatory rural appraisal was conducted to establish farmer
preferences and production constraints revealed that farmer preferred sweet-potato traits
were high yield, sweetness (taste), early maturity, high dry mass, resistance to pests and
diseases, and in-field root storability after maturity. A majority of the farmers considered
Alternaria leaf petiole and stem blight a serious production constraint causing yield loss of
over 50%. The main control measures against the disease were roguing of infected plants,
spraying with fungicides, use of healthy planting materials and planting resistant genotypes.
Thirty sweet potato land races and improved cultivars were evaluated for Alternaria blight
severity; yield, dry mass, harvest index, sweetpotato weevil (Cylas spp.) damage and
sweetpotato virus disease at two sites (Namulonge and Kachwekano) over three seasons
(2010B, 2011A, 2011B) under Alternaria inoculum and fungicide spray treatments. Landrace
Shock was more resistant to Alternaria blight than Tanzania, the resistant check. Genotypes
NASPOT 1, NASPOT 7, New Kawogo and Dimbuka were the most susceptible. Thirty two
F1 families were generated from 16 parents in two sets in a North Carolina II mating scheme.
The families were evaluated at two sites using a 5 x 7 row-column design with two
replications. There were significant (P<0.05) differences among the families in Alternaria
blight severity. Both GCA and SCA mean squares (MS) for Alternaria blight were highly
significant (P<0.001) but the predominance of GCA sum of squares (SS) for Alternaria blight
at 67.4% of the treatment SS versus 32.6% for SCA SS indicated that additive effects were
more important than the non-additive effects in controlling this trait. For the yield
components, the GCA MS were significant (P<0.05) and accounted for more than 60% of
the treatment SS except for percentage dry mass composition where SCA SS accounted for
53.0% of the treatment SS implying that non-additive genetic effects were slightly more
important than additive for this trait. Some parents that had desirable high, negative GCA
effects for Alternaria blight produced families with undesirable positive SCA effects and the reverse was also true. This implied that the best parents should not be chosen based on GCA effects alone but also on SCA effects of their best crosses. The promising F1 genotypes selected from previously evaluated crosses together with one Alternaria blight resistant check (Tanzania) and one susceptible check (NASPOT 1) were evaluated at three sites (Namulonge, Kachwekano and Serere) using a randomised complete block design with three replications. Scientists and farmers evaluated the agronomic performance and also quality traits of the genotypes before and at harvest. Genotypes G14, G16, G24, G29, G49, G59 and G69 were the most stable across the sites for low Alternaria blight severity and can, therefore, be recommended for further evaluation under both low and high disease pressure areas. Genotypes G67, G13, G14, G24, G29 and G53 were the most high yielding and stable across the sites and were therefore the most widely adapted. In the participatory selection, before harvest and at harvest, Spearman’s rank correlation of the scientists and farmers’ mean ranking of the genotypes at each site was positive and significant. This indicated that the scientists in the study were capable of selecting for farmer preferred traits. / Thesis (Ph.D.)-University of KwaZulu-Natal, Pietermaritzburg, 2012.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10700 |
| Date | January 2013 |
| Creators | Sseruwu, Godfrey. |
| Contributors | Shanahan, Paul E., Melis, Rob W. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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