Abstract
Verticillium wilt disease causes significant losses to cotton yield and varietal development for resistance is critical in combating this threat. A study was carried out to determine the underlying genetic pattern controlling disease resistance and identify suitable parental lines to use in varietal tolerance development. Five cotton varieties were crossed in a half diallel mating system to produce ten crosses and five selfed parental lines which were screened against Verticillium wilt by artificial inoculation. There were significant variations on the severity scores, vascular brown index scores, morphological and agronomical traits under Verticillium wilt pressure. The Verticillium wilt severity scores ranged from 1.22 to 3.07 and Vascular Brown Index scores from 0.78 to 2.77. The mean squares of general combining ability (GCA) for the various characteristics of the parents were significant and also for the specific combining ability (SCA) of the vascular brown index score which was 0.27 was also significant. Three parental lines were identified as breeding material with good GCA, morphological and agronomical performance and these were CRI-MS-1, SZ9314 and BC853. This study implies that varietal tolerance can be developed by incorporating genotypes with resistance genes in breeding programmes. Future work should focus on developing resistant varieties suitable for production in Verticillium wilt prone areas. Key words: Verticillium wilt, general combining ability, specific combing ability, cotton. 
Highlights
Cotton productivity has generally been low in Africa due to the effect of climatic variations, biotic stress and poor crop management by farmers (International CottonAdvisory Committee, 2012)
The plant height was significantly different at P 0.05 between the 15 cotton lines where Cotton Research Institute (CRI)-MS-2 was taller than all the genotypes
The cross between SZ9314 and CIM1 yielded the shortest progeny (121.1 cm) and this was comparable to the parental lines SZ9314, CRI-MS-1, CIM1, the crosses
Summary
Cotton productivity has generally been low in Africa due to the effect of climatic variations, biotic stress and poor crop management by farmers (International CottonAdvisory Committee, 2012). Cotton productivity has generally been low in Africa due to the effect of climatic variations, biotic stress and poor crop management by farmers In the Southern African region, cotton industry plays a pivotal role in the national economies of Zimbabwe, South Africa, Mozambique, Zambia and Malawi (Baffes, 2004). In Zimbabwe, 300 000 ha of land is put under cotton and about 250 000 tonnes of seed cotton is realised with a national average yield of around 800 kg per hectare (Mudada, 2013) which is low compared to the expected yield of 2300 kg per hectare in local varieties (Mudada and Muripira, 2010). Smallholder farmers who account for about 99% of cotton production in Zimbabwe contribute to the low yields because of poor production management practices (Mubvekeri and Nobanda, 2012).
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