Diallel analysis of resistance to Fusarium wilt Race 4 in American pima cotton

Fusarium oxysporum f. sp. vasinfectum Race 4 (FOV4) causes high seedling mortality in pima cotton (Gossypium barbadense L.) grown in the western and southwestern U.S. Cotton Belt. The objectives of this study were to analyze the combining abilities and genetic basis of FOV4 resistance in pima cotton using a half-diallel mating design. Thirty-six biparental F-3 and F-4 generations were derived from six commercial cultivars and three elite lines and artificially inoculated with FOV4 for screening for FOV4 resistance in four replicated tests, with two tests for each generation. Disease severity rating (DSR) at 28 d after inoculation (DAI) and the area under the disease progress curve (AUDPC) across 7, 14, 21, and 28 DAI were analyzed via Griffing's Method 2 and Hayman's diallel analysis. 'Pima S-6', 'PHY 76', and '8810' (resistant parents) were heterogeneous in their responses to FOV4 infections with 20 to 50% disease incidence and mortality rate; and they had significantly lower DSR and AUDPC with negative general combing ability effects compared with the susceptible parents. Variances caused by specific combining ability and nonadditive effects were higher than those caused by additive effects for FOV4 resistance, consistent with the estimates for broad-sense heritability. The results indicated a predominant nonadditive effect for FOV4 resistance within these genotypes of pima cotton. A minimum of one or two genes was estimated for FOV4 resistance based on Hayman analysis. The results represented the first study applying a diallel analysis to investigate the quantitative genetic basis of FOV4 resistance in pima cotton.