Mixed inheritance in the genetic control of ramulosis (Colletotrichum gossypii var. cephalosporioides) resistance in cotton.

Ramulosis is one of the most aggressive diseases in cotton, and understanding the genetic control of its resistance is imperative for selecting superior cotton genotypes in breeding programs. This study analyzed the inheritance pattern of this resistance using chi-square goodness-of-fit tests to determine the phenotypic proportions of the F2 generation, and a mixed inheritance approach to jointly model major gene and polygenes effects. F1, F2, Rc1, and Rc2 generations were obtained by crossing resistant (BRS Facual, CNPA 2984, or CNPA 2043) and susceptible (Delta Opal, CNPA 999, or CNPA 2161) genotypes, and were assessed under field conditions with artificial inoculation of the pathogen (Colletotrichum gossypii var. cephalosporioides). Genetic control of the trait varied among the crossings. For Delta Opal x BRS Facual and CNPA 2161 x BRS Facual, phenotypic segregations in the F2 generation did not differ from the expected proportions for the hypothesis of duplicate genes (15:1). For Delta Opal x CNPA 2043, the segregation did not differ from the expected proportions for dominant recessive epistasis (13:3). The hypothesis of genetic control by one major gene was supported only for the Delta Opal x CNPA 2043 crossing. Three other crossings showed evidence of polygenes in the inheritance of the trait. In conclusion, major genes and polygenes are likely involved in the genetic control of ramulosis resistance in cotton.

Number of repetitions for evaluating technological traits in cotton genotypes.

With the changes in spinning technology, technological cotton traits, such as fiber length, fiber uniformity, fiber strength, fineness, fiber maturity, percentage of fibers, and short fiber index, are of great importance for selecting cotton genotypes. However, for accurate discrimination of genotypes, it is important that these traits are evaluated with the best possible accuracy. The aim of this study was to determine the number of measurements (repetitions) needed to accurately assess technological traits of cotton genotypes. Seven experiments were conducted in four Brazilian States (Ceará, Rio Grande do Norte, Goiás, and Mato Grosso do Sul). We used nine brown and two white colored fiber lines in a randomized block design with four replications. After verifying the assumptions of residual normality and homogeneity of variances, analysis of variance was performed to estimate the repeatability coefficient and calculating the number of repetitions. Trials with four replications were found to be sufficient to identify superior cotton genotypes for all measured traits except short fiber index with a selective accuracy >90% and at least 81% accuracy in predicting their actual value. These results allow more accurate and reliable results in future researches with evaluating technological traits in cotton genotypes.