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Inheritance of resistance to Contarinia sorghicola Coquillet in the AF-28 sorghum variety

Reciprocal crosses were made between the sorghum varieties AF-28 resistant to the sorghum midge, and Sart susceptible. In 1975 under field conditions at Campinas, State of São Paulo, Brazil, the average damage of the Fl of these crosses was 8.5, the average damage of Sart was 9.2 and of AF-28 was 1.2 showing that susceptibility to sorghum midge is dominant or incompletely dominant. Inflorescences of F2 plants were bagged at random and F2 derived families in the generation F3 (S1 lines) were studied under field conditions in 1977. Fourteen families out of a total of 216 from the cross Sart x AF-28, did not differ significantly at the 5% level from the resistant parent AF-28. The chi-square between the observed ratio of 14:202 and the expected ratio of 14:210 was not significant. This result fits into the hypothesis of a minimum of two genes governing the resistance to sorghum midge in the variety AF-28. Three families out of a total of 109 from the cross AF-28 x Sart did not differ significantly at the 5% level from the resistant parent AF-28. The observed ratio of 3:106 does not fit properly into the expected ratio of 3:45 for two pairs of genes or into the expected ratio of 3:189 for 3 pairs of genes. The hypothesis that a minimum of 2 recessive pairs of genes are responsible for the resistance to the sorghum midge in the variety AF-28 is still sustained by the observed ratio 3:106. The lowest mean damage observed among the F3 families was 0.86 in the cross AF-28 x Sart while the mean damage of the resistant parent was 0.79. The lowest damage among the F3 families from the cross Sart x AF-28 was 0.937 while the mean damage of the resistant parent AF-28 was 0.9027. There was no F3 line among the 325 observed, which had a mean damage equal or lower than the resistant parent. This suggests that at least 2 recessive pairs of major genes are responsible for the resistance, but other genes with minor effects might be present. Some lines derived from F3 families with low mean damage showed good agronomic characteristics together with good level of resistance showing that the resistance can be transferred and recombined to produce resistant cultivars or hybrids.


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