Genetic analysis of partial resistance to black stem (Phoma macdonaldii) in sunflower as measured by a seedling test

Phoma black stem caused by Phoma macdonaldii is one of the most important diseases of sunflower in France. To determine the inheritance of partial resistance to the disease, five sunflower genotypes with a wide range of genetic variability for partial resistance were crossed in diallel fashion. Resistance was measured as reaction of seedlings to infection by P. macdonaldii. Two separate experiments were undertaken in a controlled growth chamber. In each experiment the parental genotypes and their F-1 hybrids were inoculated by one of the two selected isolates (MP8 and MP10). Analysis of variance for each of two isolates showed significant variability among parents and F-1 hybrids for disease severity score. Combined analysis of variance detected significant interactions between sunflower genotypes and Phoma isolates, suggesting that partial resistance to Phoma black stem should be isolate-specific. General and specific combining ability effects for resistance to Phoma black stem were highly significant for each of the two isolates, indicating that both kinds of gene effect were important in controlling the inheritance of partial resistance to Phoma black stem. On the basis of the results from this seedling test, the cross between M6-54-1 and ENSAT-R5 genotypes, which are the best general combiners for MP8 and MP10, respectively, could be of value in improving black stem partial resistance to these P. macdonaldii isolates.