Genetic control of partial resistance to ‘collar’ and ‘root’ isolates of Phoma macdonaldii in sunflower

Phoma macdonaldii is one of the most important pathogens of sunflower (Heliantus annuus) in France. In order to determine the inheritance of resistance to the disease, five sunflower genotypes with wide genetic variability for resistance to two ‘collar’ and two ‘root’ Phoma isolates were crossed in a diallel programme. Four separate experiments were undertaken under controlled conditions. In each one, the response of parental genotypes and their F1 hybrids were evaluated with one of the four Phoma isolates. Analysis of variance was performed to determine the effects of genotype on disease severity score when inoculated with ‘collar’ or ‘root’ Phoma isolates and showed significant variability among parents and F1 hybrids for disease severity score. Diallel analysis showed that general combining ability (GCA) and specific combining ability (SCA) effects for resistance to ‘collar’ and ‘root’ Phoma isolates were highly significant for each of the four isolates indicating that both kinds of gene effects were important in controlling the resistance. The GCA/SCA ratios were more than one for three out of four isolates showing that additive genetic effects were more important than non-additive effects for resistance to three of the studied Phoma isolates. Hence, conventional breeding methods could be recommended to achieve genetic improvement to such ‘collar’ and ‘root’ Phoma isolates