The genetics of resistance in sunflower capitula to Sclerotinia sclerotiorum measured by mycelium infections combined with ascospore tests

To enhance efficiency of breeding programmes for Sclerotinia sclerotiorum resistance in sunflower capitula, two separate resistance tests have been recommended. However, the time necessary to develop genotypes with two types of resistance makes this impractical. A strategy to combine the two tests was consequently proposed to reduce the number of seasons per selection cycle, but genetic studies were necessary to determine if it could be applied in breeding programmes. This was the objective of this work. Data from two genetically different sunflower inbred lines and their F1, F2 and backcross generations were analysed in two years to determine the genetics of resistance to S. sclerotiorum in capitula measured by application on the same plants of a mycelium test combined with an ascospore test. Effects of maternal origin were detected when the reciprocal generations were evaluated for the mycelium test. This suggests the importance of choice of the inbred line used as female in population formation by hybridisation. Progeny subjected to the mycelium test did not show any change in relative reactions to two S. sclerotiorum isolates suggesting that ranking genotypes according to the results of combined S. sclerotiorum tests is repeatable across Sclerotinia isolates and experimental seasons. Moderate narrow sense heritability indicated that selection of the best F2 plants should be effective. Genetic gain from selection is possible because a reduction of lesion areas produced by the mycelium test can be expected. Additive gene effects contribute significantly to reduction in lesion area. Consequences of results in population improvement for S. sclerotiorum resistance in capitula are discussed.