Heredity of resistance to Sclerotinia sclerotiorum in sunflowers. II. - Study of capitulum resistance to natural and artificial ascospore infections

SUMMARY Sunflower capitulum attacks resulting from infection by Sclerotinia sclerotiorum ascospores were observed under natural conditions with irrigation during flowering, and after contamination of florets with ascospore suspensions (ascospore test). The study was made on the parental inbred lines and F1 hybrids of a factorial cross of 6 male steriles and 6 restorers. There were significant genetic differences in reactions to both natural and artificial infections. According to the hybrid combinations studied, the general combining ability/specific combining ability ratio varied between 4.06 and 0.87 for natural attack and between 4.49 and 2.74 for the ascospore test. Some interaction effects were significant. The regressions between inbred lines and their hybrids were significant but much closer for the ascospore test (r = 0.83) than for natural infection (r = 0.49). The ascospore test was significantly correlated with natural infection (r = - 0.57) and it was concluded that this test may be used in breeding programmes for parents that will give hybrids with good levels of capitulum resistance under natural conditions.