Inheritance of several sources of resistance to Phomopsis stem canker (Diaporthe helianthi Munt.-Cvet.) in sunflower (Helianthus annuus L.)

The hybrids from 2 factorial crosses involving a total of 7 female lines and 11 restorers, representing the known range of reaction of sunflower to Phomopsis stem canker and most of the resistance sources used in breeding programmes, were studied in multilocational semi-natural attack trials and by artificial infections on leaves and stems in 1996, 1997 and 1998. The results confirm that resistance is quantitative, with predominantly additive effects, although there were some female x male interactions when individual trials were analysed. For lines used in both crosses, relative general combining abilities were stable. Some lines not bred for Phomopsis resistance showed good or intermediate levels of resistance. Lines from or including Yugoslavian, Rumanian and Russian sources of resistance and interspecific hybrids all provided good levels of resistance. The majority of parental lines showed the same relative reactions to semi-natural attack and artificial infections on leaves or stems indicating that their resistance is characterised by a slow rate of mycelial extension. However, one genotype showed resistance to passage of Phomopsis stem canker from the petiole to the stem and two genotypes appeared susceptible to tests but resistant to semi-natural attack, indicating that their resistance is controlled by factors not measured by the mycelium tests. The use of combinations of these resistance sources to give high levels of stable resistance to Phomopsis stem canker is discussed.