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 an d 11 restorers, representing the known range of reaction of sunflower to Ph omopsis stem canker and most of the resistance sources used in breeding pro grammes, were studied in multilocational semi-natural attack trials and by artificial infections on leaves and stems in 1996, 1997 and 1998. The resul ts confirm that resistance is quantitative, with predominantly additive eff ects, although there were some female x male interactions when individual t rials were analysed. For lines used in both crosses, relative general combi ning abilities were stable. Some lines not bred for Phomopsis resistance sh owed good or intermediate levels of resistance. Lines from or including Yug oslavian, Rumanian and Russian sources of resistance and interspecific hybr ids all provided good levels of resistance. The majority of parental lines showed the same relative reactions to semi-natural attack and artificial in fections on leaves or stems indicating that their resistance is characteris ed by a slow rate of mycelial extension. However, one genotype showed resis tance to passage of Phomopsis stem canker from the petiole to the stem and two genotypes appeared susceptible to tests but resistant to semi-natural a ttack, indicating that their resistance is controlled by factors not measur ed by the mycelium tests. The use of combinations of these resistance sourc es to give high levels of stable resistance to Phomopsis stem canker is dis cussed.