INHERITANCE OF RESISTANCE TO PHOMOPSIS (DIAPORTHE-HELIANTHI) IN SUNFLOWER

The inheritance of resistance of sunflower to Phomopsis (Diaporthe hel ianthi) was studied on 10 inbred lines representing a range of resista nce and susceptibility levels, and 25 hybrids from a factorial cross o f these lines. Analyses of observations over a 3-year period, of semi- natural infection (presence of stem lesions greater than 5 cm) on the hybrids indicated that additive gene control was predominant, with no significant interactions between parental effects. Correlations betwee n infection of parental lines and the mean of their hybrids were not a lways significant, with certain inbreds which appeared quite susceptib le giving hybrids with good levels of resistance. In order to predict hybrid values, it appears necessary to determine the general combining abilities of parental lines. Some inbreds gave very high levels of re sistance, others gave very high levels of susceptibility. A test measu ring the rate of extension of D. helianthi mycelium on leaves was sign ificantly correlated with the results of natural infections. In partic ular, it permitted distinction of the most susceptible genotypes, and thus could be used in first generations of breeding to eliminate the m ost susceptible plants.