COMPONENTS OF QUANTITATIVE RESISTANCE IN SUNFLOWER TO ALTERNARIA-HELIANTHI

Components of quantitative resistance, spore production, incubation pe riod, infection frequency and mean lesion size were measured in 17 sun flower accessions inoculated with conidia of Alternaria helianthi unde r controlled conditions. The same accessions were also rated for disea se reaction in the field in 1994 and 1995 using a generated epidemic a nd varied in their disease reactions from highly susceptible to highly resistant. Spearman's ranking of accessions was highly correlated (r = 0.9) for both years however, the ranking of components measured unde r controlled conditions with field severity was generally poor. Regres sion analysis of components with field severity ratings of the accessi ons showed that mean lesion size was highly correlated (r = 0.74) and infection frequency was moderately correlated (r = 0.58) with the fiel d severity ratings taken over the two years. Infection frequency was a lso well correlated (r = 0.75) with mean lesion size. Spore production and incubation period were poorly correlated with the field severity ratings for both years. An index based on infection frequency and mean lesion size gave a better correlation with the 1995 field severity ra tings than either component alone, but in 1994 the index was not as we ll correlated with field severity as mean lesion size alone. It is sug gested that mean lesion size, determined from plants 7-9 days after in oculation could be used to select for resistance to A. helianthi in th e greenhouse. Infection frequency could also be used as a predictor of resistance, but to a lesser degree.