Investigations into the increased rust (Puccinia helianthi) intensity on some hybrid sunflower cultivars grown in Queensland

Sunflower rust intensity in crops of several hybrid cultivars in Queensland during the 1980, 1981 and 1982 seasons ranged from 30 to 60% of leaf area. In previous years, these cultivars had rust ratings of 2-5%. Rust was also found on some crops of usually immune hybrids grown during winter in central Queensland. A number of rust differential lines were inoculated with several isolates collected from throughout Queensland during 1981 and 1982, as well as with isolates stored in liquid nitrogen from the years 1976, 1978 and 1980. Because all isolates failed to infect the differentials possessing either the R1 or R2 genes for resistance, all collections were allocated to race 1. Comparisons of the temporal isolates revealed that no differences could be detected among the generation times, pustule number and pustule size produced by isolates on a range of sunflower rust differentials and hybrid cultivars. These data indicated that the composition of the pathogenic population had not altered. The application of higher inoculum densities shortened generation times and produced more pustules; both relationships were linear. The development of race 1 rust on seedlings of the immune hybrid Hysun 30 was induced by maintaining the plants under low light intensity or short photoperiods.