Losses in sunflower yield caused by Alternaria helianthi in southern Queensland

Abstract

The effect of A. helianthi on yield of sunflower (cv. Hysun 30) was assessed in two field trials at Hermitage and Gatton in southern Queensland during the 1977-78 season. Plots were (i) inoculated with spore suspensions of A. helianthi to encourage an epidemic or, (ii) sprayed regularly with the fungicide captafol to prevent disease development. Conditions were dry at both trial sites except for one week of wet overcast weather when an epidemic of A. helianthi developed. At this time, plants in the Hermitage trial were commencing anthesis, while those in the Gatton trial were approaching maturity (4 weeks post anthesis). After the wet period the epidemic declined in the Hermitage trial, and a low level of disease was present at maturity. At Gatton the disease caused premature leaf senescence in unsprayed plots, while foliage in sprayed plots remained green. Control of the epidemic that began at anthesis increased oil yield by 28%, mainly through increased seed number. There was also a small increase in 1000 seed weight but no significant increase in oil content. Control of the epidemic that began just before maturity increased oil yield by 20%, mainly because of increased 1000 seed weight and a small increase in oil content. It appears that the plant growth stage at the time the disease epidemic occurred determined which yield attributes were most affected.