Quantifying the effects of uniconazole on growth and yield of pyrethrum in Australia

The efficacy of the plant growth regulator uniconazole (Sumagic^®) for manipulating pyrethrum canopy architecture and enhancing yield was evaluated in each of 3 years across northern Tasmania, Australia. Trials examined the efficacy of, and factors influencing, reliability and magnitude of responses, including product rate, application timing, the benefits of adding a non-ionic organosilicone surfactant (Activator^®) and combining application with urea. Results suggested that a rate of 100 g L⁻¹ uniconazole (2 L ha⁻¹ Sumagic^®) was the most efficacious and that the addition of Activator^® was unnecessary. However, at the highest rate, the cost of using the product became prohibitive and therefore trials in the latter 2 years of the study were conducted using the lower rate. Timing of application was not critical for efficacy and yield enhancements were observed when uniconazole was applied when stem length ranged from 16 to 36 cm, coinciding with a 30-day period in spring. Physiological effects of uniconazole were significant reductions in the height of stems (approximately 6 cm at flowering), increases in green leaf area and the number of flowers produced per unit area; the effects were independent of field age. This information has formed the basis for a cost-benefit analysis for the adoption of uniconazole into pyrethrum production in Australia.