Screening of South African sunflower (Helianthus annuus L.) cultivars for alachlor sensitivity

Abstract

Occasionally a herbicide that is considered safe to use at prescribed rates causes damage to crop plants at those same rates. Alachlor (Lasso EC®) is registered for use on sunflower at a rate of 1.5 to 1.92 kg ai ha⁻¹. Occasional injury to sunflower plants at these rates is usually caused by incorrect application or unsuitable weather conditions, but sometimes no explanation can be found. In many crops it has been shown that genetic differences can make specific cultivars more susceptible to alachlor injury. The objective of this study was to screen 22 of the available sunflower cultivars for sensitivity to alachlor in a glasshouse trial. Planting took place in pots filled with a sandy loam soil (20% clay). Five rates of alachlor (0, 0.96, 1.92, 3.84 and 7.68 kg ai ha⁻¹) were applied and leached into the soil with 100 ml of water. Pots were laid out in an air-conditioned glasshouse set to 28/18°C day/night in a randomised block design with three replicates. Plants were harvested 38 days after planting, at which stage plant height and mass were determined. The results indicated that cultivar differences with respect to alachlor tolerance were present. Plant height appeared to be the best predictor of alachlor activity in sunflower. Cultivars were divided into three classes (tolerant, intermediate and sensitive) based on their reaction to the herbicide.