Defoliation effects on sunflower yield reduction

Quantifying the effect of leaf area loss on sunflower (Helianthus annuus L.) yield caused by hail, pests, and diseases is important in the management of this crop, both from a technical and economic point of view. The effect of defoliation depends, however, on the foliar surface eliminated and on the growth stage at which this takes place, This effect has been quantified in the USA, but these data are lacking for specific conditions of Europe. The aim of this paper was to determine the response of sunflower cultivation to several levels of defoliation that took place at different growth stages under European growth conditions. Four field trials were conducted by applying four defoliation treatments (nondefoliated control, 33, 66, and 100%) at different growth stages ranging from preflowering (R2) to physiological maturity (R9). Crop yield loss increased with increasing level of defoliation. Preflowering stage R3 was the most sensitive. At this stage a 100% defoliation of the leaf surface resulted in 92% yield loss, reducing both the number of seeds per head and 1000-seed weight. A 100% defoliation at stage R7 (back of head a pale yellow) caused a 50% yield loss, while at stage R9 (physiological maturity) defoliation had no effect on yield. The yield loss caused by defoliation at different plant growth stages under European conditions differed slightly from the one designed for U.S, conditions.