Factors affecting the efficacy of Orobanche cumana chemical control in sunflower

Broomrapes (Orobanche spp.) are root parasites, causing severe damage to vegetable and field crops. Their early in situ detection for efficient control is crucial but difficult, because of their sub-surface development. Minirhizotron technology enables the in situ observation of host-parasite interactions. The initial underground development of Orobanche cumana on sunflower roots and its control with imazapic were investigated. Imazapic (2.4 g active ingredient ha^-1) was applied post-emergence on sunflower at the eight-true-leaf stage, when O. cumana attachments were first observed. No viable or new attachments were detected on roots of the treated sunflower after herbicide application. However, within the same time frame, the already existing O. cumana attachments in the non-treated controls continued to develop and new attachments appeared. Orobanche cumana shoot emergence was correlated with growing day-degrees (GDD, d °C). This correlation was quantified by a four-parameter logistic equation. Control efficacy was affected by depth and number of parasite attachments per minirhizotron camera frame: it was highest in the topmost layer, and decreased with depth and number of attachments. The results obtained in this study emphasise the need to develop a decision support system based on GDD that should be validated under long-term field conditions.