A growing degree day-based multiphasic model of phenological transitions in branched broomrape (Phelipanche ramosa)

Abstract

The root parasitic weed branched broomrape [Phelipanche ramosa (L.) Pomel] spends most of its life cycle underground, complicating early detection and control. A predictive model based on growing degree days (GDD) can improve management by anticipating key developmental stages. This study aimed to characterize P. ramosa phenology and develop a thermal time model to inform more effective control strategies. Two greenhouse experiments using rhizotrons were conducted over two years (2021–2022), along with a complementary pot experiment in 2022 at Davis, CA. The P. ramosa development was classified into four phenological stages: attachment, elongation, emergence, and full flowering. We recorded the thermal timing (GDD accumulation from transplanting) and number of individuals at each phenological stage and modeled the development dynamics using an inverse Weibull function. Of the 255 P. ramosa attachments recorded across two rhizotron runs, 87% advanced to elongation, 63% reached emergence, and only 49% successfully developed to full flowering. Model estimates indicated that 5% of attachments occurred at 299 GDD and 95% by 730 GDD, marking a critical control window. After the initial attachment, on average, it took about 85 GDD for an attachment to initiate the elongation process, and 385 and 815 GDDs to reach 5% and 95% elongation, respectively. Emergence followed, with 5–95% occurring from 520 to 1100 GDD. Full flowering occurred roughly 181 GDD after emergence, spanning 700 (5%) to 1280 (95%) GDD after transplanting. Model validation using data from related field experiments conducted in 2022 and 2023 in a P. ramosa-infested tomato (Solanum lycopersicum L.) field in Yolo County, CA, confirmed the model’s accuracy in predicting full flowering with Root Mean Square Error (RMSE) of 142 GDD. This thermal time model offers a valuable decision-support tool to optimize the timing of P. ramosa management, particularly in-season soil-applied herbicide programs, in tomato cropping systems.