Comparative aggressiveness and fungicide sensitivity of Phytopythium vexans and P. litorale associated with kiwifruit vine decline in Türkiye

BACKGROUND Kiwifruit vine decline syndrome (KVDS) is an economically critical disease threatening production in Türkiye. The oomycetes Phytopythium vexans and P. litorale are increasingly implicated, but their comparative roles and effective management remain poorly understood, creating an urgent need for sustainable control strategies. This study aimed to (i) compare the virulence of these two species and (ii) evaluate the in vitro versus in vivo efficacy of key fungicides to identify reliable control strategies. RESULTS Pathogenicity assays revealed that P. litorale was significantly more aggressive, causing severe disease (Disease Severity Index, DSI > 70%), whereas P. vexans induced only moderate symptoms (DSI < 42%). A critical disconnect was observed between laboratory and greenhouse fungicide performance. For instance, oxathiapiprolin, which was highly potent in vitro (EC 50 = 0.001169–0.006158 μg mL −1 ), provided only moderate disease control in vivo . Conversely, pyraclostrobin‐based fungicides delivered superior protection against the highly aggressive P. litorale , reducing the DSI to a range of 20.83–21.88% and significantly enhancing root biomass. CONCLUSION This study establishes P. litorale as a highly aggressive pathogen in KVDS etiology and demonstrates that in vitro data alone are misleading for predicting fungicide field performance. Pyraclostrobin‐based fungicides are identified as the most effective candidates for managing KVDS caused by P. litorale . These findings underscore the necessity of integrating in vivo validation in screening protocols and adopting species‐specific management approaches, providing a critical roadmap for developing sustainable solutions against this devastating disease.