Effect of SHAM on the activity of coumoxystrobin against Phytophthora litchii

Abstract Litchi downy blight, caused by Phytophthora litchii, presents significant challenges to litchi production, storage, and transportation. Previous studies have shown that coumoxystrobin exhibits effective inhibitory activity against P. litchii. Salicylhydroxamic acid (SHAM), an alternative respiratory pathway inhibitor, is commonly used to evaluate the efficacy of cytochrome respiratory pathway inhibitor like coumoxystrobin against fungal phytopathogens in vitro. In this study, the toxicity of SHAM on various developmental stages of P. litchii, including mycelial growth, sporangial germination, zoospore release, and cystospore germination, was assessed. The EC50 values for SHAM were determined as 166.72, 150.69, 333.97, and 240.91 μg/mL, respectively. Subsequently, the activity of coumoxystrobin against P. litchii was assessed in the presence of SHAM at a concentration of 50 μg/mL, which showed slight inhibition below 20% for all four developmental stages. The addition of SHAM significantly improved the inhibitory activity of coumoxystrobin against P. litchii at different stages, with reductions in EC50 values ranging from 7.55- to 122.92-fold. Moreover, respiration assays revealed that a concentration of 5 μg/mL coumoxystrobin inhibited P. litchii mycelial respiration to a lesser extent compared to the combined effect of coumoxystrobin and SHAM. SHAM also enhanced the control efficacy of coumoxystrobin against phytophthora blight development on litchi leaves. Previously, we reported that coumoxystrobin effectively controls postharvest downy mildew on litchi fruit. Consequently, coumoxystrobin holds promise as an agent for litchi downy blight control in the field and after harvest. Furthermore, similar to previous studies, SHAM, an alternative oxidase (AOX) inhibitor, was found to significantly enhance the activity of the two aforementioned QoI fungicides against P. litchii, both in vitro and in vivo. This suggests that further exploration of AOX inhibitors and the role of AOX in plant diseases could contribute to the rational use of QoI fungicides and improve control efficiency for plant diseases.