Fungitoxicity and synergism of mixtures of fluopicolide and pyraclostrobin against Phytophthora infestans

• Joint-toxicity of fluopicolide and pyraclostrobin against Phytophthora infestans was determined.
• Mixtures of fluopicolide and pyraclostrobin exhibited synergistic interactions against P. infestans.
• Mixtures of fluopicolide and pyraclostrobin exhibited good protective and curative activity.

Fluopicolide and pyraclostrobin were new systemic fungicides with highly inhibitory activity on a broad spectrum of oomycetes, but so far the fungitoxicity of their mixtures on the different developmental stages of Phytophthora infestans and whether synergism exists have not been investigated. The joint-toxicity of the mixtures of fluopicolide with pyraclostrobin was determined against the different developmental stages (mycelial growth, zoospore release, cystospore germination and sporangial germination) of P. infestans and late blight on the leaf discs and the potted plants of potato, and whether the synergistic interaction exists in the mixtures of the two fungicides were evaluated in vitro and in vivo based on the synergistic ratios calculated with the Wadley formulas. The protective activity and curative activity against P. infestans and the efficacy duration in controlling potato late blight of the synergistic mixtures of the two fungicides were examined in this study. The results showed that mixtures of fluopicolide (F) and pyraclostrobin (P) at the ratios of 10:1 and 1:4 (F:P) exhibited synergistic interactions and had excellent inhibitory activity against almost all developmental stages of P. infestans. Synergistic interactions of the mixtures at the ratios of 1:4 and 10:1 (F:P) against the mycelial growth of metalaxyl-resistant isolates were more obvious than against the mycelial growth of metalaxyl-sensitive isolates. Synergistic interactions of the mixtures at the ratio of 1:4 (F:P) against the mycelial growth of the most of the tested isolates were more obvious than those of the mixtures at the ratio of 10:1 (F:P). The highest activity of the mixtures was observed against zoospore release at the ratio of 1:4 (F:P) with EC50 value of 0.0044 μg ml−1. The strongest synergistic interaction of the mixtures was observed against zoospore release at the ratio of 10:1 (F:P) with the synergistic ratio of 5.27. The mixtures of fluopicolide and pyraclostrobin at the ratios of 1:4 and 10:1 (F:P) showed synergistic interactions against P. infestans on the leaf discs and intact plants of potato. The EC50 values for protective activity and curative activity of the mixture at the ratio of 10:1 (F:P) were 0.63 μg ml−1 and 0.87 μg ml−1, and EC50 values of protective activity and curative activity of the mixture at the ratio of 1:4 (F:P) were 0.27 μg ml−1 and 0.44 μg ml−1, respectively. At 14 days after spraying of fungicide solutions, the disease control efficacy of the mixtures at the ratios of 10:1 and 1:4 (F:P) was still over 80% at the dosages of 50 μg ml−1 and 100 μg ml−1, and the disease control efficacy of the two mixtures remained at about 97% at the dosages of 200 μg ml−1, indicating long-lasting effective control efficacy of the two fungicides against potato late blight.