Mobility of Oxathiapiprolin and Mefenoxam in Citrus Seedlings After Root Application and Implications for Managing Phytophthora Root Rot

Oxathiapiprolin is highly effective in the management of Phytophthora root rot of citrus; however, its uptake into plants after soil application is not known. This was investigated and compared with mefenoxam using potted citrus seedlings sampled 7, 10, 13, and 16 days after soil treatments. Bioassays and high-performance liquid chromatography tandem mass spectroscopy (HPLC-MS/MS) were used to quantify fungicide amounts in plant extracts. Distinct inhibition zones of mycelial growth of Phytophthora citrophthora were observed in bioassays when root, stem, or leaf extracts were added to filter paper disks on agar plates. Based on the two quantification methods, concentrations of both fungicides in the three tissue types and at all sampling times were above the mean effective concentration that provides 50% growth reduction values of the baseline sensitivities. Relative concentrations at the four sampling times sometimes varied between the two methods but, for both methods, concentrations of oxathiapiprolin were significantly higher in roots and leaves as compared with stems 10 days after treatment and statistically similar in the three tissues after 7 days. For mefenoxam, concentrations significantly increased in roots between 7 and 16 days after treatment and were significantly the highest in roots as compared with stems or leaves 16 days after treatment. Regressions of oxathiapiprolin and mefenoxam concentrations using HPLC-MS/MS on those calculated from bioassay standard curves indicated that the bioassays overestimated fungicide amounts in the extracts. The bioassay, however, can be considered an alternative option comparable with costly residue analyses in fungicide mobility studies in plants. Uptake of oxathiapiprolin at sufficient but low concentrations into plant roots provides an explanation for its long-lasting high activity in the management of Phytophthora root rot.