New Oomycota Fungicides With Activity Against Phytophthora cinnamomi and Their Potential Use for Managing Avocado Root Rot in California.

Phytophthora root rot (PRR), caused by Phytophthora cinnamomi, is the most destructive disease of avocado worldwide. In the United States, mefenoxam and phosphonate products are currently the only registered fungicides for managing avocado PRR. Four new Oomycota-specific and two registered fungicides, all with different modes of action, were evaluated. Seventy-one isolates of P. cinnamomi from avocado in California, most of them collected between 2009 to 2017, were tested for their in vitro sensitivity to the six fungicides. Baseline sensitivity ranges and mean values (in parentheses) of effective concentrations to inhibit mycelial growth by 50% (EC₅₀) for the new fungicides ethaboxam, fluopicolide, mandipropamid, and oxathiapiprolin were 0.017 to 0.069 μg/ml (0.035), 0.046 to 0.330 μg/ml (0.133), 0.003 to 0.011 μg/ml (0.005), and 0.0002 to 0.0007 μg/ml (0.0004), respectively. In comparison, the EC₅₀ value range (mean) was 0.023 to 0.138 μg/ml (0.061) for mefenoxam and 12.9 to 361.2 μg/ml (81.5) for potassium phosphite. Greenhouse soil inoculation trials with 8-month-old Zutano seedlings and 10-month-old Dusa and PS.54 clonal rootstocks were conducted to assess the efficacy of these fungicides for managing PRR. Mefenoxam and potassium phosphite were effective treatments; however, oxathiapiprolin, fluopicolide, and mandipropamid were more effective. Ethaboxam was effective in reducing PRR on the rootstocks evaluated. Oxathiapiprolin reduced PRR incidence and pathogen population size in the soil by >90%, and plant shoot growth and root dry weight were significantly increased compared with the control; thus, oxathiapiprolin was one of the best treatments overall. The high activity and performance of these new fungicides supports their registrations on avocado for use in rotation and mixture programs, including with previously registered compounds, to reduce the risk of development and spread of resistance in pathogen populations.