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

Abstract

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.

Assessing the effectiveness of oxathiapiprolin towardsPhytophthora agathidicida, the causal agent of kauri dieback disease

“…Similar to observations for other Phytophthora species, oxathiapiprolin was effective against mycelial growth and zoospore germination at concentrations far below 1 μ g/ml (Table 1). Indeed, the measured EC 50 values for mycelial growth indicate that P. agathidicida mycelia are even more sensitive to oxathiapiprolin than well-studied species such as P. capsici and P. cinnamomi (Belisle et al 2019; Ji and Csinos 2015; Miao et al 2016b). Oxathiapiprolin was also effective at delaying or preventing the infection of kauri leaves, albeit at higher concentrations (Fig.…”

“…Oxathiapiprolin is a novel oomycide that is highly active against many species of Phytophthora at extremely low concentrations (Belisle et al 2019; Gray et al 2018; Ji and Csinos 2015; Pasteris et al 2016). In this study we sought to determine its effectiveness against multiple lifecycle stages of P. agathidicida , the causative agent of kauri dieback disease.…”

“…Oxathiapiprolin is a first-in-class piperindinyl thiazole isoxazoline oomycide (Pasteris et al 2016) that is highly effective against many oomycete species when applied preventatively or curatively (Belisle et al 2019; Bittner et al 2017; Cohen 2015; Cohen et al 2018; Gray et al 2018; Humann et al 2019; Ji and Csinos 2015; Miao et al 2016b; Miao et al 2016a; Qu et al 2016a). It can be applied via foliar sprays, soil applications ( e.g .…”

Phosphonic acid: a long‐standing and versatile crop protectant

“…In California, in vitro sensitivity amongst 71 isolates of Phytophthora cinnamomi collected from avocado orchards demonstrated a broad range of EC 50 values, exhibiting bimodal distribution based on source orchard locations. In southern California where higher rates and more frequent phosphonic acid applications were made isolates were less sensitive than those from northern California where less phosphonic acid was used 74 . Research initiated in the early 1990s to monitor sensitivity of Phytophthora cinnamomi showed that after 10 years of annual trunk injections, there was limited evidence for decreased sensitivity in vitro of isolates from treated compared with untreated trees 75–77…”

Influence of oxathiapiprolin on preinfectional and early infection stages of Plasmopara halstedii, downy mildew of the sunflower

“…Several studies have revealed a very high effectiveness of oxathiapiprolin against various Phytophthora species (Ji et al. 2014;Ji & Csinos 2015;Qu et al 2016;Belisle et al 2019), Pseudoperonospora cubensis (Cohen 2015;Cohen et al 2018) and Plasmopara viticola Leonian 1922 (Pasteris et al 2016). For Plasmopara halstedii (Farl.)…”