Evidence of Phosphite Tolerance in Phytophthora cinnamomi from New Zealand Avocado Orchards

Abstract

There is a limited number of chemical control agents for managing Phytophthora root and collar rot diseases of avocado internationally, of these, phosphite is one of the most effective. To determine if prolonged phosphite use in New Zealand avocado orchards has led to decreased sensitivity of Phytophthora cinnamomi Rands to phosphite, 57 isolates were collected from phosphite-treated and -untreated avocado orchards and screened for tolerance using a mycelial growth inhibition assay. The inhibitory effect of phosphite on mycelial growth was tested in vitro using six concentrations of phosphite. Based on changes in mycelial growth using optical density measurements to calculate EC50 estimates, three phosphite susceptible (EC50 range 18.71 – 29.26 µg/mL) and three tolerant (EC50 range 81.85 – 123.89 µg/mL) isolates were selected. The effect of phosphite on the colonisation of lupin (Lupinus angustifolius) seedling roots and sporangia and zoospore production of three susceptible and three tolerant isolates were determined. The three tolerant isolates colonised lupin roots more extensively than the three susceptible isolates in the presence of 5 and 10 g/L phosphite. The tolerant isolates were able to asymptomatically colonise further above the lesion margin in the 5 g/L phosphite treated lupins relative to the phosphite susceptible isolates but no isolates were completely resistant to phosphite. The tolerant isolates produced more sporangia and consequently zoospores, in the presence of phosphite than the susceptible isolates. The detection of phosphite tolerance by Phytophthora cinnamomi in planta and in vivo is concerning for the future efficacy of phosphite to manage Phytophthora diseases.

Isolation of two triterpenoids from Phlomis purpurea, one of them with anti-oomycete activity against Phytophthora cinnamomi, and insights into its biosynthetic pathway

“…Examples of current treatments for this disease include the use of strobilurin pyraclostrobin (23.3%) (BASF) or the phenylamide mefenoxam (22%) (Syngenta), as well as host plant defense inducers like aluminum tris (O-ethyl-phosphanate) (60%) (Bayer) (Neupane et al, 2022). Long-term phosphite treatment is proposed as a method to control P. cinnamomi infestation when lacking other options (Barrett and Rathbone, 2018), but there is recent evidence of phosphite-tolerant P. cinnamomi isolates obtained from New Zealand (Hunter et al, 2023).…”

Exploring Phytophthora agathidicida Oospore Viability and Germination

“…Phosphite resistance has been reported in several Phytophthora species, although most studies report large variances between isolates (Adaskaveg et al, 2017;Beaulieu et al, 2021;Hao et al, 2018;Hunter, 2018;Hunter et al, 2022;Hunter et al, 2018;Veena et al, 2010;Wilkinson et al, 2001). The mechanisms for this resistance vary, e.g., exclusion of phosphite from entering the pathogen's cells (Griffith et al, 1993), cellular resistance to phosphite (Griffith et al, 1993;Niere et al, 1994) and efflux of phosphite from the cell (Dunstan et al, 1990).…”