Effect of potassium phosphonate on root rot of Pinus radiata caused by Phytophthora cinnamomi

Seedlings of Pinus radiata transplanted into washed river sand or sand/peat potting mix pre-inoculated with Phytophthora cinnamomi developed severe wilt and root rot symptoms within 7 weeks, and died within 12 weeks. By contrast, direct inoculation of pathogen mycelium into stem wounds produced only restricted lesions. Soil incorporation of inoculum was subsequently used to assess the ability of potassium phosphonate to protect pine seedlings against wilt and root rot caused by P cinnamomi. Three concentrations of potassium phosphonate (0.1, 1.0 and 5.0 g/L), applied as either a single foliar spray or as a single root drench at transplanting into pots containing pathogen-infested sand or sand/peat, significantly reduced the incidence of root rot symptoms. At each concentration, soil drenches were more effective than foliar sprays in delaying and suppressing dieback symptoms in container-grown plants. Phosphonate applications were more effective in sand/peat potting mix than in washed river sand. Recovery of the pathogen from diseased plant roots and potting mix was lower in pots drenched with phosphonate than from pots treated by foliar sprays. Potassium phosphonate foliar sprays at a rate of 5.0 g/L were phytotoxic, causing burning of needles and temporary branch dieback.