Abstract
Phosphite (phosphonate) is widely applied to plant communities to control the spread and impact of Phytophthora species in natural and peri‐urban woodland and forest ecosystems. Determining (1) if phosphite applications have been successfully taken up in planta, (2) how phosphite is distributed around plants across seasons, and (3) when plants need to be retreated to maintain effective pathogen control is problematic due to the time and costs associated with current methods. This paper describes a direct chemical method of rapidly and effectively estimating the concentration of phosphite in plant material using a silver nitrate reagent. Glass fiber filter papers (Whatman GF/B) are saturated with acidified silver nitrate (1 M) and dried for 2 hours at 600C. 20 uL of a PVPP treated aqueous plant extract is then adsorbed on to the filter paper and incubated in the dark at room temperature for 1 hour. The presence of phosphite in the extract reduces the silver ions to elemental silver resulting in a grey‐black precipitate that is clearly visible. The method was successfully tested on the roots and leaves of a range of exotic and Australian native plants species from different families and genera which had been treated with 0.3% phosphite. The method is rapid, sensitive and inexpensive, and can detect phosphite at concentrations of 1 mM in 20 ul of aqueous extract from 100 mg of fresh plant material, equivalent to 82 ug g‐1 fresh weight, or 20 nmol phosphite per sample. The concentrations detected by the silver nitrate method equated well with the more expensive and less rapid HPLC method that we used to confirm the accuracy of the assay.