PHOSPHONATE SENSITIVITY SHIFT IN A PHYTOPHTHORA CINNAMOMI POPULATION
MP WEINERT1, A DRENTH1, SH SOO1, JAG IRWIN1 and KG PEGG2
1CRC for Tropical Plant Pathology, The University of Queensland, Australia, 4072; 2Plant Protection Unit, Queensland Department of Primary Industries, 80 Meiers Rd., Indooroopilly, Australia, 4072
Background and objectives
The development of trunk injected phosphonates, the salts and esters of phosphorous acid, added a new dimension to Phytophthora root rot control. Phosphonates are an excellent control option as they are cheap, exhibit low mammalian toxicity and are thought to have a complex mode of action, evident from a direct effect through fungistasis and an indirect effect via stimulation of host defences .
Our aims are to examine on the effect of continuous phosphonate treatment on a P. cinnamomi population from an avocado orchard and compare the in vitro sensitivity of these isolates to ones which have never been treated. The sensitivity of isolates recovered by direct isolation from diseased roots and soil-baited isolates will also be compared. The sensitivity of a selected set of these isolates will then be examined in planta.
Materials and methods
Sensitivity tests were conducted on CMA amended with technical grade phosphorous acid buffered with potassium hydroxide, pH 6.2, prior to pouring. Final phosphonate concentrations in the media were 0,1,3,10,30,100,300 and 1000 ppm. A 5 mm agar plug was placed in the centre of each of the three replicate plates of each concentration. Colony growth was an average of two measurements per plate taken three days after inoculation. Averages were then converted to percent growth inhibition to calculate EC50 values.
Results and conclusions
As phosphonates are the major control method for Phytophthora root rot, reduced sensitivity in vitro raises the uncertainty of the future effectiveness of the direct mode of action of phosphonates for Phytophthora control. Constant monitoring and the relationship between in vitro and in planta assays are necessary to determine the durability of phosphonate controls.