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
Phytophthora cinnamomi is the most serious limiting factor to avocado (Persea americana) production in subtropical eastern Australia. The pathogen attacks and kills the white feeder roots, produced yearly by avocado trees and if untreated causes a wilt usually followed by defoliation and death.

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 [1].

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
A total of 119 P. cinnamomi isolates were tested in this study. Thirty-one isolates were collected from two trees which had never been treated with phosphonates, (14 root and 17 soil) and the remaining 88 isolates,(30 root and 58 soil) were obtained from trees which had been treated for 10 years.

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
The average EC50 values for the isolates from the untreated and treated trees were 9 ppm 1.41 and 98 ppm 90.4, respectively, representing a resistance factor (EC50treated/EC50untreated) of 10.89. Growth of all the isolates from the untreated trees was stopped by the 1000 ppm concentration whereas only 20% of isolates from the treated trees were stopped by this concentration. The EC50 values for the root isolates from treated trees was 126.5 ppm 104.6 and for the soil isolates was 84.1 ppm 79.5. These results show that the continued use of phosphonates has lead to a sensitivity shift in the P. cinnamomi population studied, for isolates from the treated trees. The lower EC50 values, for the soil isolates as opposed to the root isolates, from the treated trees provides further evidence for this.

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.

1. Guest DI, Pegg KG, Whiley AW Horticultural Reviews 17, pp.299-330.