Philip O'Brien

The systemic fungicide phosphite is increasingly being used to treat oomycete plant diseases in horticultural crops and native ecosystems around the world. The effects of phosphite application on a plant are variable. Resistance depends on the species, the time of application and the method of application. Very little is known about how phosphite is transported and stored within plants, or how these processes are affected by the physiological status of the plant. This is mainly due to the unavailability of a simple, efficient method for measuring the concentration of phosphite. We have developed an enzymatic assay that enables phosphite to be quantified as an alternative to high-performance liquid chromatography or gas chromatography. Phosphite is oxidised to phosphate by a modified phosphite dehydrogenase. NADH formed in the reaction reduces p-iodonitrotetrazolium violet (INT) via an intermediate electron carrier to produce a stable red formazan that can be measured spectrophotometrically at 490 nm. This method is being further developed to quantify phosphite in plant-tissue samples. Dried plant material is powdered and resuspended in water. Phosphite is separated from the plant extract solution by dialysis and determined with the phosphite dehydrogenase assay. The method is rapid and inexpensive and could potentially be developed to measure the phosphite concentration within plant samples in the field.

Phytophthora cinnamomi is often genetically diverse in disease fronts and diseased areas. P. cinnamomi has considerable ability to produce a wide range of pathogenic phenotypes measured from ‘plant clone by isolate by environmental factor interactions’ in each of the three pathogen clonal lineages of P. cinnamomi isolated in Australia. P. cinnamomi lineages are defined by microsatellite types. In a few disease areas, despite both mating types occurring in the same square metre of soil, or 50 g soil sample or bait plant, no evidence of sexual reproduction (genomic recombination) has been found in the field. Isolates are sexually competent in the laboratory and all lineage combinations are strongly outcrossing. Hence the substantial variation in lineages in a range of traits associated with disease development must be arising asexually. Preventing movement of any P. cinnamomi contaminated material is critical to maximising disease control and minimising risks of introducing new strains which may threaten plant communities, their diversity and the integrity of these ecosystems. Phosphite or other intended phytophthoracide usage, which does not eliminate P. cinnamomi in plant nurseries can, by camouflaging disease, increase potential risks and threats by spreading different strains in diseased materials.

Nitrification is the process where ammonium is converted to nitrate by the activity of autotrophic nitrifying bacteria. This process is important in the supply of plant available nitrogen from the decomposition of organic material in soil. This experiment examined the activity and community of nitrifying bacteria in a sandy soil amended with either lucerne hay or wheat straw during a three month incubation. Lucerne hay significantly induced autotrophic nitrification activity, the main nitrification pathway. In contrast, wheat straw suppressed the rate of nitrification. Changes in the community structure of nitrifying bacteria in soils amended with either lucerne hay or wheat straw were observed by Polymerase Chain Reaction (PCR) and hybridisation. Both treatments increased the abundance of heterotrophic and ammonium oxidising bacteria. However, no such trend was observed for the nitrite oxidiser group, even though they have important role in the nitrification process.

AFLPs are being used in L. angustifolius to construct a molecular linkage map and find molecular markers linked to traits of interest. To date, over 700 polymorphic loci have been identified in an F2 mapping population. Linkage analysis, using MAPMAKER/EXP V3.0, has generated 21 linkage groups, including 2 AFLP markers linked to Ku (early maturity). A specific PCR test has been developed for this marker that can distinguish homozygous early and late maturing plants, as well as heterozygotes. Studies are continuing to find other markers, and to expand the linkage map.