P. Scott

Phytophthora multivora sp. nov., a new species recovered from declining Eucalyptus, banksia, agonis and other plant species in Western Australia.

A new Phytophthora species, isolated from rhizosphere soil of declining or dead trees of Eucalyptus gomphocephala, E. marginata, Agonis flexuosa, and another 13 plant species, and from fine roots of E. marginata and collar lesions of Banksia attenuata in Western Australia, is described as Phytophthora multivora sp. nov. It is homothallic and produces semipapillate sporangia, smooth-walled oogonia containing thick-walled oospores, and paragynous antheridia. Although morphologically similar to P. citricola, phylogenetic analyses of the ITS and cox1 gene regions demonstrate that P. multivora is unique. Phytophthora multivora is pathogenic to bark and cambium of E. gomphocephala and E. marginata and is believed to be involved in the decline syndrome of both eucalypt species within the tuart woodland in south-west Western Australia.

Novel phosphite and nutrient application to control Phytophthora cinnamomi disease

Systemic treatment of stems with injections of phosphite liquid and novel soluble capsule implants of phosphite, PHOSCAP® (phosphorous, potassium, iron, manganese, zinc, boron, copper, magnesium and molybdenum) and MEDICAP MD® (nitrogen, phosphorous, potassium, iron, manganese, and zinc), were applied to Banksia grandis and Eucalyptus marginata trees to control Phytophthora cinnamomi. Four weeks after treatment application, excised branches were under-bark inoculated with P. cinnamomi. In B. grandis, phosphite implants and liquid injections significantly reduced lesion length compared to the control, and MEDICAP MD® implants; however, there was no significant difference in lesion length between trees treated with phosphite implants and liquid injections and PHOSCAP implants. In E. marginata, phosphite implants and liquid injections significantly reduced lesion length compared to the control, PHOSCAP® and MEDICAP MD® implants. In B. grandis and E. marginata, PHOSCAP® and MEDICAP MD® implants reduced the average lesion length compared to the control; however, the interactions were not significant. Results show that both liquid phosphite injections and novel phosphite implants are effective at controlling lesion extension in B. grandis and E. marginata, caused by P. cinnamomi. Further work is required to determine if nutrient application reduces Phytophthora disease through improving plant health.

Foliar phosphite application has minor phytotoxic impacts across a diverse range of conifers and woody angiosperms.

Phytophthora plant pathogens cause tremendous damage in planted and natural systems worldwide. Phosphite is one of the only effective chemicals to control broad-scale Phytophthora disease. Little work has been done on the phytotoxic effects of phosphite application on plant communities especially in combination with plant physiological impacts. Here, we tested the phytotoxic impact of phosphite applied as foliar spray at 0, 12, 24 and 48 kg a.i. ha(-1) . Eighteen-month-old saplings of 13 conifer and angiosperm species native to New Zealand, and two exotic coniferous species were treated and the development of necrotic tissue and chlorophyll-a-fluorescence parameters (optimal quantum yield, Fv /Fm ; effective quantum yield of photosystem II, ΦPSII ) were assessed. In addition, stomatal conductance (gs ) was measured on a subset of six species. Significant necrosis assessed by digital image analysis occurred in only three species: in the lauraceous canopy tree Beilschmiedia tawa (8-14%) and the understory shrub Dodonaea viscosa (5-7%) across phosphite concentrations and solely at the highest concentration in the myrtaceous pioneer shrub Leptospermum scoparium (66%). In non-necrotic tissue, Fv /Fm , ΦPSII and gs remained unaffected by the phosphite treatment. Overall, our findings suggest minor phytotoxic effects resulting from foliar phosphite application across diverse taxa and regardless of concentration. This study supports the large-scale use of phosphite as a management tool to control plant diseases caused by Phytophthora pathogens in plantations and natural ecosystems. Long-term studies are required to ascertain potential ecological impacts of repeated phosphite applications.

Development of a high throughput optical density assay to determine fungicide sensitivity of oomycetes

A high-throughput assay was developed to screen Phytophthora species for fungicide sensitivity using optical density measurements for unbiased, automated measurement of mycelial growth. The efficacy of the optical density assay (OD) to measure phosphite sensitivity in Phytophthora species was compared to two widely used methods, radial growth (RG) and dry weight (DW) assays. Three isolates of each of Phytophthora cinnamomi, P. multivora and P. pluvialis, with known phosphite exposure and three isolates of each species with no prior phosphite exposure, were screened for phosphite sensitivity using the three assays. Mycelial growth measurements were taken after culturing for 6, 14 and 15 days for the OD, DW and RG assays respectively. Mycelial growth inhibition at 15, 80, 200 and 500 μg/mL phosphite relative to growth on control media was used to determine effective concentration values for 50% growth reduction (EC50). The species varied in their tolerance to phosphite with P. cinnamomi being the least sensitive followed by P. multivora and P. pluvialis. No significant differences in tolerance were found between isolates within the same species using any method. The OD assay produced comparable EC50 values to the RG and DW assays. The growth of the three species was more sensitive to phosphite in the DW than the RG and OD assays, however limited sample throughput and greater variation in measuring small amounts of mycelia in the dry weight assessment increase variability and limits throughput. The OD assay offers a fast method to enable an inventory of chemical resistance and is particularly advantageous for slow growing species as it requires less time and offers greater throughput than existing RG and DW methods.