The effects of copper, boron, cobalt, manganese, molybdenum, and zinc on the production of zoosporangia by P. cinnamomi and P. drechsleri in the presence of favourable concentrations of calcium, magnesium, potassium, and iron were investigated. Copper ions were the most effective in reducing the numbers of zoosporangia formed by both fungal species. Molybdenum was also slightly inhibitory. Total inhibition of mycelial growth occurred between 1 and 5 X 10(-5) M Cu2+ whereas total inhibition of sporangial formation occurred between 1 and 5 X 10(-7) M Cu2+. At copper concentrations between 10(-5) M and 5 X 10(-7) M, many P. drechsleri zoosporangia were abnormal in appearance and nonviable. Infection of eucalypt cotyledons by P. drechsleri zoospores was inhibited by 10(-6) M Cu2+ but this inhibition was reversed by EDTA (10(-4) M). There was no evidence for interaction between Cu2+ and Ca2+, Mg2+, K+, or Fe2+ present in the solutions used in the axenic production of zoospores. Preliminary pot trials indicated that CuSO4 had a protective action for safflower seedlings to infection by P. drechsleri when CuSO4 was applied as a dilute solution over the infection period.

The effects of four cations, Ca2+, Mg2+, K+, and Fe3+, upon the production of zoosporangia by isolates of four species of Phytophthora in axenic culture have been investigated. A response surface design was used to examine main effects and possible interactions. Responses to Ca2+ and Fe3+ were strongly quadratic with the higher concentrations inhibiting sporangial production in P. cinnamomi and P. drechsleri isolates. Responses to Mg2+ and K+ were weaker and, in the case of magnesium, were linear rather than quadratic. There was no significant interaction between cations except in the case of P. cambivora where the interaction between Mg2+ and K+ was significant at the 5% level. For each cation, the optimum concentration for zoosporangial production was calculated. The processes of zoospore release and infection of plant material by P. drechsleri showed a strong linear and weak quadratic response to Ca2+, higher concentrations of Ca2+ favouring these processes. A strong quadratic response was obtained to Mg2+, K+, and Fe3+. Interaction between Ca2+ and Mg2+ was evident for all three host materials used; Mg2+ and K+ showed an interaction when Pinus radiata was used as the host material. The implications and limitations of these results are discussed.

The potential for N2 fixation by free-living bacteria using straw as a source of energy was evaluated in three soils (one from Gunnedah and two from Cowra) representative of the wheat belt in New South Wales. All three soils had a history of straw incorporation. The abilities of the respective microbial populations to use a range of carbon sources, including potential products of decomposition of straw, was determined and compared with the size and composition of each population. Neutral to alkaline (pH 7.4) soil of high (51%) clay content from Gunnedah produced higher rates of nitrogenase activity with straw than more acid (pH 5.6) lower (17%) clay containing soil from Cowra (site B). Gunnedah soil also contained a larger population of N2-fixing bacteria which used a broader range of energy sources than soil from either Cowra site B or Cowra site W (pH 5.8, clay content 34%). There was little difference in the composition of the N2-fixing populations in each of the soils except that Azotobacter spp. were absent from the acid Cowra soils. It was concluded that the difference in behaviour of the respective N2-fixing populations was primarily due to the physical characteristics of the soil affecting the numbers and activities of diazotrophic microorganisms. In addition some soil environments failed to support specific organisms.

Zoospores of Australian isolates of Phytophthora drechsleri, P. cryptogea, P. cinnamomi, P. nicotianae var. parasitica, and P. citricola were examined for their chemotactic responses to asparagine, glutamine, aspartate, glutamate, and structurally related compounds. Structural requirements for attraction include the alpha-amino-acid group with a short carbon chain terminating in an amide group. The one American isolate tested gave a different result and possible reasons for this are discussed. The pH of the environment was important, a neutral-charged molecule was more attractive than a negatively charged molecule, hence glutamine and aspartate were more attractive at pH 3.0 than pH 5.0. Zoospores tended to move away from regions with a high hydrogen ion concentration. Compounds other than amino acids were slightly attractive including several sugars and ethanol. Synergistic interactions between amino acids, ethanol, and sucrose were observed and may account for the high levels of attraction of zoospores to root exudates and extracts.

In a series of five trials, three conducted in the field and two in seed-flats in the nursery, it was shown that mulched residues from Phalaris aquatica and Triticum aestivum reduced the germination and production of Trifolium subterraneum. The deleterious effects observed with the mulched residues were not evident when the residues were incorporated into the upper soil layer. It is suggested that a reduction of the residues in pastures before the opening rains in autumn would be an essential requirement to ensure good germination and early seedling vigour from the annual legume component in mixed legume/grass pastures, especially those based on P. aquatica.