Survival of Leptosphaeria maculans in soil on residues of Brassica napus in South Australia

The survival of Leptosphaeria maculans , which causes phoma stem canker (blackleg), on oilseed rape residues ( Brassica napus ) in South Australia was investigated. Using a quantitative polymerase chain reaction (PCR) assay for L. maculans DNA, the pathogen was mainly detected in the upper 5 cm of the soil profile, including residues on the soil surface. As the size of organic matter particles in the soil decreased, so did the quantity of L. maculans detected in them. To obtain representative data for a field, at least 30 subsamples needed to be collected over the 0·81 ha area studied. In a survey of 49 commercial fields in South Australia, most L. maculans was detected in fields 1 year after oilseed rape had been grown, with less detected after 2 years and negligible amounts 3 years or more after cropping. The diagnostic DNA-based assay for L. maculans reduced the time and cost of studying L. maculans survival in soil and increased the sensitivity and accuracy of results compared with estimates of propagule number of colony-forming units on a semiselective medium.     

Genetic, phenotypic and pathogenic diversity among xanthomonads isolated from pistachio (Pistacia vera) in Australia

Repetitive extragenic palindromic polymerase chain reaction (rep-PCR), sequencing of the 16S−23S rDNA internal transcribed spacer (ITS), biochemical and physiological tests, the Biolog microplate system, polyacrylamide gel electrophoresis (PAGE) of whole-cell proteins, and pathogenicity tests were used to characterize variability among xanthomonads isolated from pistachio trees suffering from bacterial dieback in four regions of Australia. ITS sequencing and rep-PCR revealed two distinct genotypes among the strains. The ITS sequencing suggested that the pistachio strains were closely related to Xanthomonas translucens pathovars, in particular X. translucens pv . poae . Results of physiological and biochemical tests, as well as Biolog microplate analysis and protein profiling, confirmed the existence of two groups. Furthermore, pathogenicity and host-range studies indicated that the two groups were biologically different. There was an association between the two groups and the geographical origin of the strains.     

Some relations of the meat ant, Iridomyrmex purpureus (hymenoptera: formicidae) with soil in South Australia

Iridomyrmex purpureus F. Smith is common in southern Australia and nests in conspicuous. often gravel-covered mounds. The effects of the ant on some properties of soil in its nests, and the effects of soil type on the distribution of the ant were studied by comparing the distribution of particle sizes in soil of nests and undisturbed soil profiles. Particle size analysis was used also to determine the relationships of nests and more or less truncated profiles on a naturally eroding, lateritic podzolic soil. Nests are constructed by excavation of gallery systems in the upper part of the profile, with a few shafts extending downwards for 1 metre or more. Excavated material is incorporated in the mound which is usually composed of soil from the A horizon with additions from the B horizon and imported gravel. The density and rate of turnover of nests both appear to he too low for I. purpureus to have any significance to pedogenesis. But soil texture and consistency do influence the distribution of the ant. for nests are not constructed in sands or in very loose, structureless soils. Examination of the distribution of particle sizes in material from nests on the lateritic podzolic soil suggests that establishment of some existing mounds may have pre-dated erosion which has, subsequently, removed more than 20 cm of A horizon. The erosion process would probably require at least hundreds of years, and. if the interpretation of these nests is correct, suggests that they are very old. This is not inconsistent with other observations on rates of change in this population of I. purpurens.     

Hyphal density as a measure of suppression of Gaeumannomyces graminis var. tritici on wheat roots

Runner hyphae of Gaeumannomyces graminis (Sacc.) Arx & Olivier var. tritici Walker on seminal roots of wheat seedlings were photographed and their length measured. As well, their length was estimated using the line-intercept method. The correlation of 0.904 between measured and estimated lengths of hyphae was highly significant. This line intercept method was used to estimate the density (length/unit area) of hyphae on roots of plants growing in the presence and absence of a soil suppressive to G. graminis var. tritici. Estimations were made eight times during 28 days growth at 15°C. In fumigated soil (non-suppressive) inoculated with 0.1% ground oat grain infested with G. graminis var. tritici, the density of hyphae on roots started to increase at five days compared with 15 days when soil there was a 10.8% cover of the root surface after 15 days when the hyphae had reached maximum density. Suppression to G. graminis var. tritici is normally detected by a difference in disease rating of roots at 28 days but this study has shown that suppression can be demonstrated by the difference in the density of hyphae if roots are examined between seven and 19 days.     

Physical factors influencing decomposition of organic materials in soil aggregates

Glucose or starch labelled with ¹⁴C was mixed thoroughly into slurried soils. Aggregates of different sizes were obtained from the soils as they dried. The labelled substrates were considered to be distributed in both micro- and macropores in the aggregates. Control samples (labelled substrates in macropores only) were prepared by adding the labelled carbohydrates after the formation of the aggregates. The various samples were sterilized by γ-irradiation and stored at ?15°C.Samples were wetted to about ?20kPa, inoculated with soil organisms, and incubated for 4 weeks at 28°C in closed systems, which enabled regular measurement of ¹⁴CO₂ released.Based on the ¹⁴CO₂ released, it was concluded that starch was protected from microbial attack when present in micropores in aggregates made from fine sandy loam.After incubation samples were dried and rewetted. The flush of ¹⁴CO₂ released was twice as big for samples containing labelled starch compared with glucose, showing that disruption of aggregates, containing residual starch, and rearrangements of soil components are as important as chemical and biological factors in causing the flush of CO₂ resulting from wetting a soil. Mechanical disruption of the aggregates resulted in a similar flush of ¹⁴CO₂.     

Carbon loss from roots of wheat cultivars

The amounts of organic materials released into soil from roots during the first 4 weeks of growth were determined for 11 cultivars of wheat (Triticum aestivum L.). Carbon loss from roots was measured by supplying ¹⁴CO₂ continuously to the shoots and measuring the ¹⁴C content of the roots, root-free soil, water-soluble material and CO₂ flushed from the root chamber. Six cultivars were compared in each of two experiments, with the cultivar Condor common to both experiments. There were no significant differences between cultivars, relative to Condor, for ¹⁴C activity present in soil, roots, water-soluble material or rhizosphere CO₂. There was a significant difference between cultivars in experiment 1, but not in experiment 2, for the variate log₁₀ (¹⁴C lost from roots: ¹⁴C translocated to roots).There was evidence that a reduction in growth temperature, within the range 10–15°C, increased carbon loss from wheat roots into the rhizosphere.     

The penetration of soil by mycelial strands of ectomycorrhizal fungi

Soil factors affecting growth of mycelial strands from ectomycorrhizas into surrounding soil were studied experimentally. Treated soil cores were inserted into root boxes for 4–6 weeks, then infiltrated with gelatin, sectioned, and mycelial strands harvested. Very large differences in mycelial strand growth (measured by weight) occurred between different soils, some soils completely inhibiting growth of strands of the mycorrhizal fungus Rhizopogon luteolus. Compaction of soil reduced mycelial strand penetration greatly e.g. by 80 per cent in one experiment. Soil sterilization ameliorated compaction effects. High soil phosphorus tended to increase strand growth but nitrogen level and Pseudomonas fluorescens. a common soil bacterium, had small or inconsistent effects.