School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEpidemiological and physiological studies of the effects of peach rosette and decline disease on the peach, prunus persicae L. Batsch( 1975)The incidence in the field of the disease peach rosette and decline (PRD), which is of considerable economic importance in the Goulburn Valley, causing fruit loss and tree death, was shown to increase from 0.9 to 91.3% in an orchard of cv. Golden Queen in 10 years. Similar results were found with the cv. Pullars Cling, in which infection increased from 1.5 to 29.7% over five years. The pattern of spread was mainly from infected trees to contiguous uninfected trees. This is consistent with the view that the main causal agent, prune dwarf virus (PDV), is transmitted only via the transfer of infected pollen : a previous finding in cherries which was confirmed in peaches. Prunus necrotic ringspot virus (PNRV) is the other virus always present in the field in PRD-infected trees. The mode of spread of PNRV is also by pollen. Within the tree, PDV moved erratically from the first infected limb, via phloem but not xylem, into the other limbs well in advance of the appearance of symptoms. Three months after flowering, PDV was detected in 65% of main limbs adjacent to the first infected limbs but in only 30% of limbs more remotely positioned on the tree. However, removing infected limbs within four weeks of flowering, when the initial infection was presumed to have occurred, did not prevent the movement of PDV into the rest of the tree. Laterals from peach trees infected with PRD were tested for the presence of PDV, using woody virus indicators (cvs Golden Queen, Italian Prune and Elberta seedling). Golden Queen was found to be a more reliable indicator for detecting PDV than Italian prune, as the presence of PNRV with PDV killed 71% of the Italian prune buds compared to only 34% of the Golden Queen buds. Golden Queen also developed more obvious foliage symptoms of PDV infection than Elberta seedlings. The probability of failing to detect PDV in infected field trees, using all three indicator plants, was higher in the first year of infection. The rate of spread of PRD was reduced in the orchard by preventing infected trees from flowering, either by removing obviously infected trees or by deblossoming. Removing infected trees resulted in a three-fold reduction in the spread of the disease in two seasons. Removing the flowers from infected trees before pollination reduced the spread of the disease by about half. This, only partial, control of the spread of PRD by tree removal or deblossoming was attributed to the presence of up to 14.3% of trees without symptoms being latently infected with PDV. It was observed that deliberate infection with PDV by pollen also resulted in a slow expression of the symptoms of PRD. The effects of PRD on the growth of young peach trees was obvious in the first three months of growth. There were considerable varietal differences in the severity of this effect. Those varieties based on cvs. Golden Queen or Levis Cling were more severely affected than the variety Elberta. The results from shoot elongation measurements agreed with those obtained from conventional growth analysis methods. These latter experiments showed that, after three months, the dry weight and leaf area of infected Golden Queen plants were reduced by 94%. The fruit yield from mature PRD-free trees was three times that of trees infected for the first season, even though symptoms were apparent only on one limb; and six times that from chronically affected trees infected for two seasons. The effect of virus infection on the photosynthetic ability of single, attached peach leaves was studied under laboratory conditions using infra red gas analysis. The constants derived from the equations describing the relationship between net photosynthesis (Pn) and both irradiance and CO2 concentration were used to analyse the effects of infection by PRD on photosynthetic characteristics of the leaf. The asymptotic value of Pn (Pmax) in young leaves was reduced 15% by PRD-infection, mainly through an increase in the "residual resistance" to 002 diffusion and a decrease of 23% in the parameter indicating photochemical efficiency. There was also evidence that the gas phase resistance was higher in infected leaves at low levels of irradiance. Dark respiration was 51% higher in infected leaves, but this difference was not significant. PRD did not reduce Pn in 60-day-old leaves, normal leaf senescence having a predominant and greater effect. It was concluded that PRD infection had its large effects on growth via a reduction in leaf area; the effects on the photosynthetic capacity per unit leaf area being minor. An effect of PRD infection on the translocation of 14C-assimilatesout of leaves was also observed. Infected leaves retained twice the assimilates than did uninfected leaves. It is concluded that the most promising methods of control of PRD include removal of infected trees, deblossoming suspected infected trees until diagnosis is confirmed, use of virus-tested plants, the gradual destruction of infected orchards and protecting young, healthy orchards from infection either by barrier crops or deblossoming the young plants until they reach an economic bearing age.
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ItemDevelopment of management practices for control of Rhizoctonia solani Kuhn (AG 8) in Victorian cereal crops( 2003)The studies reported in this thesis showed that Rhizoctonia solani Anastomosis Group 8 was widespread in all Victorian cropping soils. In barley, the incidence of Rhizoctonia root rot at anthesis, was 54%, 70%, 93% and 97% on grey clay, red clay, red duplex and Mallee soils respectively. In wheat, the incidence at anthesis was 70%, 69%, 74% and 68% on the grey clay, red clay, red duplex and Mallee soils respectively. Over the years 1990-1992, the disease caused an average grain yield loss of 12% in barley and 5.5% in wheat, with the highest losses occurring on the Mallee and red duplex soil types. It was shown that farmers were not achieving adequate control of Rhizoctonia root rot, and that cultivation per se was not effective in reducing its incidence or severity. In barley, the average incidence of Rhizoctonia root rot at anthesis was over 90% in crops grown on Mallee or red duplex soil types, regardless of tillage practice. Other researchers have reported tillage to reduce the incidence and severity of Rhizoctonia root rot. In wheat, incidence at anthesis was over 60% in crops grown on all soil types, regardless of tillage practice. The distribution of R. solani (AG 8) was related to both biological and physical factors of the different soil types. The disease was more severe in soils where organic matter levels were low, and there was little natural suppression of the fungus by other organisms. It is postulated that increasing the amount of organic matter in the soil would lead to a higher microbial mass in the soil and the resulting competition would reduce the survival of R. solani (AG 8). The root and hypocotyl rot strain of R. solani AG 2-1 (ZG 6) that occurs on pulses, was found on wheat and barley roots, and the cruciferous strain AG 2-1 (ZG 5) was found on both wheat and lupin roots and hypocotyls. These strains of R. solani could therefore increase under intensive rotations in the Wimmera and North-Central areas. Results from the field experiments demonstrated that the use of "cultivate deep - sow shallow" methods of crop establishment in conservation farming systems are as effective in controlling Rhizoctonia root rot, as traditional cropping systems. However, control could be improved with a strategic cultivation in years with an early autumn break to maximise destruction of the fungus networks of the fungus in the soil. Deep banding fertiliser, 5-10 cm under the seed, with the depth-modified combine at sowing reduced the severity of Rhizoctonia root rot and increased grain yield in 5 out of 9 field experiments. This indicates that deep banding fertiliser is viable option for farmers, and further studies in this area with a range of fertilisers would confirm these results. As a consequence of the studies represented in this thesis it was concluded that further research into the management of R. solani (AG 8) should concentrate on developing host plant resistance with transgenes. Other diseases: Cereal cyst nematode (Heterodera avenae) was detected in all soil types, and caused annual yield losses of 6.0% in barley and 6.6% in wheat. The nematode was most common on Wimmera grey clays, and was recorded for the first time on the red duplex soils of the North-Central district. Further studies are required to determine if changes in cultural practice through the use of gypsum, stubble retention and direct drilling to improve soil structure will increase the incidence of cereal cyst nematode. The re-introduction of susceptible cultivars into the Wimmera could lead to major outbreaks of cereal cyst nematode. Take-all (Gaeumannomyces graminis var. tritici) was detected in crops growing on all soil types. Yield losses were 4.9% in barley and 5.1% in wheat, and the pathogen was most prelevent in Mallee and red duplex soils. It is recommended that further studies should be undertaken to determine whether the adoption of conservation farming practices to improve soil structure will reduce the incidence and severity of take-all.
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ItemThe impact of root and stolon rot of white clover in Northern Victorian dairy pastures( 2001)An investigation into the impact of root and stolon rots on white clover commenced in July 1995 and concluded in June 1998. This involved a survey of root and stolon disease over three seasons and the isolation of fungal and nematode species that were detected in the plant tissue and in the surrounding soil. The fungal isolates found were then subsequently tested for pathogenicity on white clover seedlings and adult plants. The project aimed to: (i) develop methods for root and stolon rot disease monitoring and assessment, (ii) determine and quantify the impact of root and stolon rot diseases on white clover dairy pasture production, and (iii) identify fungi that invade root and stolons of white clover in pastures and test their pathogenicity. In order to properly describe and identify rots on roots and stolons, descriptive symptom codes were developed with collaborators in New South Wales and New Zealand. These codes were used as standards in order to describe the status of stolons and root in all studies conducted during this project. In a field experiment used to evaluate the effect of selective fungicides and a nematicide on white clover decline there were usually no effects of treatments on pasture dry matter production or white clover content. Also agronomic assessments suggest application of fungicides and a nematicide did not increase total dry matter production. In the later stages of the experiment (mid year 2 - year 3), the application of fungicides reduced (P<0.05) dry matter production relative to the control. Within the first two to three weeks after the last or previous chemical applications, pasture dry matter and white clover content were higher (P<0.05), and stolon and root rot diseases were reduced (P<0.05) in either the fungicides alone, nematicide alone or the combination compared to the untreated control. The nematicide treatment showed significantly higher white clover content than the other treatments. There was no association between root and stolon rot diseases and either pasture dry matter or white clover content throughout the experimental period. A pathogenicity test for seedlings of white clover was developed and proved to be an ideal method to rapidly determine the pathogenicity of large numbers of fungi. This is the first time the author has noted this method used on white clover exclusively. Of 400 fungi isolated, 350 isolates were tested on seedlings and six week old white clover plants for their ability to cause disease. Rhizoctonia spp. and Fusarium spp. (including F. oxysporum, F. subglutinans, and F. solani) were among the highly pathogenic fungi. The pathogenicity tests provided evidence of fungi as a cause of white clover root rot. A photographic study of the rots on stolons of white clover found that most of the fungi tested infected the stolons when they were wounded. Only Rhizoctonia spp. Group 2 and Fusarium oxysporum had the capacity to form lesions on non-wounded stolons. Nematodes found in the field study included Helicotylenchus spp., Psilenchus spp., Quinisulcius spp., Heterodera trifola, Tylenchus spp. and Tylencholamius spp. Nematicide application reduced populations of Helicotylenchus spp. and Tylenchus spp. relative to the untreated control. The increase in pasture dry matter production in the nematicide treatment indicated that these ecoparasites could have a role in the root disease complex. The severity of root and stolon rot symptoms was not correlated with dry matter production or pasture composition. Results obtained from this research suggest "root and stolon diseases caused by soilborne fungi do not reduce the productivity of white clover in dairy pastures". It is more likely that white clover decline is pnmarily induced by abiotic stresses that act as predisposing factors for a secondary invasion by soilborne fungi. Nematodes, in addition to the damage they cause on their own, also provide entry points for secondary invading soilborne fungi.
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ItemThe codling moth in Williams pears : a report of an investigation into the problem in the Goulburn Valley, 1936-19( 1944)The codling moth (Cydia pomonella,L.) is the most serious pest of pears in the Goulburn Valley irrigation area. In the Shepparton, Kyabram, and Ardmona districts, there are approximately 3500 acres of Williams Pon Chretien pears grown principally for canning purposes. For some years past, the losses of pears due to the codling moth have been considerable. In 1936 it was estimated, by executives of the Northern Victoria Fruitgrowers' Association, that codling moth losses accounted for as much as 20% of the total pear crop from the area, that is, a loss of at least �20,000 per annum was being experienced by the industry. No investigation of the codling moth had ever been conducted in the Goulburn Valley; and, although a considerable amount of research had been done elsewhere on the control of the pest in apples, little information was available on combatting the codling moth in pears. Ward (1936) conducted a survey of the pear-growing districts of the Goulburn Valley and found that there was a complete lack of uniformity among growers as to the control measures being used against the codling moth. Having no precise information on the seasonal activity of the pest, growers were not In a position to apply their sprays at the correct time, nor was there an efficient spray schedule in general use. In response to a request from the Northern Victoria Fruitgrowers' Association, and with funds made available from a Commonwealth grant for research and advisory work in relation to the apple and pear industry, an investigation of the problem of codling moth in 'NEC pears was commenced in 1936 by the Victorian Department of Agriculture. For the first two years of the investigation, portion of the Oriental Peach Moth Laboratory at the Ardmona Cannery was kindly made available by the Council for Scientific and Industrial Research, and so facilitated a study of the bionomics of the codling moth under Goulburn Valley conditions.
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ItemSclerotinia minor in sunflowers : onset of disease and bioprotection with gliocladium virens( 1994)Sclerotinia stem rot (Sclerotinia minor) is a serious constraint to sunflower production in south-eastern Australia, where the lack of cultivars with adequate levels of resistance and the costs of chemical control make biocontrol an attractive option. Onset of disease in the field occurs at bud development. Field trials with the highly susceptible inbred line, PacF2582, confirmed the absence of disease during vegetative growth. A study of root development found a marked proliferation of lateral roots at GS 3.1-3.3 (early to mid-bud) in field-grown sunflowers and slightly earlier in glasshouse plants. Glasshouse studies demonstrated the susceptibility of vegetative stage plants to infection of lateral roots by one or 2 pre-germinated sclerotia. Onset of disease in soil inoculated to a high density with sclerotia was also delayed, generally until GS 1.5 (8 leaf), suggesting that occasional contact between roots and sclerotia is not sufficient for disease initiation. A model system was developed to examine the effects of surface microflora and exogenous nutrient on sclerotial germination. Field sclerotia, incubated on water agar disks, required surface sterilisation to relieve fungistatic dormancy. Root sap and exudates from sunflower lines stimulated germination of partially surface sterilised field sclerotia, as did root sap from non-host species. Germination rates were significantly higher with root nutrient from plants during vegetative growth than from plants at mid-bud stage. Exposure to root sap for more than 3 days was required for stimulation of germination in vitro. The germination response of soilconditioned culture sclerotia to nutrients was less consistent than that of field sclerotia. The results indicate that root nutrients have potential to stimulate germination during vegetative growth, but onset of disease awaits root proliferation at bud development. It was proposed that a threshold of root activity prolongs the exposure of sclerotia to root exudates, while overlapping rhizospheres may provide a critical concentration of nutrient for germination and mycelial development. An isolate of Gliocladium virens from disease-affected soil proved highly antagonistic to S. minor in culture. Seed treatment with G. virens suppressed disease induced by inoculation of lateral roots in pasteurised potting medium and in field soil. Significant disease control was also obtained by sowing seed into a plug of G. virens growing on V8-vermiculite or by pre-germination of seed with a culture of G. virens. Bioprotection by seed treatment with G. virens was evaluated at two field sites, and was effective in field microplots where disease incidence was less than 50%. Since G. virens, applied on seed, did not affect root growth and did not grow into the lateral root zone, it was concluded that seed treatment with G. virens creates a "cordon sanitaire" around the upper tap root, sufficient for bioprotection of inoculated plants in pots and field microplots. If seed priming techniques can be adapted to allow germination of G. virens on seed before sowing, it may be possible to enhance the cordon sanitaire and delay onset of disease in the field to obtain commercial yields.
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ItemAn investigation of potato gangrene in Victoria : including the identification and comprehensive study of the causal organism, Phoma foveata Foister and a comparison of the Scottish and Victorian isolates with P. eupyrena Sacc., P. solanicola Prill et Del. and Phomopsis tuberivora Guss et Fost( 1957?)In October 1963 attention was drawn to the possibility of an hitherto unknown storage disease of potatoes in Victoria. Diseased tubers if the variety ton ware brought to the Plant Research Laboratory, Burnley, from a property at Clarke's Hill, via Ballarat, and a pycnidial-forming fungus was isolated consistently from lesions by Anderson (1953). Later that month, apparently identical isolates were obtained from diseased tubers of two other varieties which were received from the same property. In consequence, an investigation of dry rots in storage was commenced in September, 1954. One of the original objectives was to determine whether this pycnidial-forming fungus was a causal organism of tuber rot. As the investigation proceeded, it became evident that the fungus in question was responsible for a particular type of rot. In other countries similar types of tuber disease have been attributed to species of the Phomaceae. In the detailed study that followed, attempts were made to identify Victorian isolates with, or to distinguish them from, some of these other storage pathogens. As a result of the investigation, the pycnidial forming fungus has been identified as Phoma foveats Foister.. This fungus is responsible for the disease known as Gangrene of tubers in the British Isles. It has not been recorded previously outside that area. Although most of the work described in this report is laboratory in nature, one of the main aims has been to collect information which may may assist in determining an approach to the control of the disease.
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ItemStudies on the cereal cyst nematode (Heterodera avenae Woll.) in Victoria( 1973)Ten populations of the cereal cyst nematode (Heterodera avenge Will.) from the Victorian Mallee and Wimmera districts, were tested for their variation in pathogenicity, using a selected range of cereal species and cultivars as.indicators. The tests were made in a glasshouse between 1967 and 1969. Of the oats used, Avena sterilis and A.strigosa were resistant, but all other cultivars and species were susceptible to each of the populations. Two barley cultivars, Morocco, and Marocaine 079 were resistant to all populations, but all other barleys used were susceptible. None of the cultivars of wheat was resistant; the spring wheat cv. Loris was very susceptible. Rye cv. South Australian was resistant to all populations. The results indicate that a single biotype of H. avenge is present in Victoria. It appears to be more "aggressive" than any of the biotypes known in Europe. Two hundred and forty cereal species and cultivars (wheat, barley, oats, rye and Triticales) were field . tested for resistance to H. avenge at Sea Lake in the Victorian Mallee between 1966 and 1969. All wheats tested were susceptible to very susceptible, and all Triticales were susceptible. Two barley cultivars Morocco, and Marocaine 079 were resistant and two others moderately resistant. Avena sterilis and A. strigosa were resistant and fifteen cultivars of A. sativa were moderately resistant. Rye cv. South . Australian, posseses a high degree of resistance in contrast to European ryes. The systemic nematicides "Temik" and "Lannate", and the fumigant nematicides ethylene dibromide and dibromochloropropane, were tested for control of H. avenge in a field experiment with Wheat at Sea Lake in the 1968-69 season. All chemical treatments reduced white cyst production, and four of the treatments increased grain yield, and reduced nematode carryover. "Temik" (11 kg/ha broadcast), gave the best nematode control and more than trebled grain yield. "Lannate" (13'kg/ha broadcast) was almost as effective. Dibromochloropropane was an effective nematicide, but was phytotoxic at 56 1/ha. The systemic nematicides "Temik", "Lannate", "Nemafos", "Vydate", "Nemac ur P", and the contact nematicide "Mocap" were tested for control of H. avenge in field experiments with wheat; at Greenland Dam in the Wimmera (1969-70), and at Sea Lake in the Mallee (1969-70 to 1971-72). In one series of experiments all the nematicides were compared on a broadcast basis. All chemical treatments reduced white cyst production and nematode carryover. Plants from plots treated with "Temik" (9 kg/ha) were always cyst-free. "Mocap" when used at the same rate was severely phytotoxic. Grain from plots treated with "Temik" and "Lannate" at the higher rate (9 kg/ha) were analysed for the presence of chemical residues. No "Lannate" residues were detected, and "Temik" residues were less than 0.1 ppm. In a second series of experiments, a comparison was made of methods and rates of application of "Temik" and "Lannate". All broadcast nematicide applications, except those at Sea Lake in 1969-70, reduced white cyst production and nematode carryover. All nematicide applications with superphosphate in the drill row, except for "Lannate" 0.3 kg/ha at Sea Lake in 1971-72, also reduced white cyst production and nematode carryover. Significant grain yield increases were obtained in each experiment. "Temik" (9 kg/ha broadcast, and 2.2 kg/ha in the drill row) consistently gave the best control of H. avenge. Plants from plots receiving these treatments were always cyst-free. In all experiments "Lannate" dusted wheat seed provided little, if any, control of H. avenge. When used at comparable rates of application, "Temik" was a more effective nematicide than "Lannate".
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ItemThe occurrence of brittleheart in Eucalyptus regnans and its effect on various wood properties( 1990)This project is mainly concerned with a description of anatomical and ultrastructural characteristics of cell wall deformations in brittleheart of E. regnans, development of methodology for quantification of percentage of broken fibre pieces (PBFP) in E. regnans, and physical and anatomical properties of E. regnans in relation to the occurrence of brittleheart. Two 1939 regrowth E. regnans butt logs and one mature growth E. regnans butt log removed from a tree approximately 120 years old were used in the study. The anatomical and ultrastructural characteristics of cell wall deformations were examined with bright field, polarized light, and scanning electron microscopy. The microscopic compression lines were found to consist of slip planes, minute compression failures, corrugations, and buckles. The width of microscopic compression lines along the longitudinal fibre axis ranged from one single fold in the cell wall up to 7 times the average fibre width. The length of the microscopic compression lines ranged from ones which only crossed a few fibres to ones which crossed up to 200 fibres. The severity of microscopic compression lines appeared to be dependent on the type of cell wall deformations and degree of compaction in the deformed zone. In the methodology studies it was found that for a pulp sample prepared from a 2 mm by 2 mm by 15 mm wood chip approximately 400 cellular elements in 8 out of 16 areas on a slide needed to be counted in order to obtain PBFP with less than 10% error. For macerated thin sections, all the cellular elements on 2 slides each carrying about 600 fibres and fibre pieces needed to be counted. A highly significant correlation was found between the length of microscopic compression lines per unit area and the microscopically determined PBFP. A highly significant correlation was found between the theoretical PBFP and the microscopically determined PBFP. These findings confirm that maceration of wood containing cell wall deformations results in broken fibres and verifies the validity of the maceration technique for quantifying the amount of microscopic compression lines. PBFP was found to increase with macerating time. A maceration time around 10 hours appears sufficient to cause fibres having cell wall deformations to break but longer times cause fibres without cell wall deformations to also break due to overmaceration. A significant relationship was found between PBFP determined after 5 hours and after 10 hours of maceration at the same temperature. It was found that parameters extracted or derived from cell length distributions produced by the Kajaani FS-200 may be used to determine the amount of fibre fragments in pulp samples. These parameters were the high peak, the length weighted average, and the mass weighted average of the cell length distributions for pulps which had PBFP greater than 10. Based on the maceration technique, brittleheart was detected in both the mature and the 1939 regrowth logs. Brittleheart was more severe and occupied a larger area in the mature growth than in the regrowth wood. The PBFP was found to be mostly below 5 in the regrowth wood although relatively high PBFP values of 21 and 30 were observed. PBFP values as high as 85 was found in the mature growth wood. In general, PBFP was found to be higher nearer the pith, decreasing toward the bark, and dropping to zero before reaching the two-thirds theoretical point where the stress is assumed to be 0. A large circumferential variation in PBFP was observed in four adjacent growth rings of the mature log. A large variation in PBFP was also found within a volume of 1000 mm^3 for both the mature and the 1939 regrowth wood. The earlywood PBFP was found to be significantly higher than the latewood PBFP for the mature and regrowth logs. A total of 72 green and 132 12% MC Izod specimens were prepared from the two 1939 regrowth logs and tested for impact strength. The mean Izod value was found to be 9.9 ft.lb for the 132 12% MC Izod specimens and 9.2 ft.lb for the 72 green Izod specimens. For side-matched Izod specimens, the mean Izod value was found to be 9.2 ft.lb at green and 9.7 ft.lb at 12% MC and the mean for the 12% MC specimens did not reflect the expected increase in strength with moisture loss. Excessively low Izod values (eg. 1.8 ft.Ib) were found in the 12% MC Izod specimens located near the pith. For 76 12% MC Izod specimens, their PBFP, fibre length, and specific gravity were also measured. Significant relationships (p=0.01) were found between Izod values and specific gravity, PBFP, and fibre length for these 76 12% MC Izod specimens. Specific gravity and fibre length positively contribute to the impact strength whereas PBFP negatively affects the impact strength. Brash-break specimens showed a low mean Izod value, a low mean specific gravity, the presence of brittleheart, and a short mean fibre length.
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ItemStudies of barley yellow dwarf virus infection in ryegrass (Lolium spp.)( 1989)Measurements were made of the effect of barley yellow dwarf virus (BYDV) infection on the early growth of four commercial cultivars of ryegrass (Lolium spp.) under two different temperatures (24C and 16C). At 24C, BYDV infection was associated with reduced root dry weight (30-40%) in all cultivars; the effect of infection on shoot dry weight and leaf area was variable. At 16C, the effect of BYDV infection was variable, being associated with increases in both leaf area and root and shoot dry weight in one cultivar (Grasslands Arild) and decreases in another (Victorian). In two other cultivars, both leaf area and root and shoot dry weight were not significantly affected (p > 0.05) by infection with BYDV. There was no evidence of visible symptoms associated with BYDV infection. At 240C, reductions in root dry weight associated with BYDV infection were not reflected in reductions in the relative growth rates of the roots. Up to at least 28 days after inoculation (46-50 days after germination) reductions in root dry weight were associated with both aphid-feeding damage and virus infection. Experiments with the cultivar Victorian showed that shoot dry weight was not significantly affected (p > 0.05) by feeding with either viruliferous (BYDV) or non-viruliferous aphids (Rhopalosiphum padi L.). At 16C, changes in root and shoot dry weight were associated with changes in the respective relative growth rates. Both visual scoring and changes in plant dry weight were found to be unreliable criteria for assessing disease severity. Two assay systems, an enzyme-linked immunosorbent assay (ELISA) and a nucleic acid, or complementary DNA (cDNA) probe, were developed for the detection of BYDV in ryegrass. The conditions for detecting BYDV with the ELISA (the "double sandwich" variant) were optimized. Exposure of the virus to extracts of ryegrass leaf tissue for 30 min or more resulted in reductions in absorbance, with the amount of the reduction dependent on the antigen (virus) incubation temperature. The optimization of conditions for the immunoglobulin (IgG) used for coating microtitre plates (coat IgG), the IgG - alkaline phosphatase conjugate (conjugate IgG) and the antigen (virus), in conjunction with the use of polyethylene glycol - 6 000 in the preparation of the conjugate buffer, gave a rapid and reliable ELISA procedure for the consistent detection of BYDV at levels of 2 ng/ml. Both ELISA and cDNA probe techniques were used to study the incidence of BYDV isolates in perennial ryegrass at three sites (Balmoral, Mininera and Hamilton) in south-western Victoria. BYDV was detected in perennial ryegrass cultivars from the three sites, with different levels of incidence of BYDV isolates between cultivars. While the PAV-related isolate was prevalent at the three sites, the incidence of the MAV- and RPVrelated isolates varied with the site and the cultivar. One difficulty with the ELISA was that absorbance values did not fall in to distinct infected and uninfected groups within the cultivars, and at Mininera, the incidence of BYDV varied significantly between the various positive - negative ELISA thresholds that were used. At Balmoral, the incidence of BYDV determined with each threshold was compared with that determined by cDNA probing using a Spearman correlation test. A threshold of double the mean negative (uninfected ryegrass) control showed the highest correlation with the cDNA probe for a number of replicate plots at the site. The incidence of the PAV-related isolate was compared for the two cultivars Ellett and Victorian, and significant (p < 0.01) reductions in its incidence in cv. Ellett compared with cv. Victorian at the three sites, suggested that cv. Ellett is resistant to the PAVrelated isolate of BYDV. The comparative resistance of cv. Ellett to at least this isolate of BYDV indicated the feasibility of breeding and selecting for resistance to other isolates of BYDV in perennial ryegrass. In attempts to quantify resistance in perennial ryegrass, the ELISA was further developed to allow measurement of BYDV titre in ryegrass shoot tissue. In twenty selections of cv. Victorian, the performance, as measured by changes in both root and shoot dry weights, of individual selections to infection with BYDV, did not significantly (p < 0.05) correlate with virus titre measurements. Experiments with twenty selections of cv. Ellett indicated that its resistance to BYDV that was e' ident at the three field sites was not associated with resistance to aphid (Rhopalosiphum path) feeding, and consequently, transmission of the virus. Attempts to inoculate cv. Ellett with either the PAV- or RPVrelated isolates were of limited success, and it was speculated that its resistance to the virus may be due to reductions in the level of virus replication.
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ItemDecline of river red gum (Eucalyptus camaldulensis Dehnh.) on grazing lands in Western Victoria( 1988)River Red Gum (Eucalyptus camaldulensis Dehnh.) is a common and extensively distributed tree in western Victoria. Widely-spaced stands of this species are a prominent feature of pastoral landscapes in that area. Many of these remnant stands are in a state of decline, with individuals being lost through deliberate clearing, senescence or the effect of rural dieback disease syndromes. This thesis reports on a study of the decline of one stand of E.camaldulensis trees growing on part of a grazing property near Buangor, in west-central Victoria. The rate of tree loss from agricultural land in the Buangor district was assessed from aerial photographs taken in 1947 and 1980. The average compounding rate of tree loss over that period was 1.1% p.a.. Rates of tree loss were greater from the remnant box-stringybark stands (1.9% p.a.) occurring in the district than from remnant River Red Gum stands (0.8% p.a.). The size and crown condition of all trees in the stand under investigation were assessed. Dieback-affected trees had relatively small and open crowns, that often had many dead branchlets and branches. The crowns of dieback-affected trees were largely of secondary or epicormic origin. Tree height and stem girth were not affected by contemporary crown health. The age of one tree was estimated by radiocarbon dating to be 440. ± 110 y B.P.. Detailed investigations of trees classified as either "healthy", "average" or "unhealthy" were undertaken. The amount of organic matter in soil declined as the quality of tree cover changed from "healthy" to "unhealthy" tree to open pasture. This change was probably linked with reduced additions of organic material through litterfall as tree crown cover deteriorated. The possible consequences of reduced litterfall and declining soil organic matter levels were that; (i) it may have disrupted biogeochemical nutrient cycling; and (ii) it may have increased the susceptibility of trees to root damage and other problems associated with animal traffic. Measurements of soil strength and bulk density indicated that soil under open pasture had been compacted, presumably by animal traffic. Soil under trees was harder during winter than soil under open pasture and was therefore less susceptible to puddling. Soil under trees was not as strong as soil under open pasture during spring and hence provided more favourable conditions for root growth. Investigations into the mineral nutrition of study trees found that "unhealthy" trees did not appear able to mobilize some macronutrient elements (N,P, K, Mg) to the same extent as "healthy" trees. The apparent nutrient deficiency may have been due to disruptions to the biogeochemical nutrient cycle as dieback progressed. The nutrient deficiency may exacerbate this disruption, since it was associated and possibly linked with delayed new foliage production in "unhealthy" trees and reduced area growth in that foliage. Measurements of leaf water potential, diffusive conductance and transpiration showed that atmospheric, rather than soil water deficits had the greatest influence on water relations in the study trees. The measurements showed that "unhealthy" and "average" trees did not greatly restrict transpiration during periods of high evaporative demand. "Healthy" and "average" trees transpired more water and assimilated more carbon than did "unhealthy" trees. Differences in gas exchange were due only to large differences in leaf area. Soil under "healthy" and "average" trees remained drier than soil under "unhealthy" trees and open pasture throughout the study period. However, there was little difference in soil water depletion during the summer, even though the annual pasture had died off. Measurements provide evidence of deep infiltration of water beyond plant root zones, particularly under open pasture and "unhealthy" trees. The healthier trees appeared to have a beneficial impact on the local water balance. Continued rural dieback and/or tree loss is likely to exacerbate disruptions to the local water balance that commenced with clearing for agriculture.