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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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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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.