School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemStudies of Ascochyta rabiei in Australia( 2005)Ascochyta rabiei (teleomorph: Didymella rabiei) which causes ascochyta blight is the most serious disease of chickpea (Cicer arietinum) in Australia as it causes significant losses in crop yield and quality. Although A. rabiei is heterothallic and genetically diverse elsewhere in the world, a study carried out on Australian isolates collected between 1995 and 2000 identified only one mating type and a low level of genetic diversity within the Australian A. rabiei population. In 2002, ascospores of Didymella rabiei, the sexual state of A. rabiei, were trapped in a discharge chamber, from chickpea stubble naturally infected with ascochyta blight in Western Australia. Examination of the stubble revealed pseudothecia typical of Didymella rabiei. The reported presence of the teleomorph in Western Australia indicated that the second mating type had been introduced into Australia or that the pathogen was capable of a low level of homothallic compatibility, previously undetected. The aims of this research were, to undertake a new survey of Australian A. rabiei isolates, to test for the presence of the second mating type, to determine if there has been a change in the diversity of the Australian population and to investigate if pathogenic variability was displayed among isolates. Sixty-seven isolates collected from chickpea fields in South Australia, New South Wales, Queensland, Victoria and Western Australia during the 2003 cropping season were single spored and confirmed as A. rabiei using a PCR test. The isolates were typed for mating type using MAT gene specific PCR primers and compared with tester isolates from USA. This test revealed that all the 67 isolates belonged to mating type 2 (MAT 1-2), thus, the presence of mating type 1 (MAT 1-1) in Australia could not be confirmed. Sequence Tagged Micro Satellite (STMS) markers were used to examine the genetic diversity of the A. rabiei isolates. The isolates were assessed for allelic variation at 19 microsatellite loci, each of which amplified a single locus. Seven of the loci were polymorphic across all the 67 isolates, while the remaining twelve were monomorphic. Based on the allele profiles at the seven polymorphic loci, 19 distinct A. rabiei haplotypes were identified with a total of 33 alleles. One haplotype constituted 35.8 % of the population and was found in the collections from South Australia, New South Wales, Queensland and Victoria. Cluster analysis did not show a clear distinction between isolates based on the state from which they were collected. Polymorphism across the 19 microsatellite loci revealed a slight elevation in diversity in the 2003-2004 population (Ht = 0.07; compared to 0.02 in the 1995 to 2000 collection) and an increase in the number of haplotypes compared with that detected in the previous study of Australian isolates. To examine the pathogenic variability of the Australian population of A. rabiei, nine isolates were inoculated on five chickpea differentials, ranging from highly susceptible to resistant, under controlled conditions optimal for A. rabiei growth and infection. Eight of the isolates were virulent on the susceptible and intermediate chickpea cultivars but not the resistant cultivar and one isolate was only virulent on the susceptible cultivar. Based on these results the isolates were classified into two pathotype groups. The results obtained from the study of the population structure and the pathogenic variability of A. rabiei in Australia will enable the Australian chickpea breeders to understand the A. rabiei population better for formulating management and breeding strategies.
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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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