Agriculture and Food Systems - Theses

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    Studies of Ascochyta rabiei in Australia
    Pradhan, Prashanti ( 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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    A new virus disease of carrots : its transmission, host range, and control
    Stubbs, Lionel Leslie ( 1948)
    During 1940 commercial vegetable growers in the Melbourne market garden area requested an investigation of a disease of spring sown carrots, which had been occurring over a number of years. Summer sowings were not affected, but the failure, or partial failure, of spring sowings had made the production of this crop uneconomic, and as a result, the continuity of carrot supplies to the Melbourne market had been disrupted. The principal objects of the investigation described in this paper have been the elucidation of the factors responsible for the disease, and the development of practical disease control measures. A preliminary report of the investigation, which commenced in 1941, has been published elsewhere (Stubbs & Grieve, 1944).
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    Studies on the etiology and control of parsnip canker, caused by itersonilia pastinacae channon
    Smith, Peter Raymond ( 1968)
    In Victoria, parsnip canker is caused chiefly by Itersonilia pastinacae Channon and, less frequently by Phoma sp.. Inoculation of both fungi into parsnip roots produced typical symptoms of the disease. The morphology of the local isolates of I. pastinacae was compared with others described and the taxonomy of the genus discussed. The fungus was shown to be seed-borne, the infection being carried in plant debris contaminating the seed and also externally on the seed itself. An aerated-steam heat treatment was developed to eliminate this seed-borne infection. In field studies I. pastinacae in naturally infected roots survived at least 12 months burial in soil. When the root was mutilated by removing the stem apical meristem, to promote rapid decomposition, the fungus survival period was reduced to 6 months. In laboratory studies the fungus, impregnated in fibre glass mesh, survived 6 months burial in unsterile soil, whereas the survival period in sterile soil was at least 12 months. Survival was shown to be due to the presence of chlamydospores; mycelium and ballistospores being rapidly lysed. Chlamydospores germinated directly to form ballistospores. I. pastinacae, also had a low competitive saprophytic ability. Field and laboratory studies demonstrated that ballistospore discharge was influenced chiefly by high relative humidity, the presence of rain being more effective than dew. Spore discharge followed a diurnal periodicity, the maximum discharge occurring between 08.00 and 10.00 hr.. Low temperature, however, reduced spore discharge even at optimum R.H. Ballistospore germination studies demonstrated soil fungistasis an effect destroyed by sterilization of the soil. Germination of ballistospores was also influenced by nutrients, either diffusates from parsnip roots and leaves or by a 2% glucose solution. Strepstomyces sp. and Bacillus subtilis were isolated from soil and antagonism towards I. pastinacae exhibited. Cell-free culture filtrates of both antagonists lysed I. pastinacae while incorporation of the filtrates into media reduced the growth rate of the fungus. Sterile soil inoculated with the antagonists also lysed I. pastinacae. Foliage applications of fungicides, particularly those containing copper, reduced the incidence of parsnip canker in the field. Control was also achieved in the field by keeping the developing crowns covered with soil during the growth of the crop.
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    A study of pathogens associated with stunted patches and deadheads in Victorian wheat crops
    Price, Robert Daniel ( 1968)
    Between 1959-1966 seven species of root-rotting fungi and barley yellow dwarf virus were consistently associated with stunted patches and "deadheads" in Victorian wheat crops. The fungi occurring in order of frequency were: Fusarium culmorum (W.G.Sm) Sacc., Curvularia ramosa (Bainier) Boedijn, Helminthoaporium sativum Pammell, King and Bakke, Pythium ultimum Trow var. ultimum, Rhizoctonia solani Kuhn, Ophiobolus graminis Sacc. and Fusarium graminearum Schwabe. Barley yellow dwarf virus was found each season in the higher rainfall areas of the State; in 1961 and 1965 it occurred in epidemic proportions in all districts. These organisms may be divided into three groups based upon their ability to reduce yield of wheat under controlled glasshouse experiments. (1) MAJOR REDUCTION BY: Barley yellow dwarf virus; Ophiobolus graminis; Helminthosporium sativum. (2) MINOR R RUCTION BY: Fusarium graminearum, Fusarium culmorum; Rhizoctonia solani; Pythium ultimum. (3) NO REDUCTION BY: Curvularia ramosa Ophiobolus graminie and barley yellow dwarf virus are the most important of these pathogens in Victoria. Both caused yield losses of approximately 45-60% in the field. Under controlled glasshouse conditions they caused highly significant yield losses. Five other fungi investigated caused some losses, but during the period 1959-66 were not of great economic importance. The incidence of these pathogens could be related to climatic conditions before and during the wheat growing season. (1) Highest incidence after a dry winter and wet spring: Ophiobolus graminis; Pythium ultimum. (2) Highest incidence after a mild winter and dry spring: Barley yellow dwarf virus; Fusarium graminearum; Fusarium culmorum; Helminthosporlua ; Curvularia ramosa. (3) Highest incidence when crop sown under wet conditions: Rhizoctonia solani; Pahiuta ultimum.
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    A study of the fungi associated with the decline of subterranean clover pastures in Victoria
    Edgerton, James Pitt ( 1972)
    A general study of the problem of decline of subterranean clover (Trifolium subterraneum L.) in Victorian pastures indicated that Fusarium spp., especially F. oxysporum are the most common fungi associated with rotted roots of affected plants. Moisture stress due to waterlogging was shown to be conducive to the fungal root attack of subterranean clover in pot studies, indicating that this is an important factor causing clover decline in the field. Evidence was obtained supporting the recognition of Fusarium avenaceum (Fr.) Sacc. as an important seedling pathogen of subterranean clover. Studies of the pathogenicity of this species at a range of temperatures were conducted in controlled environment chambers. The results suggested that increasing temperature has a negligible effect on the incidence of infection, but increases the severity of disease after infection.
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    Studies on ascochyta, botrytis and seed 'spot' of vicia faba in Tasmania
    Geard, Ian Dudley ( 1961)
    The investigation, commenced as a study of the Ascochyta disease of tick beans, was extended to include a study of Botrytis fabae and to determine the true cause of seed "spot", previously attributed to Ascochyta, but found to be damage by the bug Calocoris norvegicus. Tick beans, grown mainly as a side line crop on a restricted rotation, are used for stock feed and sowing green manure crops. Ascochyta disease: A. fabae infects V. faba and its varieties. Leaf lesions are roughly circular, 5-8 mm. in diameter, greyish-brown, with a definite, though not raised or thickened margin. Pycnidia, grouped in roughly concentric rings near the centre of the lesion are visible to the naked eye. Vicia tetrasperma, V. sativa, V. hirsute and V. angostifolia are also infected. A. fabae, f requently isolated from infected material, was inoculated onto V. faba and other hosts, producing typical symptoms and re-isolated. The writer could not infect beans with A. pisi nor peas with A. fabae. The disease has been attributed by various authors to A. pisi, A. fabae and A. pisi var. fabae. The writer reviews the literature and the results of his own experiments, concluding that retention of the name A. fabae is justified. No ascigerous stage has been found. Consistent morphological and cultural differences between A. fabae and A, pisi are recorded. A. pisi produces less mycelium and sporulates more freely than A. fabae and its spores are smaller (A. pisi average 12.6 X 3.4 ?, A. fabae 16.9 X 5.1 ?). A. fabae sporulated poorly on standard agar media but well on steamed bean foliage and on propylene oxide sterilised bean flour with water agar. Inoculum was prepared by transferring pieces of agar culture to fresh medium. Cultures incubated outside sporulated freely but were heavily contaminated. In culture A. fabae produced 0.5 - 2.5 X 106 spores/pycnidium. The thermal death cure of A. fabae gets determined. Mycelium heated in water was killed at 40.7C in 10 minutes and at 51.70 in 2 minutes. Some infection occurred if foliage remained wet for 8 hours after inoculation but 100 infection needed more than 24 hours. Inoculation of imported varieties and growers' lines showed no consistent evidence of resistance. Infected seed was rarely found but the organism was isolated from seeds with a general reddish discolouration of the seed coat or a yellowish brown zoned lesion 2-3 mm in diameter. No direct evidence of symptomless infection was found but it probably occurs. The main carryover in most seasons was concluded to be in infected debris. Spores maintained 2% viability in infected material for 5 months at 70% R.H. and 22C. Carryover in infected seed and on infected vetches is concluded to be of secondary importance. Of sporadic seasonal occurrences the disease was minor in the years 1955 to 1960. A severe outbreak was recorded in 1949 and evidence is presented indicating that unusually frequent and prolonged humid conditions in October were responsible. A number of control methods were considered and it was decided initially to produce disease free seed by heat treatment and propagation in isolated conditions. The literature on heat treatment of seed is reviewed and experiments on the effect of heat treatments on germination of tick beans are reported. Isolation trials using hot water treated seed are described and the results examined statistically. It is concluded that since seed infection is much less common than formerly supposed heat treatment was probably unnecessary. Botrytis disease: A species of Botrytis commonly found on tick beans caused reddish-brown, circular lesions 2 - 4 mm in diameter with a prominent, darker, raised or thickened border. Under prolonged moist conditions "progressive" infection causes blighting of flowers, leaves and shoots. Infection of V. sativa and V. tetrasperma was also observed. The Botrytis from beans was only weakly pathogenic to other plants and is concluded to be specialised to V. faba. Consistent differences in cultural characteristics and spore and sclerote sizes between normal B. cinerea and the bean form are described. The Tasmanian bean form does not correspond entirely with B. fabae described by Sardina and some isolates of B. cinerea (e.g. from grapes) were equally pathogenic to beans. It is suggested that all forms of Botrytis attacking V. faba might be regarded as B. cinerea, although the form described in this study is called B. fabae for convenience. Imported varieties and growers' lines were inoculated but none proved to be resistant. B. fabae was isolated from seeds with a reddish-brown discolouration of the seed coat or a yellowish-brown zonate lesion 2 - 3 mm in diameter. When Botrytis infected seeds were hot water treated the organism was not recovered after 30 minutes at 60.5C. Less severe treatments eliminated the organism after pre-soaking in tap water for 16 hours and exclusion of hard seeds. Seed "spot": seed "spot" appears, in mature seeds, as a dark coloured spot on the seed coat, half - 1 mm in diameter and sometimes perforated at the centre. Commercial samples had up to 80% of seeds affected and none free of the disorder was found. Germination is not adversely affected by seed "spot" which was considered to be important because of its supposed association with Ascochyta and export restrictions on affected seed. Evidence collected from a number of sources proved that Ascochyta was not involved and the possibility of a nutrient deficiency was rejected. It was concluded that the disorder was very probably an insect injury and a search was made for the type of insect believed to be responsible. In a crop showing light brown necrotic spots of the immature seed coats but with scarcely detectable signs of injury to the adjacent pod, a sparse population of very active bugs was found and their feeding habits observed to be such that a small number of insects could cause extensive damage. Specimens collected and caged on individual pods produced typical symptoms of the disorder. The insect was identified as Calocoris norvegicus Gmel. The significance of this finding is discussed and the disorder is shown to be very similar to seed pitting of lima beans, also caused by bug injury.
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    Bark disorders and manganese nutrition of some fruit trees in the Goulburn Valley
    Grasmanis, Vilhelms Oskars ( 1961)
    Some Delicious apple trees growing in the Goulburn Valley and adjacent areas show "measles", an internal bark necrosis with elevated pustules distributed over the bark surface. Wood which is one or more years old is affected. The pustules increase in number and size as the disorder becomes more severe. In the cortex underneath the bark epidermis, necrotic spots are found in great numbers, being largest beneath the epidermal elevations. In severe cases, cracking and splitting of the bark occurs, mainly on older wood; the affected trees have fewer and smaller leaves than normal; twigs and limbs die back; and eventually the whole tree may die. Another bark disorder, known as "papery bark", is fairly common on Josephine pears in the Goulburn Valley. The symptoms are an internal bark necrosis with cracking and rolling back of the bark epidermis on shoots and twigs one or more years old. The symptoms of a further disorder, known locally as "leaf spotting are more severe on these trees affected with "papery bark", and in addition fruit on such trees or limbs shows more "russeting" than is normal. Die-back of shoots and limbs often follows. The cause of these nutritional disorders, "bark measles" on Delicious apples and "papery bark" on Josephine pears, was investigated. The analysis of fresh tissues showed increased amounts of manganese in leaves, fruit and, bark of affected apple trees and in the leaves, bark, buds and fruit of affected pear trees. To demonstrate that excess uptake of manganese was the cause of these disorders, hydrated manganese sulphate was injected into trees of healthy appearance showing low manganese content in their fresh tissues. After this injection, disorders similar to "bark measles" and "papery bark" were produced on apples and pears respectively. Soil acidification with chemicals in the field and soil waterlogging in pots also produced "papery bark" on pears, while soil waterlogging in pots produced "bark measles" on apples. Analysis showed increased amounts of soluble manganese in treated soils and in the tissues of affected plants growing on treated soils. Laboratory trials with soils from orchards, both healthy and with manganese toxicity (hereafter called "toxic soils"), showed that drying, heating and waterlogging, especially at temperatures of 20 degree Celsius or more, also contribute to the release of high amounts of soluble manganese. These trials showed the importance of moisture conditions and pH in the recovery of toxic soils.
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