School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemA study of pathogens associated with stunted patches and deadheads in Victorian wheat crops( 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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ItemStudies on Ophiobolus graminis and 'deadheads' of wheat( 1970)The virulence of 300 monosporous field isolates of Ophiobolus graminis was compared on wheat. All were strongly pathogenic when first Isolated, but some became weakly pathogenic when maintained in culture. Symptom expression was studied in wheat infected with one of the isolates. Infections affected root length and numbers, plant height, fresh weight, had development and yield. : The so-called empty "deadhead" was only one of several head symptoms which ranged from complete suppression to empty and poorly-filled heads. Overall symptoms were increased in severity by reducing light intensity, days length and soil moisture; by restricting root development, and by earlier inoculations. Symptoms were also influenced by temperature in the test range 20- 28C, being more severe at the lower temperatures when plants were inoculated within eight weeks of seeding. Lower temperatures and shorter day lengths also retarded plant maturity, thus indirectly increasing severity of final symptoms. Symptom expression was unaffected by nitrogenous, phosphatic, potassic and complete fertilizers, although nitrogenous dressings increased yield. Symptoms caused by the isolate in wheat were compared with those by Fusarium graminearum, HeIminthosporium sativum and Pythium ultimum under spring glasshouse conditions. Swelling of root tips was caused by P. ultimum and root discolouration by the remainder, but only O. graminis significantly reduced plant height, fresh weight, numbers of functional roots and yield. Nuclear distribution in hyphae of the isolate was compared with that of Curvularia ramosa, C. spicifera, Fusarium avenaceum, F. culmorum, F. graminearum, Helminthosporium sativum and Wojnowicia graminis. Tip cells of O. graminis were predominantly uni-nucleate whilst those of other fungi were generally multi-nucleate. When opposed to other fungi in culture, O. graminis was slightly antagonistic towards Fusarium avenaceum, F. culmorum. Helminthosporium satiyum and Wojnowicia graminis, whereas Pythium ultimum exhibited antagonism towards O. graminis. The combination of O.graminis with Curyularia ramosa was mutually antagonistic, but no antagonism was evident in combination with either C. spicifera or Fusarium graminearum. The saprophytic survival of the isolate was examined on artificially-infected wheat straw buried in unsterilized soil. Survival was prolonged by nitrogenous soil amendments, unaffected. by superphosphate dressings and reduced by growing various plant species over infected straw. O. graminis was isolated from all species, including dicotyledons, sown over straw. Three methods for estimating survival on various hosts were compared, viz., those involving artificially-infected straws previously sterilized by either propylene oxide or autoclaving and inverted sods of infected hosts. The use of sods was the most satisfactory and indicated O. graminis survived better on barley than wheat if inoculated at seeding and inverted after four weeks. Delaying inversion further increased survival on barley but not on wheat. Survival was also increased by increasing plant density, the effect being relatively greater with barley than wheat. However, survival was greater on wheat than barley when inoculation was delayed until eight weeks after seeding. O. graminis also survived better on Lolium perenne, Bromus mollis and B. sterilis than on Avena fatua and Vulpia myuros; and on Bromus gussonii and Hordeum hystrix than on PhalarIs minor, Avena sterilis, Ehrharta erecta and linseed when inoculated at seeding and inverted eight weeks later.
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ItemStudies on ergot (Claviceps purpurea) of wheat( 1984)Ergot [Claviceps purpurea (fr.) Tul.] is an important disease of cereals and grasses in Australia, as grain contaminated with ergot is rejected or heavily discounted in value because of its toxicity to animals and man. Yield losses from the disease are small. The ergots most commonly detected in grain samples are from ryegrass and only rarely are the much larger cereal ergots present. Laboratory studies showed that C. purpurea can be cultured on several synthetic media, however, yeast malt dextrose agar was the best. On this medium the fungus grew well, sporulated abundantly and pathogenic isolates maintained their virulence. Pathogenicity tests on wheat showed there was a wide range in the virulence of individual isolates. Field studies suggested that direct infection of wheat by ascosporic inoculum was unlikely, but that primary infection of ryegrass by ascospores was possible. Surveys showed that ryegrass was often infected with ergot and glasshouse tests proved that these isolates were able to infect wheat. This suggests that ergot survives from year to year on infected ryegrass and can pass to wheat by conidial transfer when conditions are favourable. The level of ergot contamination of grain was reduced to a negligible level by controlling ryegrass within a crop. Victorian wheat cultivars vary in their reactions to C. purpurea, the cultivars Kewell and Olympic being very susceptible to ergot while the cultivars Halberd, Zenith and Kalkee are moderately resistant. No cultivars tested have shown immunity. The results of these studies show ergot infection of wheat can be avoided by planting seed free of ergot, burning and deep ploughing of ergot infested land, controlling ryegrass within crops and the sowing of resistant cultivars.