School of Agriculture, Food and Ecosystem Sciences - Theses
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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.