Bibliography: leaves 121-141.

Bibliography: leaves 118-132. This thesis examines the structure and gene expression of barley yellow dwarf viruses (BYDVs)-PAV in order to gain a better understanding of the interaction between the virus and the Yd2 resistance gene. The protein products of open reading frame (ORF)3, ORF4 and ORF5 are expressed in bacterial cells, in order to characterise the BYDV-PAV virion-associated proteins. The effect of the Yd2 resistance gene on the expression of the BYDV-PAV viral proteins in infected cells is also studied.

Includes bibliographical references (leaves 265-296). Pathogens associated with root damage were investigated in the Murray Mallee region of South Australia over the 1987-1989 growing seasons. Occurence of fungal species and the root lesion nematode (Pratylenchus neglectus) was assessed, and related to the appearance and severity of symptoms on the roots. Field experiments were supplemented with innoculation tests in the glasshouse and laboratory.

Mycosphaerella graminicola (anamorph Septoria tritici) causes Septoria tritici blotch, a globally important disease of winter wheat. Resistance and pathogenicity generally vary quantitatively. The pathogen reproduces both sexually and asexually, and the pathogen population is highly genetically variable. Several unresolved questions about the epidemiology of this pathosystem are addressed by this research. Among them are whether cultivar-isolate specificity exists, how partial host resistance affects pathogen aggressiveness and sexual reproduction, and how host genotype mixtures influence epidemic progression and pathogenicity. At its release in 1992, the cultivar Gene was highly resistant to M. graminicola, but that resistance had substantially dissolved by 1995. Six of seven isolates collected in 1997 from field plots of Gene were virulent to Gene seedlings in the greenhouse, while 14 of 15 isolates collected from two other cultivars were avirulent to Gene. Gene apparently selected for strains of M. graminicola with specific virulence to it. In a two-year experiment, isolates were collected early and late in the growing season from field plots of three moderately resistant and three susceptible cultivars, and tested on seedlings of the same cultivars in the greenhouse. Isolates were also collected from plots of two susceptible cultivars sprayed with a fungicide to suppress epidemic development. Isolate populations were more aggressive when derived from moderately resistant than from susceptible cultivars, and more aggressive from fungicide-sprayed plots than from unsprayed plots of the same cultivars. Over 5,000 fruiting bodies were collected in three years from replicated field plots of eight cultivars with different levels of resistance. The fruiting bodies were identified as M. graminicola ascocarps or pycnidia, or other. In all three years, the frequency of ascocarps was positively correlated with cultivar susceptibility, as measured by area under the disease progress curve, and was also positively associated with epidemic intensity. For three years, four 1:1 mixtures of a moderately resistant and a susceptible wheat cultivar were planted in replicated field plots. Isolates from the plots were inoculated as bulked populations on greenhouse-grown seedlings of the same four cultivars. Mixture effects on disease progression varied among the years, and were moderately correlated with mixture effects on pathogenicity. / Graduation date: 2002

Graduation date: 2003

Despite many investigations genotype by environment interaction remains one of the least understood factors in plant improvement. Understanding genotypic differences responsible for such interactions could assist in making more informed breeding decisions. The components of yield being less complex than grain yield per se may be useful for selection to improve adaptation of genotypes and enhance grain quality. However, the potential compensatory response among the components of yield could compromise their usefulness as selection criteria. To evaluate this aspect fifteen cultivars, including Soft White, Hard White and Hard Red wheats, were planted at three diverse locations over two growing seasons. Genotypes were ranked based on measurements for specific traits in each environment. The genotype by environment interaction for grain yield, protein concentration and hardness were investigated according to the AMMI model. Influence of environmental factors and genotype by environment interactions on associations among selected traits were determined. The results of this study showed that genotypic differences in adaptation to the Pacific Northwest and resistance to Septoria spp. were responsible for interactions for grain yield. However, no consistent patterns of response were found among the similar cultivars for quality traits. Those environments that favored expression of biomass and grain weight potential provided for enhanced performance of the adapted cultivars. The environments with suppressed biomass accumulation and grain filling interacted positively with the unadapted cultivars. A large compensatory relationship between tillering and apical growth was detected. No or low compensation was observed between grains per spike and thousand kernel weight. Two different strategies were proposed for improvement of Soft and Hard White wheat cultivars under the conditions of the Pacific Northwest. Both strategies accentuate importance of increasing biomass while maintaining harvest index. When selecting for higher yielding Soft White cultivars, plants with larger leaves, stronger stems, larger spikes and heavier grains should be emphasized. For Hard Whites - higher number of tillers and grains m����� is desired, as they provide for harder grains with higher protein content. / Graduation date: 2000

Graduation date: 1996

Graduation date: 1996

No description available.

M. Tech. Agriculture / This study was conducted to establish whether genetic resistance to root-knot nematode is present in local available Amaranthus-, Spinacea oleracea-, Beta vulgaris-, Daucus carota-, and Capsicum genotypes. Host suitability trials for the relevant vegetable genotypes were conducted in separate greenhouse studies. Different nematode parameters were used to select for root-knot nematode resistance but reproduction factors values (Rf) were used as main criterion.