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The thesis aims to characterise the basis of naturally occuring resistance to BYDV in cereals in three ways: i. A map-based approach to the isolation of the Yd2 gene for BYDV resistance from barley. -- ii. Determining if a BYDV resistance gene in rice is orthologous to Yd2. -- iii. Establishing if other BYDV resistance genes in non-Ethiopian barleys are allelic to Yd2.
The disease: symptomatology and host range of barley yellow dwarf; Distribution and economic importance of barley yellow dwarf; Epidemiology of barley yellow dwarf in Europe; Epidemiology of barley yellow dwarf in Australasia; Interactions of barley yellow dwarf viruses: cross-protection and interactions with other pathogens and with abiotic factors; The viruses: purification and viron characterization of barley yellow dwarf viruses; Genome structure and function of barley yellow dwarf viruses; Taxonomy of barley yellow dwarf viruses; The vectors: biology and taxonomy of vectors of barley yellow dwarf viruses; Aphid transmission of barley yellow dwarf viruses: interactions between viruses, and host plants; Control: barley yellow dwarf: diagnostic procedures and reagents; Cultural chemical, and biological methods for the control of barley yellow dwarf; Host plant tolerance or resistance for control of barley yellow dwarf; Prospects for genetically engineered resistance to barley yellow dwarf viruses.
Studies were conducted on artificially infected susceptible and resistant barley varieties in an attempt to determine the effect of barley yellow dwarf virus on the growth and development of these plants. A resistant and a susceptible variety were infected at four stages of growth and harvested at two stages. Roots, and the above ground portions of the plants were measured and weighed and the number of tillers were counted. Thirty barley varieties and three oat varieties were grown in greenhouse soilbeds and infected with barley yellow dwarf virus. The height of the plants was measured at weekly intervals until heading, and again at harvest. Yield components were also determined at harvest as well as total seed yield. Seventeen crosses between thirteen resistant and susceptible barley varieties were made, and the F2 and F3 generations evaluated for resistance to the barley yellow dwarf disease. The possibility of an association between resistance and the characters of kernel row number, kernel color, rachilla hair length, and the length of outer glumes was investigated. These investigations indicated that the barley yellow dwarf disease apparently exerts a primary effect on root development, resulting in an inadequate supply of moisture and nutrients to the plant. While the root development of susceptible plants was restricted when plants were infected at early stages of growth, infection of resistant plants at the 3- and 5-leaf stages of development resulted in stimulation of root development. Resistant varieties reacted differently to infection by barley yellow dwarf virus. While all infected plants of resistant varieties grew approximately the same in height as the uninfected plants, the components of yield were affected. One yield component of a resistant variety would be increased while another was suppressed. In another resistant variety the reverse reaction was observed. A few varieties were observed in which the all yield components were equal or superior to those of uninfected plants of the same variety, resulting in increased total yields of grain. Genetic investigations indicated that one gene segregated for resistance, and that the same gene segregated in the varieties CI 3208-4, CI 9654, and CI 9795. The gene which segregated for resistance was assumed to be the Yd2 gene even though this gene was expressed as a recessive in all crosses between a resistant and susceptible variety. The difference in behavior of this gene was explained by the fact that the susceptible variety, Hannchen, exhibits a low level of resistance to the disease. No association was found between yellow dwarf resistance and any other characters studied. An association was found between the character of red kernel color in the Abate variety and kernel row number, indicating that kernel color was controlled by the Re2re2 gene on chromosome V.A recombination value of 20.42 ± 4.67% was obtained.
This collection reviews advances in the key areas required to achieve durable disease resistance in cereal crops, from advances in understanding pathogen biology/epidemiology and plant pathogen interactions to identifying sources of resistance and advance techniques for breeding new varieties.