Multiple aphid resistance from alien sources and its chromosomal location in bread wheat

Wheat (Triticum aestivum) is a very important cereal crop and is cultivated worldwide on more than 200 million hectares annually, with an average grain yield of about 3 t/ha. A number of diseases and pests are known to affect wheat production, with aphids being important insect pests. The aphid species that commonly attack wheat are Schizaphis graminum, Rhopalosiphum padi, Sitobion avenae, Diuraphis noxia and Metopolophium dirhodum. These aphids can reduce wheat yields by up to 40% solely due to feeding and by over 60% when their feeding transmits viral diseases. One way to reduce aphid damage is through plant breeding and growing resistant varieties. The aims of this thesis were to: 1) identify novel sources of resistance to multiple aphid species in a wheat-alien genetic stock; 2) determine the utility of those resistance sources in the field; 3) review the utility of rye as a source of resistance to biotic stresses in wheat; and 4) locate genomic regions associated with aphid resistance in a synthetic hexaploid wheat (SHW). Under laboratory conditions, certain wheat genotypes carrying the 1R chromosome from rye reduced both R. padi and S. avenae growth, the most resistant ones by 24 and 34% relative to the control, respectively. Certain Aegilops speltoides-derived wheat lines displayed hardly any chlorosis due to S. graminum and reduced aphid colony weight by up to 68% compared with the control. The results of laboratory and field evaluations were in good agreement. The most resistant wheat-rye genotype reduced R. padi field population development by 33% relative to the control, while the A. speltoides-derived line reduced S. graminum field population development by up to 75%. Certain rye-derived genotypes carrying resistance to one or two aphid species also showed resistance to fungal diseases such as powdery mildew and Septoria tritici blotch. Five quantitative trait loci (QTL) associated with aphid resistance were found in the SHW mapping population. One QTL for R. padi antibiosis is located on chromosome arm 4BL, two QTL for R. padi tolerance on 5AL and 5BL, and two QTL for S. graminum resistance on 2DL and 7DL. An epistatic interaction that enhanced R. padi tolerance was also detected. The sources of resistance identified here have potential applications in wheat breeding programmes aiming to incorporate aphid resistance