Potential strategies for control and treatment against powdery mildew of wheat

Improvement of both quantity and quality of wheat production is of great importance for food security as wheat is the most traded cereal crop and third most produced globally. Wheat diseases represent a constant threat affecting grain production, therefore cost-effective, sustainable control strategies are required. Airborne powdery mildew (Blumeria graminis f. sp. tritici) is the sixth most damaging pathogen of wheat and can reduce yield from 13-30%. Characteristics such as short propagation period, easy dispersal and rapid establishment on the host makes management of this pathogen difficult. A thorough understanding of wheat powdery mildew control with a focus on sustainable, non-chemical approaches is a research high priority. This thesis presents studies which explore two such sustainable means of reducing the disease ‚Äö- genetic resistance and use of symbiotic fungi which alter plant response to disease. To facilitate marker assisted selection for breeding resistant wheat varieties, a review of current knowledge was undertaken to produce a genetic map-based reference which integrates records of all known powdery mildew resistance genes and quantitative trait loci (QTL). Over 200 powdery mildew genes (permanently and temporally designated genes) and QTL were mapped to 21 chromosomes of common bread wheat, which is expected to benefit future molecular resistance breeding. A genome-wide association study (GWAS) was conducted on a total set of 329 wheat varieties with diverse genetic backgrounds. Based on disease incidence data from three Tasmanian trials from 2016 to 2018, and variety genotypes obtained by single-nucleotide polymorphism (SNP) array, 14 QTL associated with wheat powdery mildew resistance were identified on 11 different chromosomes. Out of these, four QTL on chromosome 3A, 3B, 6D and 7D were believed to be novel. High confidence gene candidates underlying new QTL involved in wheat powdery mildew resistance include a member of major facilitator superfamily (MFS) which is a new class of plant-defence related proteins, genes encoding disease resistance protein and regulating early response to fungal infection. In addition to exploring genetic resistance, the influence of beneficial fungi on the expression of resistance was also investigated. Arbuscular mycorrhizal fungi (AMF) are widely known for their benefits in plant growth, as well as reported evidence of disease mitigation. This thesis presents a study conducted in three wheat cultivars that were inoculated with the AMF species Rhizophagus irregularis. Results showed that AMF inoculation had no significant effects on wheat powdery mildew incidence or plant growth. These results contrasted from previous studies which found that this AMF species was associated with a 34% decrease in powdery mildew incidence or severity in wheat. This difference may be explained by the different wheat genotypes used in this study, which highlights the specific of plant-pathogen-AMF interactions. The findings provide insight into wheat powdery mildew control. Greater understanding in genetic resistance, in particularly durable resistance, may enable more utilization of identified QTL. The current findings also suggest the impact of an AMF species on disease cannot be generalised to all host genotypes, but rather AMF effects should be evaluated with a case by case basis.