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Yellow rust, caused by the biotrophic fungus Puccinia striiformis f. sp. tritici (Pst), poses a major challenge for wheat breeders and growers globally. The past two decades have seen the rise of Pst populations that are more genetically diverse, more aggressive and that have adapted to warmer temperatures. These features, likely further aided by increased international travel, have led to important epidemic outbreaks and have jeopardised wheat yellow rust resistance levels in the main wheat-producing regions globally. Widespread epidemics have been further facilitated by the deployment of genetically uniform material underpinned by major resistance over large areas. With such a rapidly changing Pst population landscape, disease resistance breeding strategies must adapt accordingly, and this starts with the continued characterisation of adequate yellow rust resistance loci and accompanying molecular and genomic tools. It is crucial that these loci are of direct relevance to breeding programmes for rapid varietal deployment. To this end, I used the NIAB Elite Multi-parent Advanced Generation Inter-Cross (MAGIC) population, a multi-founder population that captures 80 % of the genetic variation present in key representative varieties used in UK wheat breeding (1970-2010s), to identify and characterise genetic loci controlling yellow rust resistance in replicated multi-environmental field trials, in both leaves and ears. This approach has further opened up the avenue for dissecting disease resistance beyond the limited scope of single varieties. I found that nine Quantitative Trait Loci (QTLs) conferred resistance to yellow rust, with four consistently detected across environments and explaining nearly 50 % of the phenotypic variation, and the other five explaining 15-20 % with inconsistent detection across environments. There was a strong indication of additivity effects between the four strong-effect QTL. Furthermore, all founders but the most susceptible one contributed towards resistance, indirectly demonstrating that UK breeding germplasm has high resistance potential against yellow rust. In the second part of my thesis, I focus on the physical interval of the eight most significant QTL previously identified, by examining gene annotations from the recently published IWGSC RefSeq v1.0 genome assembly. Five QTLs were characterised by NBS-LRR clusters. The presence of NBS-LRR-encoding genes with integrated domains revealed the potential for effector triggered immunity based on indirect recognition for a subset of those yellow rust resistance loci. The other three QTL were characterised by the absence of NBS-LRR-encoding genes in their physical interval, potentially indicating the role of non-race specific yellow rust resistance in the MAGIC population. Finally, I focus on glume infection, a phenotypic trait largely overlooked in QTL mapping studies, despite repeated reports of outbreaks. Despite high heritability (72 %), the five QTLs detected explained between 3 and 6 % of the phenotypic variation. Three QTLs co-located with QTLs for foliar resistance. The other two were associated with flowering time, suggesting that earlier ear emergence potentially leads to increased susceptibility to yellow rust in the glumes.

Parastagonospora nodorum is a necrotrophic fungal pathogen of wheat (Triticum aestivum L.) and the causative agent of stagonospora nodorum blotch (SNB). P. nodorum mediates host cell death using proteinaceous effectors. Three P. nodorum effectors have been cloned: SnToxA, SnTox1 and SnTox3. SnTox3 susceptible wheat cultivars possess a dominant susceptibility allele at the Snn3-B1 locus; breeding to remove SnTox3 sensitivity could help increase P. nodorum disease resistance. The research presented here falls into three main areas: 1. Genetic mapping of SnTox3 sensitivity: the major SnTox3 sensitivity locus Snn3-B1 was fine-mapped using an association mapping panel, the Avalon x Cadenza population and a MAGIC population. These defined a 6.2 kb region on chromosome 5B explaining ≥ 32 % of the phenotypic variation. The most significantly associated SNP was converted to a KASP marker. Additionally, nine minor SnTox3 sensitivity QTL were identified on chromosomes 1B, 2A, 2B, 3B, 4D, 6A, 6B, and 7B, each accounting for 2.6-6.0 % of the phenotypic variation. 2. Gene expression analysis: combining RNA-seq and genomic analysis in a time-course experiment using a SnTox3 sensitive (Cadenza) and insensitive variety (Avalon), two treatments (+/- SnTox3 infiltration) and 0, 4, 8, 12, and 24 hour time-points allowed investigation of gene expression before and after SnTox3 infiltration. Two Snn3-B1 candidate genes were identified, a protein kinase and wall-associated kinase, with analysis of genome-wide patterns of gene expression providing insights into the molecular mechanism of the SnTox3-Snn3-B1 pathway. 3. Adult plant resistance: The Avalon x Cadenza bi-parental population was used to investigate adult plant P. nodorum resistance. First, an improved Avalon x Cadenza genetic map was developed using publicly available genotype data. This was combined with field trial data to identify a major QTL on chromosome 5A for leaf and glume blotch and a further seven QTL on chromosomes 1B, 2D, 3B, 5A, 5B and 7B. Of these, field resistance QTL were found to co-locate with the effector sensitivity QTL Snn1 and Snn7. Collectively, this research provides wheat breeders and researchers with knowledge and molecular resources regarding sensitivity to the P. nodorum effector, SnTox3, as well as field QTL for both leaf and glume blotch.

Gaeumannomyces tritici is a soil-borne, highly destructive, wheat root pathogen, causing take-all disease. Some modern, elite, winter wheat cultivars possess a genetic trait promoting low take-all inoculum build-up (LowTAB). This leads to reduced disease if wheat is grown in the same field the next year. This PhD aimed to test if genetics (LowTAB) and chemistry will individually or synergistically influence take-all fungal inoculum build-up in first wheats as methods to control second wheat take-all disease. The underlying mechanism, epidemiology, agronomy and genetics of the TAB (take-all build-up) trait in eight first wheat field trials was investigated. This identified two minor QTLs conferring the LowTAB trait, in a doubled haploid mapping population. This PhD also confirms the highly complex cultivar-year-field interactions that underpin this trait. Root phenotyping experiments in the field and laboratory highlight that the TAB trait is not likely to be the result of root system architecture variation. Future field trials are planned to confirm the QTLs identified and to test for links between TAB and root-soil-microbial interactions. The effect of foliar applied chemistry (fungicide: Amistar, active ingredient: azoxystrobin and plant growth regulator: Moddus, a.i. trinexapac-ethyl) combined with genetics (TAB) on first wheat take-all inoculum build-up and second wheat disease was investigated. To complement this, laboratory screens were performed checking for common target site mutations to the azoxystrobin fungicide, in new and historic G. tritici isolates. For the first time, legacy effects of first wheat foliar chemistry on second wheat disease were identified, however no synergy with genetics were found. Early first wheat Amistar sprays reduced second wheat take-all disease, whilst later sprays and plant growth regulator, Moddus; had no effect. However, first wheat inoculum reduction by Amistar, could not be directly linked to the second wheat disease outbreaks observed. No evidence of fungicide resistance was found in 40 UK isolates, thus the varied efficacy of Amistar is linked to soil dose rate at the different application times. The collective PhD findings of the effect of first wheat chemistry and genetics make a significant contribution to the control of take-all disease in commercial second wheat crops.

The use of nitrogen (N) fertilizers is both costly to farmers and contributes to environmental degradation. N applied to wheat accounts for 18% of N applied to farmland globally, making it a prime target for reducing and optimizing N application. Chapter I is a review on nitrogen use efficiency (NUE) in wheat, with emphasis on breeding efforts and genetic resources available to increase NUE. The concept of effective use of nitrogen (EUN) as yield per unit N applied as a measure of N use, is also introduced. Chapter II is a study using two bi-parental double haploid families to evaluate genetic variability of both the genetic main effects (intercept) and linear response to N (slope) and determine the feasibility of selection for EUN in wheat. Using cross validation, a genomic prediction accuracy of 0.68 for intercept and 0.50 for slope was found, indicating that EUN is under genetic control and can be selected for. The prospect of breeding for EUN under limited resources, i.e., using fewer N rates and fewer experimental plots, is also explored. It was found that two different N treatments can be used to produce accurate predictions of intercept and slope as high as 0.98 and 0.95, respectively. Chapter III uses the same population described in chapter II to further investigate feasibility of selection for EUN using a normalized difference vegetation index (NDVI) obtained from multi-spectral aerial images gathered throughout the growing season. Cumulative photosynthesis across the growing season was estimated by integration across the NDVI curve, and compared to grain yield estimates to determine the efficacy of aerial imaging to identify high EUN lines. NDVI values and the area under the NDVI curve were able to predict yield and had the strongest ability to predict yield in moderate to low N treatments, with R2 values as high as 0.81 and 0.78 respectively.

Stagonospora nodorum blotch (SNB) is an economically important wheat disease caused by the necrotrophic fungus Parastagonospora nodorum. SNB resistance in wheat is controlled by several quantitative trait loci (QTLs). Thus, the identifying of novel resistance/susceptibility QTLs is crucial for continuous improvement of the SNB resistance. Here, the hard winter wheat association mapping panel (HWWAMP) comprising accessions from breeding programs in the Great Plains region of the US, was evaluated for SNB resistance and necrotrophic effectors (NEs) sensitivity at the seedling stage. A genome-wide association study (GWAS) was performed to identify singlenucleotide polymorphism (SNP) markers associated with SNB resistance and effectors sensitivity. We found seven significant associations for SNB resistance/ susceptibility distributed over chromosomes 1B, 2AL, 2DS, 4AL, 5BL, 6BS, and 7AL. Two new QTLs for SNB resistance/ susceptibility at the seedling stage were identified on chromosomes 6BS and 7AL, whereas five QTLs previously reported in diverse germplasms were validated. Allele stacking analysis at seven QTLs explained the additive and complex nature of SNB resistance. We identified accessions (‘Pioneer-2180’ and ‘Shocker’) with favorable alleles at five of the seven identified loci, exhibiting a high level of resistance against SNB. Further, GWAS for sensitivity to NEs uncovered significant associations for SnToxA and SnTox3, co-locating with previously identified host sensitivity genes (Tsn1 and Snn3). Candidate gene analysis for SNB resistance revealed 35 genes of putative interest with plant defense response-related functions. The QTLs identified and validated in this study could be easily employed in breeding programs using the associated markers to enhance the SNB resistance in hard winter wheat.

Wheat (Triticum aestivum, L) and barley (Hordeum vulgare) are widely grown as winter annual grains in a double crop rotation with soybean (Glycine max, L. Merr.) in much of the U.S. Improved management strategies and the development cultivars that meet the quality requirements of higher value end-use markets is important to increase production and profitability of winter annual grains and the double crop rotation in the Eastern U.S. In Chapter I, fifteen commercially relevant winter wheat genotypes ranging in maturity were sown in a split-plot design (sowing date=main plot, genotype=subplot) at three different sowing dates (considered to be 'very early' (20-28 days before recommended), 'early (6-11 days before recommended)', or 'recommended') and replicated three times at eight environments (site-year) from 2015-2018 in VA and KY. Grain yield, tiller estimation, heading date, protein, and 1000-kernel weight were assessed for each yield plot. At all environments, sowing earlier in the fall achieved an earlier (P 3.0) with relevant malt quality traits. Phenotypic variation for malt quality was largely and consistently explained by QTL on chromosomes 1H, 5H, and 7H in the Endeavor / VA09B-34 population and by two separate QTL on 1H in the Violetta / VA09B-34 population. A region on 4H corresponding with QDp.DiMo-4H, explained between 12.1 - 42.2% (Endeavor / VA09B-34) and 30.0 - 55.7% (Violetta / VA09B-34) of the phenotypic variation for diastatic power (DU). These QTL are recommended for MAS in order to aid breeding strategies that aim to select for improved malting characteristics in Eastern U.S. malt barley breeding material.

The study reported in this thesis was carried out during 2018/2019 at Lincoln University, New Zealand, to investigate olive growth and development and wheat yield performance in an olive wheat intercropping system.This experiment was laid out in a split design with three replicates. The main treatments were olive (Olea europaea) intercropped with wheat (Triticum aestivum), olive intercropped with wheat undersown with white clover (Trifolium repens) and fallow used as a control. Sub plots were three olive cultivars Frantoio, Leccino and Barnea. The main intercropping treatments had no significant effect on olive shoot elongation, leaf apperance, fresh and dry leaf weights and leaf nutrient contents. However, there were significant differences in leaf weight and nutrient content among cultivars. Leccino had the highest fresh and dry leaf weights. In terms of nutrient content, Barnea leaves contained the lowest nitrogen, phosphorus, potassium. Frantoio had the highest content of potassium and Leccino had the highest phosphorus content. It suggests that olive leaf could be used as animal fodder due to their high mineral content. This research suggests that undersown clover had no effect on wheat final yield and grain quality. When wheat is intercropped with olive, the wheat yield is likely to reduce by 25% because the tree canopy reduced the available radiation to wheat. However, it is likely that wheat grain quality will be improved by tree shading. Wheat canopy and phenology modelling suggests that in the Canterbury region at least, in an olive – wheat intercropping system, earlier wheat sowing is likely to result in a higher wheat yield. Late sowing is likely to increase the grain quality but also increase the nutrient competition between the wheat and olive trees.

Disease resistance is critical in soft red winter wheat (Triticum aestivum L.) cultivars. Leaf rust caused by Puccinia triticina Eriks and stripe rust caused by Puccinia striiformis Westend. f.sp. tritici Eriks. are destructive pathogens of wheat. From 2014 to 2015 phenotypic data was collected at diverse locations for resistance to leaf rust (North Carolina, Texas, and Virginia) and stripe rust (Arkansas, North Carolina, Georgia, Texas, and Virginia) in a Pioneer ‘25R47’ /‘Jamestown’ (P47/JT) population composed of 186 F5:9 recombinant inbred lines (RILs). Analysis of the P47/JT population identified two quantitative trait loci (QTL) for leaf rust resistance on chromosome 5B and two QTL for stripe rust resistance on chromosomes 3B and 6A. Phenotypic variation (%) explained by the putative leaf rust resistance QTL of Jamestown on 5B was as high as 22.1%. Variation explained by the putative stripe rust resistance QTL of Jamestown on 3B and 6A was as high as 11.1 and 14.3%, respectively. Jamestown is postulated to contain gene Lr18. Seedlings of 186 F5:9 recombinant inbred lines from the P47/JT population and 200 F2 seedlings from eight other crosses including Jamestown and/or the Lr18 host differential line RL6009 (Thatcher*6/Africa 43) were screened with P. triticina race TNRJJ. Genetic analysis of the populations was conducted to validate the presence of Lr18 in Jamestown. Results of linkage analysis identified SNP maker IWB41960 linked within 5 cM of gene Lr18 in all three populations. From 2016 to 2017 phenotypic data was collected at diverse locations for resistance to leaf rust (Illinois, North Carolina, and Virginia) in a ‘2013412’ (PI 667644) / VA10W-21 (PI 676295) population (412/21) composed of 157 doubled haploid (DH) lines. The 412/21 DH lines were genotyped via genotyping by sequence (GBS). Analysis of the 412/21 population identified one quantitative trait loci (QTL) region associated with adult plant resistance to leaf rust on chromosome 1B. Phenotypic variation (%) explained by the putative leaf rust resistance QTL of 2013412 on 1B was as high as 40.1%. Kompetitive allele-specific (KASP) markers KASP_S1B_8414614 and KASP_S1B_8566239 were developed as markers for use in marker assisted selection.

In multiple species, genome-wide association (GWA) studies have become an increasingly prevalent method of identifying the quantitative trait loci (QTLs) that underlie complex traits. Despite this, relatively few GWA analyses using high-density genomic markers have been carried out on highly quantitative traits in wheat. We utilized single-nucleotide polymorphism (SNP) data generated via a genotyping-by-sequencing (GBS) protocol to perform GWA on multiple yield-related traits using a panel of 329 soft red winter wheat genotypes grown in four environments. In addition, the SNP data was used to examine linkage disequilibrium and population structure within the testing panel. The results indicated that an alien translocation from the species Triticum timopheevii was responsible for the majority of observed population structure. In addition, a total of 50 significant marker-trait associations were identified. However, a subsequent study cast some doubt upon the reproducibility and reliability of plant QTLs identified via GWA analyses. We used two highly-related panels of different genotypes grown in different sets of environments to attempt to identify highly stable QTLs. No QTLs were shared across panels for any trait, suggesting that QTL-by-environment and QTL-by-genetic background interaction effects are significant, even when testing across many environments. In light of the challenges involved in QTL mapping, prediction of phenotypes using whole-genome marker data is an attractive alternative. However, many evaluations of genomic prediction in crop species have utilized univariate models adapted from animal breeding. These models cannot directly account for genotype-by-environment interaction, and hence are often not suitable for use with lower-heritability traits assessed in multiple environments. We sought to test genomic prediction models capable of more ad-hoc analyses, utilizing highly unbalanced experimental designs consisting of individuals with varying degrees of relatedness. The results suggest that these designs can successfully be used to generate reasonably accurate phenotypic predictions. In addition, multivariate models can dramatically increase predictive accuracy for some traits, though this depends upon the quantity and characteristics of genotype-by-environment interaction.

Fusarium Head Blight (FHB) is a devastating fungal disease of wheat (Triticum aestivum L.) and results in significant economic losses due to reductions in grain yield and the accumulation of mycotoxins, such as deoxynivalenol (DON) and nivalenol (NIV). As a result, breeding programs have been working to identify resistance genes in wheat varieties known to be resistant to FHB. Some of the major quantitative trait loci (QTL) for FHB resistance identified to date have been from exotic sources such as 'Sumai3' and the Chinese landrace Wangshuibai, and native resistance has been identified in North American cultivars such as Ernie and Truman which are being used in breeding programs. This study was conducted to characterize and map QTL for resistance to FHB in the soft red winter wheat cultivar Jamestown and to identify tightly linked DNA markers associated with those QTL so that marker-assisted selection (MAS) can be used in pyramiding these and other known QTL into elite backgrounds. Types of resistance assessed in this study include: Type I (resistance to initial infection; incidence), Type II (resistance to spread in wheat spike; severity), and decreases in mycotoxin accumulation (DON) and percentage of Fusarium damaged kernels (FDK). A population composed of 186 F5:7 recombinant-inbred lines (RILs) from the cross Pioneer Brand '25R47'/Jamestown were used to evaluate these traits in six environments (MD, NC, and VA in 2011 and 2012). This study identified a QTL for resistance to DON accumulation and FHB severity on the wheat chromosome 1B. The QTL accounted for 12.7% to 13.3% of the phenotypic variation in DON accumulation and 26.1% of the phenotypic variation in FHB severity. The most diagnostic marker for the QTL on chromosome 1B associated with resistance to FHB severity and DON accumulation is Xwmc500.6 located 7.2 cM from the QTL peak and flanked by markers Xwmc500.7 and Xgwm273.2 (28.2 cM interval).

Powdery mildew, caused by the pathogen Blumeria graminis (DC) Speer (Syn. Erysiphe graminis DC) f. sp. tritici, is a major disease of wheat (Triticum aestivum L.). Race-specific resistance is easily identified in the field due to its qualitative phenotype and it is easy to incorporate because it is inherited as a single gene. Unfortunately, this type of resistance is easily overcome by the pathogen. Traits associated with quantitative trait loci (QTL) such as adult-plant resistance (APR), have become popular with plant breeders because of their durability over a wide geographic range and time. Due to the quantitative nature of these genes, they are difficult to study requiring multiple assessments of disease development under natural conditions in more than one location over a period of several weeks. Numerous QTL for APR to powdery mildew have been mapped in independent studies in different wheat backgrounds. The wheat cultivar Massey has been the subject of several studies due to its APR to powdery mildew that has remained effective for several decades. However, it has been difficult to identity simple sequence repeat (SSR) markers that are tightly linked to the QTL for APR in Massey. Such markers give breeders an advantage by allowing them to quickly identify and select for traits that would be difficult to distinguish in the field among breeding progeny from several backgrounds. Therefore, identification of tightly linked markers associated with APR to powdery mildew is necessary so that these traits can be selected for reliably in progeny.

Cyanobacteria are a tantalising prospect for research into biofertilisers and plant growth promotion. With photolithic autotrophy, broadly distributed capacities for biological N2 fixation and phytohormone production, cyanobacteria have all the necessary requirements for enhancing plant growth in cultivated soils. Research into the use of cyanobacterial inoculants for wheat cultivation, and particularly for salt stress amelioration, has never been assessed in Australia. Therefore, this thesis aimed to explore the potential of Australian cyanobacteria to ameliorate the effects of irrigation salinity on Australian wheat cultivars. This thesis describes the first investigation into the diversity of cyanobacteria from Australian wheat fields and cycad rhizospheres. It also characterises the nitrogenase activity and salt physiology of selected isolates and evaluates their application in ameliorating the salt stress of Australian wheat. The results of this study indicate that there is only a weak relationship between cycad cyanobionts and their associated rhizosphere cyanobacteria. A Cylindrospermum wheat field isolate had the highest nitrogenase activity, being almost double that of Nostoc. Microcoleus vaginatus was also isolated from a wheat field and was found to have nitrogenase activity and to contain cyanobacterial nifH. Analysis of cyanobacterial salt physiology determined that salinity generally lowered O2 evolution and nitrogenase activity of cyanobacteria though the relationships between salinity, O2 evolution and nitrogenase activity were more complicated for the individual isolates studied. The chlorophyll fluorescence parameters failed to explain the observed effects of salinity on O2 evolution. In pot trials, inoculation with cyanobacteria negatively affected the biomass and nutrient concentrations of wheat seedlings at all salinities, though the effects were fewer as salinity increased. The most likely explanation of these results is the sorption of nutrients to cyanobacterial extracellular polymeric substances, making them unavailable for plant uptake. These results indicate that Australian cyanobacteria do not ameliorate salt stress for wheat seedlings but may have a role to play in the stabilisation and improvement of degraded agricultural soils.

Wheat stem rust caused by the fungus Puccinia graminis f. sp. tritici (Pgt) has historically been a devastating disease of wheat. In 1998, a race of Pgt was detected in Uganda that is virulent to the majority of wheat cultivars being grown. This new race, commonly referred to as Ug99, was characterized as race TTKSK based upon the North American stem rust differentials. Race TTKSK has recently spread throughout eastern and southern Africa, and into the Middle East. Data suggest that Ug99 can alternatively be described as Pgt race PTKSK, based upon demonstrated avirulence to stem rust resistance gene Sr21. Accessions of wild relatives of wheat, Triticum monococcum, T. urartu, and Aegilops tauschii were screened with Pgt race TTKSK. Crosses among resistant and susceptible accessions of T. monococcum led to the characterization of two new stem rust resistance genes effective to Pgt-TTKSK. Preliminary screening of 700 accessions of spring wheat led to the identification of 88 accessions with resistance to Ug99. Examination of the genetics of resistance in one of these accessions, SD 1691, identified Sr28 as conferring a high level of resistance. Molecular markers linked to Sr28 were identified on chromosome arm 2BL. Examination of the genetics of resistance in Gabo 56 indicated the presence of a single dominant gene. This gene was temporarily designated as SrGabo56 and mapped on chromosome arm 2BL. Segregation of resistance in the progeny of the cross between SD 1691 and Gabo 56 indicated that Sr28 and SrGabo56 are linked. Characterization of the genetics of resistance to Pgt-TTKSK in synthetic wheat TA4152-37, resulted in the identification of Sr13 in TA4152-37. Molecular markers linked to Sr13 in hexaploid wheat were identified on chromosome arm 6AL. Overall, 3 new stem rust resistance genes effective to Pgt-TTKSK were identified. Molecular markers linked to SrGabo56, Sr13, and Sr28 were described. The identification of markers linked to multiple stem rust resistance genes will facilitate the combination of these genes in breeding lines. The identification of the new sources of resistance to Pgt-TTKSK will provide plant breeders with additional tools to protect wheat from this dangerous race.

Fusarium graminearum (Schwabe), causal organism of fusarium head blight (FHB), has become a major pathogen of wheat (Triticum aestivum L.) throughout North America. Since its discovery in the United States, the disease has spread south and east until at present it is an annual threat for growers of winter wheat in the Mid-Atlantic region. Yield losses for soft red winter (SRW) wheat averaged 908 kg ha-1 in the FHB outbreak of 1998 (Griffey et al., 1999). The economic loss from this single FHB epidemic was an estimated 8.5 million dollars. Environmental conditions favorable for FHB development, including above average rainfall and temperatures during anthesis, have become more common in the Upper-Midwestern wheat-growing region over the past decade, leading to substantial losses in wheat and barley crops. This, coupled with low prices being paid for wheat, has prompted research toward solving the problem of FHB across the nation. The majority of labor and financial resources devoted to FHB research are dedicated to incorporating FHB resistance into adapted wheat lines. While this is a prudent method of combating this disease, this process will take many years to complete. We have examined all FHB assessment parameters, which include FHB incidence, FHB severity, FHB index, percentage fusarium damaged kernels (percentage FDK), and 15-acetyl deoxynivalenol toxin (DON toxin) accumulation, to ascertain which assessment parameters best quantify FHB resistance levels in addition to grain yield and grain volume weight (GVW) losses. FHB index provides the most reliable in-field assessment of a genotype's resistance level, whereas percentage FDK provides a reliable measure of a genotype's resistance level post-harvest. FHB index and percentage FDK are also the most predictive assessment parameters with regard to grain yield and GVW loss. A wide range in both level and type of resistance was observed among genotypes examined in this study. The cultivars Agripro Patton, Ernie, INW9824, Roane, and the experimental line NY87048W-7388 consistently had lower scores for FHB assessment parameters and lower losses of grain yield and GVW.

The epidemiology of Mycosphaerella graminicola (Fuckel) Schroeter was studied for two seasons and the role of ascospores as primary inoculum of the disease was examined as a basis for the development of control measures. Ascospores were released following rain and there were two peaks of ascospore release. The first peak occurred during April and May and it was concluded from spore trapping and bait plant exposures that these ascospores provided the inoculum source for primary infection. The second peak occurred during the middle of November. Since secondary spread by means of rain-splashed pycnidiospores occurs during crop growth after primary infection, the significance of this second peak for epidemic development is not fully understood. Speckled leaf blotch infection in the field was detected during late autumn and sporulation first occurred during tillering. The disease was prevalent during late autumn and winter, and spread rapidly in rainy periods. The effect of disease on leaf area was again demonstrated in this research study. The non-random distribution of disease on leaves of unequal size necessitated the use of a weighted mean to fully describe disease. Furthermore, a carry-over effect of early disease on leaf area could be better accounted for by expressing disease as the total green leaf area per shoot. Absolute green leaf area may be more appropriate for all studies in which the mechanism of yield reduction is to be analysed. Several protectant fungicides strongly inhibited pycnidiospore germination in laboratory tests. Many systemic fungicides tested did not prevent spore germination. Once germination had taken place, systemic fungicides were highly effective in inhibiting mycelial growth. In the 1982-83 efficacy trial, foliar applications with the systemic fungicides triadimefon, PP450 and propiconazole controlled disease and resulted in significant yield increases in Kopara wheat crops. Two wheat trials were conducted in 1981-82 and 1982-83 to study the effects of natural and fungicide modified epidemics of speckled leaf blotch and stripe rust on yield and yield potential. In these trials, seed treatment with triadimenol and a triadimefon foliar spray applied at the 6 or 7 leaf stage proved to be highly effective in controlling both diseases, resulting in significant yield increases: Detailed analysis of epidemic development aided the identification of the duration of fungicide protection and the necessity for additional sprays. From knowledge of the development of yield potential, it was shown that the early phase of the epidemic was the most damaging, with reductions to the grain number per ear and ear number per unit area as well as to the individual grain weight. The results can serve as a basis for optimal timing of fungicide applications rather than routine spraying where applications may sometimes be unwarranted.