Tar spot complex (TSC) is one of the most destructive foliar diseases of maize (Zea mays L.) in tropical and subtropical areas of Central and South America, causing significant grain yield losses when weather conditions are conducive. To dissect the genetic architecture of TSC resistance in maize, association mapping, in conjunction with linkage mapping, was conducted on an association-mapping panel and three biparental doubled-haploid (DH) populations using genotyping-by-sequencing (GBS) single-nucleotide polymorphisms (SNPs). Association mapping revealed four quantitative trait loci (QTL) on chromosome 2, 3, 7, and 8. All the QTL, except for the one on chromosome 3, were further validated by linkage mapping in different genetic backgrounds. Additional QTL were identified by linkage mapping alone. A major QTL located on bin 8.03 was consistently detected with the largest phenotypic explained variation: 13% in association-mapping analysis and 13.18 to 43.31% in linkage-mapping analysis. These results indicated that TSC resistance in maize was controlled by a major QTL located on bin 8.03 and several minor QTL with smaller effects on other chromosomes. Genomic prediction results showed moderate-to-high prediction accuracies in different populations using various training population sizes and marker densities. Prediction accuracy of TSC resistance was >0.50 when half of the population was included into the training set and 500 to 1,000 SNPs were used for prediction. Information obtained from this study can be used for developing functional molecular markers for marker-assisted selection (MAS) and for implementing genomic selection (GS) to improve TSC resistance in tropical maize. Abbreviations: BLUP, best linear unbiased prediction; DH, doubledhaploid; DTMA, Drought Tolerant Maize for Africa; FDR, false discovery rate; GBS, genotyping-by-sequencing; GS, genomic selection; LD, linkage disequilibrium; LOD, logarithm of odds; MAF, minor allele frequency; MAS, marker-assisted selection; PCA, principle component analysis; PVE, phenotypic variation explained; QTL, quantitative trait loci; r MG , genomic prediction accuracy; SNP, single-nucleotide polymorphism; TSC, tar spot complex. Core Ideas• Association and linkage mapping are effective for dissecting genetic architecture of complex traits in maize.• TSC resistance in maize is controlled by a major QTL and several minor QTL.• Major QTL on bin 8.03 confirmed by association and linkage mapping.• TSC resistance in tropical maize could be improved by MAS and GS individually or stepwise.

Leaf rust (caused by Puccinia triticina Erikss. [Pt]) is increasingly impacting durum wheat (Triticum turgidum L. var. durum) production with the recent appearance of races with virulence to widely grown cultivars in many durum producing areas worldwide. A highly virulent P. triticina race on durum wheat was recently detected in Kansas. This race may spread to the northern Great Plains, where most of the US durum wheat is produced. The objective of this study was to identify sources of resistance to several races from the United States and Mexico at seedling stage in the greenhouse and at adult stage in field experiments. Genome-wide association study (GWAS) was used to identify single-nucleotide polymorphism (SNP) markers associated with leaf rust response in a worldwide durum wheat collection of 496 accessions. Thirteen accessions were resistant across all experiments. Association mapping revealed 88 significant SNPs associated with leaf rust response. Of these, 33 SNPs were located on chromosomes 2A and 2B, and 55 SNPs were distributed across all other chromosomes except for 1B and 7B. Twenty markers were associated with leaf rust response at seedling stage, while 68 markers were associated with leaf rust response at adult plant stage. The current study identified a total of 14 previously uncharacterized loci associated with leaf rust response in durum wheat. The discovery of these loci through association mapping (AM) is a significant step in identifying useful sources of resistance that can be used to broaden the relatively narrow leaf rust resistance spectrum in durum wheat germplasm. Abbreviations: AM, association mapping; ARS, Agricultural Research Service; CDL, Cereal Disease Laboratory; GWAS, genome-wide association study; IT, infection type; LD, linkage disequilibrium; MAF, minor allele frequency; MR, moderately resistant; MS, moderately susceptible; MSD, mean squared difference; NSGC, National Small Grain Collection; PC, principal component; PCA, principal component analysis; pFDR, positive false discovery rate; Pt, Puccinia triticina Erikss.; QTL, quantitative trait loci; R, resistant; S, susceptible; SNP, single-nucleotide polymorphism; SSR, simple-sequence repeat. Core Ideas• Thirteen durum wheat accessions showed resistance to all Puccinia triticina races tested• GWAS revealed 88 SNPs (37 loci) associated with leaf rust response in durum wheat• Associations were identified on all chromosomes except 1B and 7B• GWAS revealed 14 previously uncharacterized loci for leaf rust resistance

Tar spot complex (TSC), caused by at least two fungal pathogens, Phyllachora maydis and Monographella maydis , is one of the major foliar diseases of maize in Central and South America. P. maydis was also detected in the United States of America in 2015 and since then the pathogen has spread in the maize growing regions of the country. Although remote sensing (RS) techniques are increasingly being used for plant phenotyping, they have not been applied to phenotyping TSC resistance in maize. In this study, several multispectral vegetation indices (VIs) and thermal imaging of maize plots under disease pressure and disease-free conditions were tested using an unmanned aerial vehicle (UAV) over two crop seasons. A strong relationship between grain yield, a vegetative index (MCARI2), and canopy temperature was observed under disease pressure. A strong relationship was also observed between the area under the disease progress curve of TSC and three vegetative indices (RDVI, MCARI1, and MCARI2). In addition, we demonstrated that TSC could cause up to 58% yield loss in the most susceptible maize hybrids. Our results suggest that the RS techniques tested in this study could be used for high throughput phenotyping of TSC resistance and potentially for other foliar diseases of maize. This may help reduce the cost and time required for the development of improved maize germplasm. Challenges and opportunities in the use of RS technologies for disease resistance phenotyping are discussed.

Leaf rust, caused by Puccinia triticina, is one of the main fungal diseases limiting durum wheat production. This study aimed to characterize previously undescribed genes for leaf rust resistance in durum wheat. Six different resistant durum genotypes were crossed to two susceptible International Maize and Wheat Improvement Center (CIMMYT) lines and the resulting F1, F2, and F3 progenies were evaluated for leaf rust reactions in the field and under greenhouse conditions. In addition, allelism tests were conducted. The results of the study indicated that most genotypes carried single effective dominant or recessive seedling resistance genes; the only exception to this was genotype Gaza, which carried one adult plant and one seedling resistance gene. In addition, it was concluded that the resistance genes identified in the current study were neither allelic to LrCamayo or Lr61, nor were they related to Lr3 or Lr14a, the genes that already are either ineffective or are considered to be vulnerable for breeding purposes. A complicated allelic or linkage relationship between the identified genes is discussed. The results of the study will be useful for breeding for durable resistance by creating polygenic complexes.

The emergence and spread of new crop diseases threatens the global food security situation. Phyllachora maydis, one of the three fungal pathogens involved in Tar Spot Complex (TSC) of maize, a disease native to Latin American countries, was detected for the first time in the United States of America (USA) in 2015. Although TSC has been previously reported to cause up to 50% of yield losses in maize in Latin America, the impact of P. maydis alone on maize yield is not known yet. However, there is a possibility that Monographella maydis, the second most important pathogen involved in TSC, would be introduced to the USA and would become associated with P. maydis and both pathogens could form the devastating complex disease in the country. The first objective of this study was to identify the TSCvulnerable maize-producing regions across the USA by applying a climate homologue modeling procedure. The second objective was to quantify the potential economic impact of the disease on the maize industry in the USA. This study showed that even a 1% loss in maize production caused by the disease could potentially lead to a reduction in maize production by 1.5 million metric tons of grain worth US$231.6 million. Such production losses will affect not only the maize-related industries in the USA but also the food security in a number of low-income countries that are heavily dependent on US maize imports. This, in turn, may lead to increased poverty and starvation and, in some cases, to social unrest due to increased prices of maize-based staple foods. The study is intended to raise public awareness regarding potential TSC outbreaks and to develop strategies and action plans for such scenarios.