Tar Spot Complex Research Articles

Effect of gypsum and potassium on corn yield and on the exchangeable bases of an acid soil in La Frailesca, Chiapas

Objective: To evaluate the residual effect of gypsum in corn crops (2 years after its application in a previously limed soil), as well as the result of a new addition of gypsum combined with potassium in La Frailesca, Chiapas, Mexico. Design/Methodology/Approach: We used a composite factorial design. The initial arrangement (2017) consisted of four levels of gypsum (0, 1.25, 2.5, and 5 t ha-1) and four levels of potassium (0, 60, 120, and 180 kg K2O ha-1). In 2019, the gypsum-treated plots were divided in half: the same amount of gypsum applied in 2017 was added to the first half and the other half was used to assess the residual effect of the initial treatment. The potassium doses were the same as the original. Corn grain yield, pH, exchangeable bases, and aluminum saturation percentage were measured at 0 to 7 and 7 to 14 cm below ground level. Results: The greatest effect on yield was obtained with 2.5 t ha-1 of gypsum applied in 2017; no significant increases were recorded with higher gypsum doses. The exchangeable calcium content and pH level increased, while magnesium, potassium, and aluminum in the soil decreased. Study Limitations/Implications: Suspected presence of Tar Spot Complex was diagnosed. Findings/Conclusions: An excessive application of gypsum generates an imbalance in exchangeable potassium and magnesium in the soil; therefore, producers must exercise caution in the use of these products as part of their fertilization plan.

Association mapping of resistance to tar spot complex in maize

AbstractTar spot complex (TSC) is a major fungal disease of maize in Mexico and several Central and South American countries. The causal agents of the disease are Phyllachora maydis and Monographella maydis. Yield losses of up to 58% associated with this disease have been reported previously. The majority of commercial maize germplasm in the United States is considered highly susceptible to TSC. In order to accelerate the disease resistance improvement process, genes for resistance to TSC and molecular markers linked to these genes, need to be identified and characterized. Here, we used an association mapping technique to detect quantitative trait loci (QTLs) for TSC resistance using genome‐wide association study (GWAS)‐based genotyping‐by‐sequencing (GBS) and Diversity Arrays Technology Sequencing (DArTseq) single‐nucleotide polymorphisms. An association mapping panel of 228 CIMMYT maize lines was used for the GWAS. One major QTL explaining 27.66% of the phenotypic variance and four minor QTLs explaining 10.18%, 12.72%, 10.14% and 13.38% of the phenotypic variance, respectively, were identified by the GBS‐based association mapping. Two QTLs were identified by DArTseq‐based association mapping explaining 1.07% and 18.29% of the phenotypic variance, respectively. These QTLs are expected to facilitate subsequent maize improvement using marker‐assisted selection.

Mining alleles for tar spot complex resistance from CIMMYT's maize Germplasm Bank

The tar spot complex (TSC) is a devastating disease of maize (Zea mays L.), occurring in 17 countries throughout Central, South, and North America and the Caribbean, and can cause grain yield losses of up to 80%. As yield losses from the disease continue to intensify in Central America, Phyllachora maydis, one of the causal pathogens of TSC, was first detected in the United States in 2015, and in 2020 in Ontario, Canada. Both the distribution and yield losses due to TSC are increasing, and there is a critical need to identify the genetic resources for TSC resistance. The Seeds of Discovery Initiative at CIMMYT has sought to combine next-generation sequencing technologies and phenotypic characterization to identify valuable alleles held in the CIMMYT Germplasm Bank for use in germplasm improvement programs. Individual landrace accessions of the “Breeders' Core Collection” were crossed to CIMMYT hybrids to form 918 unique accessions topcrosses (F1 families) which were evaluated during 2011 and 2012 for TSC disease reaction. A total of 16 associated SNP variants were identified for TSC foliar leaf damage resistance and increased grain yield. These variants were confirmed by evaluating the TSC reaction of previously untested selections of the larger F1 testcross population (4,471 accessions) based on the presence of identified favorable SNPs. We demonstrated the usefulness of mining for donor alleles in Germplasm Bank accessions for newly emerging diseases using genomic variation in landraces.

Identification and fine mapping of a major QTL (qRtsc8-1) conferring resistance to maize tar spot complex and validation of production markers in breeding lines

Key messageA major QTL of qRtsc8-1 conferring TSC resistance was identified and fine mapped to a 721 kb region on chromosome 8 at 81 Mb, and production markers were validated in breeding lines.Tar spot complex (TSC) is a major foliar disease of maize in many Central and Latin American countries and leads to severe yield loss. To dissect the genetic architecture of TSC resistance, a genome-wide association study (GWAS) panel and a bi-parental doubled haploid population were used for GWAS and selective genotyping analysis, respectively. A total of 115 SNPs in bin 8.03 were detected by GWAS and three QTL in bins 6.05, 6.07, and 8.03 were detected by selective genotyping. The major QTL qRtsc8-1 located in bin 8.03 was detected by both analyses, and it explained 14.97% of the phenotypic variance. To fine map qRtsc8-1, the recombinant-derived progeny test was implemented. Recombinations in each generation were backcrossed, and the backcross progenies were genotyped with Kompetitive Allele Specific PCR (KASP) markers and phenotyped for TSC resistance individually. The significant tests for comparing the TSC resistance between the two classes of progenies with and without resistant alleles were used for fine mapping. In BC5 generation, qRtsc8-1 was fine mapped in an interval of ~ 721 kb flanked by markers of KASP81160138 and KASP81881276. In this interval, the candidate genes GRMZM2G063511 and GRMZM2G073884 were identified, which encode an integral membrane protein-like and a leucine-rich repeat receptor-like protein kinase, respectively. Both genes are involved in maize disease resistance responses. Two production markers KASP81160138 and KASP81160155 were verified in 471 breeding lines. This study provides valuable information for cloning the resistance gene, and it will also facilitate the routine implementation of marker-assisted selection in the breeding pipeline for improving TSC resistance.

Genomic Prediction of Resistance to Tar Spot Complex of Maize in Multiple Populations Using Genotyping-by-Sequencing SNPs.

Tar spot complex (TSC) is one of the most important foliar diseases in tropical maize. TSC resistance could be furtherly improved by implementing marker-assisted selection (MAS) and genomic selection (GS) individually, or by implementing them stepwise. Implementation of GS requires a profound understanding of factors affecting genomic prediction accuracy. In the present study, an association-mapping panel and three doubled haploid populations, genotyped with genotyping-by-sequencing, were used to estimate the effectiveness of GS for improving TSC resistance. When the training and prediction sets were independent, moderate-to-high prediction accuracies were achieved across populations by using the training sets with broader genetic diversity, or in pairwise populations having closer genetic relationships. A collection of inbred lines with broader genetic diversity could be used as a permanent training set for TSC improvement, which can be updated by adding more phenotyped lines having closer genetic relationships with the prediction set. The prediction accuracies estimated with a few significantly associated SNPs were moderate-to-high, and continuously increased as more significantly associated SNPs were included. It confirmed that TSC resistance could be furtherly improved by implementing GS for selecting multiple stable genomic regions simultaneously, or by implementing MAS and GS stepwise. The factors of marker density, marker quality, and heterozygosity rate of samples had minor effects on the estimation of the genomic prediction accuracy. The training set size, the genetic relationship between training and prediction sets, phenotypic and genotypic diversity of the training sets, and incorporating known trait-marker associations played more important roles in improving prediction accuracy. The result of the present study provides insight into less complex trait improvement via GS in maize.

CP-Elvia 3, new white maize variety resistant to tar spot complex for Mexican subtropical areas

En la producción de maíz, el agricultor se encuentra ante la alternativa de sembrar semilla comercial de maíz híbrido o de polinización libre, incluyendo variedades mejoradas o nativas. La semilla de maíz híbrido es costosa y es necesario comprarla en cada nuevo ciclo de siembra, mientras que una variedad mejorada de polinización libre ofrece varias ventajas ya que puede sembrarse por varios años, puede competir en rendimiento con los híbridos, es de menor costo y se difunde entre agricultores. Una variedad sintética ofrece ventajas adicionales sobre otras variedades incluyendo uniformidad en floración y fenotipo, mejor rendimiento de grano y resistencia a enfermedades. En éste trabajo se muestra la obtención de la variedad sintética de maíz de grano blanco CP-Elvia 3, con buen comportamiento agronómico y resistente al complejo de mancha de asfalto y que se obtuvo mediante un programa de selección recurrente de familias S1.

Application of Remote Sensing for Phenotyping Tar Spot Complex Resistance in Maize.

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.

Identification of Fungal Communities Within the Tar Spot Complex of Corn in Michigan via Next-Generation Sequencing.

Tar spot is a fungal disease complex of corn that has been destructive and yield limiting in Central and South America for nearly 50 years. Phyllachora maydis, the causal agent of tar spot, is an emerging corn pathogen in the United States, first reported in 2015 from major corn producing regions of the country. The tar spot disease complex putatively includes Monographella maydis (syn. Microdochium maydis), which increases disease damage through the development of necrotic halos surrounding tar spot lesions. These necrotic halos, termed "fish-eye" symptoms, have been identified in the United States, though Monographella maydis has not yet been confirmed. A recent surge in disease severity and loss of yield attributed to tar spot in the United States has led to increased attention and expanded efforts to understand the disease complex and how to manage it. In this study, next-generation sequencing of the internal transcribed spacer-1 (ITS1) ribosomal DNA was used to identify fungal taxa that distinguish tar spot infections with or without fish-eye symptoms. Fungal communities within tar spot only lesions were significantly different from communities having fish-eye symptoms. Two low abundance OTUs were identified as Microdochium sp., however, neither were associated with fish-eye symptom development. Interestingly, a single OTU was found to be significantly more abundant in fish-eye lesions compared to tar spot lesions and had a 91% ITS1 identity to Neottiosporina paspali. In addition, the occurrence of this OTU was positively associated with Phyllachora maydis fish-eye symptom networks, but not in tar spot symptom networks. Neottiosporina paspali has been reported to cause necrotic lesions on various monocot grasses. Whether the related fungus we detected is part of the tar-spot complex of corn and responsible for fish-eye lesions remains to be tested. Alternatively, many OTUs identified as Phyllachora maydis, suggesting that different isolate genotypes may be capable of causing both tar spot and fish-eye symptoms, independent of other fungi. We conclude that Monographella maydis is not required for fish-eye symptoms in tar spot of corn.

Inducción de resistencia a Puccinia sorghi y complejo mancha de asfalto (Phyllachora maydis y otros) en maíz (Zea mays)

Para demostrar la eficiencia de productos inductores de la resistencia a enfermedades en maíz, en 2016 y 2017 se establecieron ensayos en el Campo Experimental de la Universidad Autónoma del Estado de México, Campus Toluca, con el híbrido de maíz comercial BG1384W, donde se estudió la eficiencia de seis agroquímicos como inductores de resistencia, incluyendo Fosetil-Al, Acibenzolar-S-metil, Bacillus subtilis, Tebuconazole + Trifloxystrobin, Proteína Harpin y Clotianidin + Bacillus firmus, para controlar la roya común (Puccinia sorghi Schw.) y el complejo de la mancha de asfalto (Phyllachora maydis Maubl. y otros), con dos formas de aplicación (al suelo y foliar) y tres dosis (comercial recomendada, mitad de la recomendada y recomendada más 50%) para cada agroquímico. Se registraron datos agronómicos en un ensayo de rendimiento y la severidad de las mencionadas enfermedades. En 2016, la severidad no fue afectada por ninguno de los agroquímicos, aunque se incrementó el rendimiento con las aplicaciones de Fosetil-Al y Acibenzolar-S-metil. En 2017, Bacillus subtilis disminuyó la severidad de la roya, mientras que Fosetil-Al redujo la severidad del complejo mancha de asfalto, pero mostró el rendimiento más bajo.

Threats of Tar Spot Complex disease of maize in the United States of America and its global consequences

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 TSC-vulnerable 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.

Genome-Wide Analysis of Tar Spot Complex Resistance in Maize Using Genotyping-by-Sequencing SNPs and Whole-Genome Prediction.

Tar spot complex (TSC) is one of the most destructive foliar diseases of maize ( 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.

Use of remote sensing technology in the assessment of resistance of maize to tar spot complex

Assessment of Tar Spot Complex (TSC) severity in maize breeding experiments is conducted visually and may sometimes result in inconsistencies due to human interpretation. Disease scoring using remote sensing technologies may help bring more precision to the phenotyping process. An experiment for assessment of grain yield losses due to TSC was conducted at the Aguafria Experimental Station of the International Center for Wheat and Maize Improvement – CIMMYT in Mexico. Twenty-five maize genotypes were planted in spring of 2016 under a fungicide treatment to control TSC development and no fungicide treatment in a square lattice design with three replications. Four flights were carried out using an Unmanned Aerial Vehicle (UAV) equipped with a multispectral (550, 660, 735, 790 nm) and a thermal camera, simultaneously with the visual disease scorings and the yield was measured after harvesting. The preliminary results of the study indicated that the use of remote sensing in disease resistance phenotyping may be as effective as visual disease scoring since both correlate highly with the grain yield. Structural and chlorophyll vegetation indices (VIs) proved to be a good alternative for the estimation of yield losses caused by TSC in experimental field conditions, which may be potentially used for screening for resistance to this disease in maize genotypes, hypothetically reducing the need for visual disease scoring in the field.

Combined linkage and association mapping identifies a major QTL (qRtsc8-1), conferring tar spot complex resistance in maize

A major QTL ( qRtsc8 - 1 ) conditioning resistance to tar spot complex of maize and occurring at a frequency of 3.5% across 890 maize inbred lines. Tar spot complex (TSC) is a highly destructive disease of maize found in some countries in America. Identification of TSC resistant germplasm and elucidating the genetic mechanism of resistance is crucial for the use of host resistance to manage this disease. We evaluated 890 elite maize inbred lines in multiple environments and used genome wide association analysis (GWAS) with genotypic data from Illumina MaizeSNP50 BeadChip containing 56K SNPs to dissect the genetics of TSC resistance. GWAS results were validated through linkage analysis in three bi-parental populations derived from different resistant and susceptible parents. Through GWAS, three TSC resistance loci were identified on chromosome 2, 7 and 8 (-log10 (p)>5.99). A major quantitative resistance locus (QTL) designated qRtsc8-1, was detected on maize chromosome bin 8.03. qRtsc8-1, was confirmed in three independent bi-parental populations and it accounted for 18-43% of the observed phenotypic variation for TSC. A rare haplotype within the qRtsc8-1 region, occurring at a frequency of 3.5% increased TSC resistance by 14%. Candidate gene analysis revealed that a leucine-rich repeat receptor-like protein (LRR-RLKs) gene family maybe the candidate gene for qRtsc8-1. Identification and localization of a major locus conditioning TSC resistance provides the foundation for fine mapping qRtsc8-1 and developing functional markers for improving TSC resistance in maize breeding programs. To the best of our knowledge, this is the first report of a major QTL for TSC resistance.

First Report on the Presence of <i>Phyllachora</i> sp. in Corn Crops at Toluca, Estado de Mexico

Symptoms of Tar Spot Complex (TSC) in corn appear in the form of black spots surrounded by a chlorotic halo known as fish-eye. The crop with these symptoms was found at San Pablo Autopan, Estado de Mexico. The stromal region of foliar lesions was cut longitudinally, showing perithecia, asci and ascospores typical of Phyllachora maydis. The results were confirmed by PCR-ITS with ITS 1 and ITS4 initiators, cloned by pJET, before being sequenced and compared against NCBI’s data base. Phyllachora identity was confirmed. This is the first report where TSC pathogen has been detected under conditions that had not been previously reported, for instance, at 2600 meters above the sea level.

Evaluación de híbridos tri-lineales de maíz (Zea mays L.), por su reacción al Complejo Mancha de asfalto causada por los patógenos (Phyllachora maydis, Monographella maydis y Coniothyrium phyllacorae)

Durante 2013 se instaló un ensayo con el objetivo de identificar híbridos de maíz por su reacción a la enfermedad foliar conocida como Complejo Mancha de asfalto5. El ensayo se condujo en trece localidades de diferentes departamentos de El Salvador, teniendo nueve de ellas antecedentes de la enfermedad. La evaluación se desarrolló entre los meses de mayo a noviembre.Se utilizó el diseño experimental alpha-látice 4*5, con tres repeticiones y 20 tratamientos. Los tratamientos se conformaron de 17 híbridos tri-lineales experimentales y tres híbridos comerciales como testigo. Se realizó un análisis de varianza por localidad y combinado, especialmente para porcentaje de mazorcas con mala cobertura; porcentaje de pudrición de mazorcas totales y porcentaje de mazorcas podridas afectadas por el complejo y rendimiento. Se utilizó el modelo AMMI Biplot GGE-SREG para determinar la interacción genotipo-ambiente, encontrándose significancia para dicha interacción; así como para los factores individuales en las que existió significancia al 1% de probabilidad. En diez de las 13 localidades en la que se realizó el estudio, no existió presencia de la enfermedad. Pero en las restantes, la enfermedad incidió drásticamente sobre el rendimiento. El híbrido con mayor provecho en localidades sin presencia de la enfermedad fue DK-357 con 8.35 t ha-1, seguido por HES-3 y HES-1 con 7.75 y 7.1t ha-1 respectivamente. En las localidades donde hubo presencia de la enfermedad, los mejores híbridos fueron: HES-3, HES-17 y DK-357, con rendimientos de 4.04, 4.01 y 4t ha-1 respectivamente.Los porcentajes de mazorcas afectadas por la enfermedad para estos híbridos fueron 50.3%, 46.8% y 60.7%, respectivamente. El híbrido menos afectado por la enfermedad fue HES-1, con 42.9% de mazorcas infectadas y rendimiento de 3.85 t ha-1. El análisis de estabilidad reportó como los híbridos más estables a HES-27 y HES-13. Los resultados obtenidos muestran a HES-1 como híbrido tolerante y a HES-3 como resistente.Producción Agropecuaria y Desarrollo Sostenible, Vol. 3, 2014: 77-88

Sequential development of pathogens in the maize tarspot disease complex

The tarspot complex is caused by the interaction of Phyllachora maydis and Monographella maydis. Coniothyrium phyllachorae, possibly a mycoparasite, is found in older ascostromata of P. maydis, which always appears first causing tarspot. M. maydis follows and is responsible for the damaging “fisheye” symptom. The fisheye symptom is always associated with a tarspot in the center of the lesion, whereas 12 to 20% of the Phyllachora ascostromata remained free of M. maydis. Inoculations of maize leaves with the Microdochium anamorph of the Monographella (usually produced in lesions) failed to produce infections. Some infections with M. maydis were, however, obtained under unusual conditions in the field. Inoculations onto tarspots in the laboratory were unsuccessful, but in field experiments inoculations with conidia of M. maydis enhanced severity of the tarspot complex. Fisheye symptoms of the complex naturally appear 2 to 7 days after the manifestation of P. maydis. This is followed a week later by the appearance of M. maydis which became predominant in the lesions and is associated with empty perithecia of P. maydis. In the early stages of the tarspots pycnidia of the anamorph of P. maydis, Linochora sp., could occasionally be observed. Ascomata of M. maydis were rare in the field. Of the 36 genetic materials of CIMMYT tested, 30 developed the fisheye symptom, 4 tarspots only and 2 remained free of symptoms

Inheritance of Resistance to Tar Spot Complex in Maize

Tar spot complex (TSC) of maize can cause serious yield losses in Latin American countries. Phyllachora maydis is the most important pathogen associated with TSC. Little is known about the genetics of resistance to this disease. This study was carried out to elucidate the inheritance of resistance to P. maydis found in S 2 lines from a CIMMYT (International Maize and Wheat Improvement Center) maize population. Segregation and diallel analyses were performed in three environments (...)