Phyllachora Maydis Research Articles

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.

First Report of Tar Spot on Corn Caused by Phyllachora maydis in Georgia, United States.

Tar spot is a major foliage disease of corn (Zea mays) caused by the fungus Phyllachora maydis. This disease can reduce the quality of silage, stover, husks, and grain (Valle-Torells et al. 2020). It was identified in the United States for the first time in 2015 in northern Illinois and Indiana (Ruhl et al. 2016). As of 2020, it has also been confirmed in Iowa, Michigan, Wisconsin, Ohio, Florida, Missouri, Minnesota and Pennsylvania (Telenko et al. 2020, Collins et al. 2021). In early August 2021, tar spot symptoms and signs were observed in late-planted, hybrid corn in southwestern Georgia in both Tift and Calhoun Counties. Where it occurred, incidence was nearly 100% and severity ranged from approximately 1% to 20% leaf coverage. As of November 2021, Tar spot has been confirmed in 13 counties throughout South Georgia. The symptoms on leaves were chlorotic lesions and the signs were black, raised, circular to irregular-shaped structures (stromata or clypei) ranging from 0.2 mm to 8 mm on the surface of the leaves. The stromata were present on both green and necrotic leaf tissue. Southern corn rust (Puccinia polysora) was often, but not always observed on leaves with tar spot. Microscopic observations of stromata included single and clustered ascomata (60 × magnification) that contained cylindrical, unitunicate asci with ellipsoid, hyaline, aseptate ascospores and abundant filiform paraphyses (400 × magnification). For molecular confirmation, stromata were surface sterilized with 95% ethanol and separated from the leaf tissue, then DNA was extracted using the Qiagen DNeasy Plant Mini-Kit (Qiagen, Inc., Valencia, CA). PCR was conducted with the universal ITS1 and ITS4 primers to amplify the internal transcribed spacer (ITS) region of the rDNA. The amplicons were Sanger sequenced (Genewiz, Inc., South Plainfield, NJ) and a consensus sequence of a representative lesion was deposited in GenBank (accession number OK649959). Using the BLASTN algorithm our sequence shared 100% and 99.2% coverage, and 97.2% and 98.4% similarity to P. maydis GenBank accessions MG881848 and MK184990, respectively. Phyllachora maydis is an obligate parasite that cannot be cultured, so Koch's postulates were not performed. Inoculum of P. maydis may have been transported to Georgia from Florida by wind or other weather events, but this has not been confirmed.

Identification of resistance for Phyllachora maydis of maize in exotic‐derived germplasm

AbstractTar spot of maize (Zea mays L.), caused by the obligate biotroph Phyllachora maydis Maubl., is an emerging disease in the United States and Canada, and the identification of sources of resistance for tar spot will enable the development of resistant hybrids. In 2019 and 2020, 25 accessions from the germplasm enhancement of maize (GEM) project containing exotic introgressions in elite backgrounds were evaluated in nine environments for tar spot severity. Environmental conditions had a major influence on disease development, as tar spot severity varied across locations with only four of the nine locations showing moderate to high levels of disease. In five environments, disease levels were low and disease severity data was not collected or used. Accessions were visually evaluated for tar spot during reproductive growth stages in three environments and during vegetative growth stages in one environment. There was a strong correlation between resistance to P. maydis across locations where accessions were evaluated in reproductive growth stages. Two accessions, GEMS‐0066 and GEMS‐0226, were the most resistant and could prove useful for tar spot resistance breeding. These accessions are publicly available and able to be directly used in breeding programs.

Phyllachora maydis (black spot of maize)

Phyllachora maydis (black spot of maize)

Fungicide Efficacy on Tar Spot and Yield of Corn in the Midwestern United States

Tar spot of corn caused by Phyllachora maydis has recently led to significant yield losses in the eastern corn belt of the Midwestern United States. Foliar fungicides containing quinone outside inhibitors (QoI), demethylation inhibitors (DMI), and succinate dehydrogenase inhibitors (SDHI) are commonly used to manage foliar diseases in corn. To mitigate the losses from tar spot, 13 foliar fungicides containing single or multiple modes of action (MOA/Fungicide Resistance Action Committee groups) were applied at their recommended rates in a single application at the standard VT/R1 (tassel/silk) growth stage timing to evaluate their efficacy against tar spot in a total of eight field trials in Illinois, Indiana, Michigan, and Wisconsin during 2019 and 2020. The single MOA fungicides included either a QoI or DMI. The dual MOA fungicides included a DMI with either a QoI or SDHI, and fungicides containing three MOAs included a QoI, DMI, and SDHI. Tar spot severity estimated as the percentage of leaf area covered by P. maydis stroma on the ear leaf of the nontreated control at dent growth stage ranged from 1.6 to 23.3% in the trials. Averaged across eight field trials all foliar fungicide treatments significantly reduced tar spot severity, but only Delaro 325SE (prothioconazole + trifloxystrobin), Revytek 3.33LC (mefentrifluconazole + pyraclostrobin + fluxapyroxad), and Veltyma 3.34S (mefentrifluconazole + pyraclostrobin) significantly increased yield over the nontreated control. When comparing groups of fungicide treatments by the number of MOAs, foliar fungicide products that had two or three MOAs decreased tar spot severity over not treating and products with one MOA. In addition, the fungicide group that contained all three MOAs significantly increased yield over not treating with a fungicide or using a single MOA group.

Fungicide Efficacy During a Severe Epidemic of Tar Spot on Corn in the United States and Canada in 2021

A 2021 epidemic of tar spot of corn caused by Phyllachora maydis led to significant yield losses in the midwestern United States and Ontario, Canada. Uniform fungicides trials consisting of nine foliar fungicides applied at the tassel (VT) or silk (R1) growth stage were evaluated for tar spot management in five field trials in the midwestern United States and Ontario, Canada, in 2021. All nine foliar fungicide treatments significantly reduced tar spot severity, but only Delaro Complete (prothioconazole + fluopyram + trifloxystrobin), Revytek (mefentrifluconazole + pyraclostrobin + fluxapyroxad), and Veltyma (mefentrifluconazole + pyraclostrobin) protected yield compared with the nontreated control.

Contour-Based Detection and Quantification of Tar Spot Stromata Using Red-Green-Blue (RGB) Imagery.

Quantifying symptoms of tar spot of corn has been conducted through visual-based estimations of the proportion of leaf area covered by the pathogenic structures generated by Phyllachora maydis (stromata). However, this traditional approach is costly in terms of time and labor, as well as prone to human subjectivity. An objective and accurate method, which is also time and labor-efficient, is of an urgent need for tar spot surveillance and high-throughput disease phenotyping. Here, we present the use of contour-based detection of fungal stromata to quantify disease intensity using Red-Green-Blue (RGB) images of tar spot-infected corn leaves. Image blocks (n = 1,130) generated by uniform partitioning the RGB images of leaves, were analyzed for their number of stromata by two independent, experienced human raters using ImageJ (visual estimates) and the experimental stromata contour detection algorithm (SCDA; digital measurements). Stromata count for each image block was then categorized into five classes and tested for the agreement of human raters and SCDA using Cohen's weighted kappa coefficient (κ). Adequate agreements of stromata counts were observed for each of the human raters to SCDA (κ = 0.83) and between the two human raters (κ = 0.95). Moreover, the SCDA was able to recognize “true stromata,” but to a lesser extent than human raters (average median recall = 90.5%, precision = 89.7%, and Dice = 88.3%). Furthermore, we tracked tar spot development throughout six time points using SCDA and we obtained high agreement between area under the disease progress curve (AUDPC) shared by visual disease severity and SCDA. Our results indicate the potential utility of SCDA in quantifying stromata using RGB images, complementing the traditional human, visual-based disease severity estimations, and serve as a foundation in building an accurate, high-throughput pipeline for the scoring of tar spot symptoms.

First Report of Tar Spot of Maize (Zea mays) Caused by Phyllachora maydis in Pennsylvania

First Report of Tar Spot of Maize (<i>Zea mays</i>) Caused by <i>Phyllachora maydis</i> in Pennsylvania

Recovery Plan for Tar Spot of Corn, Caused by Phyllachora maydis

Tar spot is a foliar disease of corn threatening production across the Americas. The disease was first documented in Mexico in 1904 and is now present in 15 additional countries throughout Central America, South America, and the Caribbean. Researchers and growers in Central America, South America, and the Caribbean consider tar spot to be a disease complex caused by multiple fungal pathogens. When environmental conditions are conducive for infection, these regions have experienced yield losses that can reach up to 100%. In 2015, tar spot was detected in the United States for the first time in Illinois and Indiana. Since that time tar spot has spread across the U.S. corn-growing region, and the disease has been found in Florida, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, Pennsylvania, and Wisconsin. In 2020, tar spot was also found in southwest Ontario, Canada. Losses in the United States due to tar spot totaled an estimated 241 million bushels from 2018 to 2020. With the potential to continue to spread across the U.S. corn-growing states, much greater losses could result when environmental conditions are conducive.

An Observation of Corn Tar Spot Dispersal from Agricultural Fields to an Isolated Urban Plot

Tar spot of corn is a fairly new disease to the United States, and since its introduction in 2015 it has spread to over 310 counties across nine states. Little is understood of the biology and epidemiology of the causative agent of tar spot of corn, Phyllachora maydis. Published research from Latin America indicates that spores can only travel a distance of 75 m; however, rapid progression and spread in the United States, in addition to widespread observations of top-down infection, indicate distal dispersal is likely more important. We observed an isolated plot of decorative corn in an urban setting, isolated from agricultural fields. We assessed nearby fields for crop, tar spot, cropping history, and distance from the urban site. Based on our observations, the propagules infecting the urban plot arrived from at least 560 m if from corn residue, and from at least 1,249 m if from active infections from nearby corn. Although this observation is not replicated, it does indicate that dispersal values from Latin America may not reflect the potential dispersion in Midwest topographies.

Tar Spot: An Understudied Disease Threatening Corn Production in the Americas.

Tar spot of corn has been a major foliar disease in several Latin American countries since 1904. In 2015, tar spot was first documented in the United States and has led to significant yield losses of approximately 4.5 million t. Tar spot is caused by an obligate pathogen, Phyllachora maydis, and thus requires a living host to grow and reproduce. Due to its obligate nature, biological and epidemiological studies are limited and impact of disease in corn production has been understudied. Here we present the current literature and gaps in knowledge of tar spot of corn in the Americas, its etiology, distribution, impact and known management strategies as a resource for understanding the pathosystem. This will in tern guide current and future research and aid in the development of effective management strategies for this disease.

Draft Genome Sequence Resource for Phyllachora maydis-An Obligate Pathogen That Causes Tar Spot of Corn with Recent Economic Impacts in the United States.

Phyllachora maydis is an important fungal pathogen that causes tar spot of corn and has led to significant yield loss in the United States and other countries. P. maydis is an obligate biotroph belonging to the Sordariomycetes class of Ascomycota. Due to the challenges posed by their obligate nature, there is no genome sequence available in the Phyllachora genus. P. maydis isolate PM01 was collected from a corn field in Indiana and the genome was determined by next-generation sequencing. The assembly size is 45.7 Mb, with 56.46% repetitive sequences. There are 5,992 protein-coding genes and 59 are predicted as effector proteins. This genome resource will increase our understanding of genomic features of P. maydis and will assist in studying the corn-P. maydis interaction and identifying potential resistant candidates for corn breeding programs.

First Report of Tar Spot of Corn Caused by Phyllachora maydis in Minnesota

HomePlant DiseaseVol. 104, No. 6First Report of Tar Spot of Corn Caused by Phyllachora maydis in Minnesota PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Tar Spot of Corn Caused by Phyllachora maydis in MinnesotaD. K. Malvick, D. E. Plewa, D. Lara, N. M. Kleczewski, C. M. Floyd, and B. E. ArenzD. K. Malvick†Corresponding author: D. K. Malvick; E-mail Address: [email protected]http://orcid.org/0000-0002-0131-8411Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108Search for more papers by this author, D. E. PlewaDepartment of Crop Sciences, University of Illinois, Urbana, IL 61801Search for more papers by this author, D. LaraDepartment of Crop Sciences, University of Illinois, Urbana, IL 61801Search for more papers by this author, N. M. Kleczewskihttp://orcid.org/0000-0001-5671-6727Department of Crop Sciences, University of Illinois, Urbana, IL 61801Search for more papers by this author, C. M. FloydDepartment of Plant Pathology, University of Minnesota, St. Paul, MN 55108Search for more papers by this author, and B. E. ArenzDepartment of Plant Pathology, University of Minnesota, St. Paul, MN 55108Search for more papers by this authorAffiliationsAuthors and Affiliations D. K. Malvick1 † D. E. Plewa2 D. Lara2 N. M. Kleczewski2 C. M. Floyd1 B. E. Arenz1 1Department of Plant Pathology, University of Minnesota, St. Paul, MN 55108 2Department of Crop Sciences, University of Illinois, Urbana, IL 61801 Published Online:7 Apr 2020https://doi.org/10.1094/PDIS-10-19-2167-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Tar spot, caused by Phyllachora maydis Maubl., can be a significant disease of corn (Zea mays L.). Commercial corn hybrids with typical signs and symptoms of tar spot were observed and collected in fields from Fillmore and Winona Counties, Minnesota, in late September and early October 2019. The disease in Fillmore County appeared to be focused in one area of the field about 10 m in diameter with incidence estimated at 90% and severity on individual leaves at and above the ear ranging from 1 to 34%. The field in Winona County had leaves with a similar level of severity on field edges. Signs and symptoms on the epidermis of leaves in both fields consisted of small black, raised, circular stroma (0.5 to 4 mm), primarily on the upper surfaces of leaves. All lesions were assessed, and less than 3% of tar spots were associated with fisheye lesions or chlorotic halos. Microscopic examination of the stroma from both locations revealed asci and oval to ovoid, hyaline ascospores of characteristic size and appearance for P. maydis as described by Parbery (1967). Specifically, 40 randomly selected stroma were dissected from the Fillmore County sample, and ascospores within had an average length of 9.8 μm and width of 6.6 μm. Identity of the stroma from samples in both counties was further confirmed by DNA sequencing. Genomic DNA was extracted from stroma, and a portion of the RNA gene (491 bases) was amplified by PCR using ITS4/ITS5 or ITS1F/ITS4 primers (White et al. 1990). The sequence similarity using BLASTN for all samples was 96 to 100% for four accessions of P. maydis (NCBI GenBank accessions MG881846.1, MG881847.1, KU184459.1, and MK184990.1). Due to the biotrophic, obligate nature of P. maydis, Koch’s postulates were not conducted (Parbery 1967). This confirms for the first time that tar spot of corn caused by P. maydis occurs in Minnesota. These results document an increased distribution of tar spot in the United States since it was first found in 2015 (Ruhl et al. 2016), which includes at least two counties in Minnesota, where it could cause significant economic losses for corn producers.The author(s) declare no conflict of interest.

Phyllachora maydis Ascospore Release and Germination from Overwintered Corn Residue

Tar spot of corn, caused by Phyllachora maydis, has been reported in several upper Midwest states in the United States. This has led to expanded efforts to more thoroughly understand the biology of P. maydis and the epidemiology of tar spot. This study determined the potential for P. maydis ascospore release and germination from overwintered P. maydis-infected corn residues from various locations in the upper Midwest. Corn residue samples collected in the spring of 2019 from 12 fields in four states were examined. Ascospore release and germination were observed in all residue samples collected. The mean total number of ascospores released per milliliter of water ranged from 3.6 × 103 to 4.8 × 106 after 4-h incubation and 3.7 × 103 to 4.4 × 106 after 24-h incubation. The mean percent spore germination ranged from 0.7 to 24.1% after 4-h incubation and 2.4 to 24.9% after 24-h incubation. There was a significant inverse relationship between total numbers of ascospores released and the percent germination of the ascospores. Samples from Illinois consistently yielded the greatest total ascospore release but were also consistently among the samples with the lowest percent ascospore germination. Samples from Wisconsin and Indiana were among the lowest for total ascospore numbers but were among the highest for total ascospore germination. These findings provide evidence that P. maydis can overwinter in multiple areas of the upper Midwest. Future research should focus on reducing infested corn residue to reduce initial infection by P. maydis.

Documenting the Establishment, Spread, and Severity of Phyllachora maydis on Corn, in the United States

AbstractTar spot on corn, caused by the fungus (Phyllachora maydis Maubl. [Phyllachorales: Phyllachoraceae]), is an emerging disease in the United States. In 2018 and 2019, significant but localized epidemics of tar spot occurred across the major corn producing region of the Midwest. After being first detected in 2015, tar spot was detected in 135 and 139 counties where the disease was not previously detected in 2018 and 2019, respectively, and is now established across 310 counties across the United Sates. Foliage with signs (stromata) of P. maydis and symptoms of tar spot were collected from 128 fields in 2018 and 191 fields in 2019, across seven states. Samples were assessed for severity of fungal stromata (percent leaf area covered with stromata) on foliage and the incidence of fisheye lesions (proportion of lesions with fisheye symptoms) associated with fungal stromata. Stromatal severity on samples in 2018 ranged from 0.5 to 67% and incidence of fisheye lesions ranged from 0 to 12%, whereas in 2019, stromatal severity ranged from 0.1 to 35% and incidence of fisheye lesions ranged from 0 to 80%, with 95% of samples presenting less than 6% incidence of fisheye lesions. Tar spot has spread substantially from where it was first reported in the United States. Collaborative efforts to monitor the spread and educate clientele on management are essential as this disease spreads into new areas.

Corn Yield Loss Estimates Due to Diseases in the United States and Ontario, Canada, from 2016 to 2019

Annual reductions in corn (Zea mays L.) yield caused by diseases were estimated by university Extension-affiliated plant pathologists in 26 corn-producing states in the United States and in Ontario, Canada, from 2016 through 2019. Estimated loss from each disease varied greatly by state or province and year. Gray leaf spot (caused by Cercospora zeae-maydis Tehon &amp; E.Y. Daniels) caused the greatest estimated yield loss in parts of the northern United States and Ontario in all years except 2019, and Fusarium stalk rot (caused by Fusarium spp.) also greatly reduced yield. Tar spot (caused by Phyllachora maydis Maubl.), a relatively new disease in the United States, was estimated to cause substantial yield loss in 2018 and 2019 in several northern states. Gray leaf spot and southern rust (caused by Puccinia polysora Underw.) caused the most estimated yield losses in the southern United States. Unfavorable wet and delayed harvest conditions in 2018 resulted in an estimated 2.5 billion bushels (63.5 million metric tons) of grain contaminated with mycotoxins. The estimated mean economic loss due to reduced yield caused by corn diseases in the United States and Ontario from 2016 to 2019 was US$55.90 per acre (US$138.13 per hectare). Results from this survey provide scientists, corn breeders, government agencies, and educators with data to help inform and prioritize research, policy, and educational efforts in corn pathology and disease management.

First Report of Tar Spot of Maize (Zea mays), Caused by Phyllachora maydis, in Ohio

HomePlant DiseaseVol. 103, No. 7First Report of Tar Spot of Maize (Zea mays), Caused by Phyllachora maydis, in Ohio PreviousNext DISEASE NOTESFirst Report of Tar Spot of Maize (Zea mays), Caused by Phyllachora maydis, in OhioF. Dalla Lana, D. E. Plewa, E. S. Phillippi, D. Garzonio, R. Hesterman, N. M. Kleczewski, and P. A. PaulF. Dalla Lana†Corresponding author: F. Dalla Lana; E-mail Address: felipedallalana@gmail.comhttp://orcid.org/0000-0002-3112-6197Department of Plant Pathology, The Ohio State University, Wooster, OH, 44691Search for more papers by this author, D. E. PlewaDepartment of Crop Science, University of Illinois, Urbana, IL, 61801Search for more papers by this author, E. S. PhillippiDepartment of Crop Science, University of Illinois, Urbana, IL, 61801Search for more papers by this author, D. GarzonioGROWMARK, Inc., Bloomington, IL, 61701Search for more papers by this author, R. HestermanGROWMARK, Inc., Bloomington, IL, 61701Search for more papers by this author, N. M. Kleczewskihttp://orcid.org/0000-0001-5671-6727Department of Crop Science, University of Illinois, Urbana, IL, 61801Search for more papers by this author, and P. A. Paulhttp://orcid.org/0000-0002-7912-3067Department of Plant Pathology, The Ohio State University, Wooster, OH, 44691Search for more papers by this authorAffiliationsAuthors and Affiliations F. Dalla Lana1 † D. E. Plewa2 E. S. Phillippi2 D. Garzonio3 R. Hesterman3 N. M. Kleczewski2 P. A. Paul1 1Department of Plant Pathology, The Ohio State University, Wooster, OH, 44691 2Department of Crop Science, University of Illinois, Urbana, IL, 61801 3GROWMARK, Inc., Bloomington, IL, 61701 Published Online:14 May 2019https://doi.org/10.1094/PDIS-01-19-0070-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat In October 2018, maize (Zea mays L.) leaf samples with symptoms commonly associated with tar spot were received from Hancock, Wyandot, Fulton, Clark, Van Wert, and Crawford Counties in Ohio. Disease severity on individual leaves ranged from <1 to 10%, depending on the hybrid and location. Lesions were black, raised, and rounded, and ranged in size from 0.9 to 3.93 mm by 0.72 to 2.0 mm (n = 40, average 2.28 by 1.38 mm). A few of the lesions (<1%) presented a tan halo surrounding the black spot, resembling the fisheye lesions often considered typical of tar spot. Microscopic observations of leaf pieces sectioned through lesions showed ascomata occurring singly or in groups within a clypeus. Ascomata contained cylindrical asci with hyaline, aseptate, ellipsoid ascospores, ranging in size from 5.0 to 7.5 by 7.5 to 13.75 µm (n = 40, average 5.9 by 10.9 µm), and filiform paraphyses. Based on symptoms and fungal morphology, the disease was identified as tar spot and the pathogen as Phyllachora maydis Maubl. For molecular confirmation, DNA was extracted from ascomata on leaf samples from Hancock County, and amplicons of the internal transcribed spacer regions were generated using primers ITS5 and ITS4 (White et al. 1990). The results confirmed the fungus as P. maydis, showing 97% sequence homology to GenBank deposit accession MG881848.1 (McCoy et al. 2018), with 100% query cover, max score of 830, and E value of 0.0. Because P. maydis is an obligate biotroph (Müller and Samuels 1984), and as such nonculturable (Ruhl et al. 2016), Koch’s postulates were not attempted. The sequence from Hancock County was deposited in GenBank (accession no. MK184990). Although previously reported in the United States in Indiana, Illinois, Florida, Iowa, Michigan, and Wisconsin (McCoy et al. 2018; Ruhl et al. 2016), tar spot is still an emerging disease in the United States for which the economic impact is unknown. However, there have been reports of significant yield loss in Illinois, Indiana, Michigan, and Wisconsin in 2018 (N. M. Kleczewski, personal communication). It has been suggested that significant losses occur in Central and South America when plants are coinfected with P. maydis and Monographella maydis (Mottaleb et al. 2018). The latter species has not been reported in the United States, but evidence from preliminary studies indicates that P. maydis alone may be capable of impacting grain yield (N. M. Kleczewski, personal communication).The author(s) declare no conflict of interest.

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.