Tar spot is a major foliar disease of corn caused by the obligate fungal pathogen Phyllachora maydis, first identified in Indiana in 2015. Under conducive weather conditions, P. maydis causes significant yield losses in the U.S. and other countries constituting a major threat to corn production. Relatively little is known about resistance to tar spot other than a major quantitative gene that was identified in tropical maize lines. To test for additional sources of resistance against populations of P. maydis in North America, twenty-six parental inbred lines of the Nested Associated Mapping (NAM) population were evaluated for tar spot resistance in Indiana in replicated field trials under natural infection for 3 years. Tar spot disease severity was scored visually using a 0 to 100 % scale. Maximum disease severities (MDS) for tar spot scoring at reproductive growth stage ranged from 0 to 48.3 % with 0% being most resistant and 48.3% being most susceptible. Nine inbred lines were resistant to P. maydis with MDS ranging from 0 to 5.0%, six were moderately resistant (5.2 to 10.6% MDS), two were moderately susceptible (11.7 to 26.0% MDS) and the remaining eight inbred lines were rated as susceptible (30.0 to 48.3% MDS). There was some variability between years, due to higher disease pressure after 2019. Inbred B73, the common parent of the NAM populations, was rated as susceptible with MDS of 30.0%. The nine highly resistant lines provide a potential source of new genes for genetic analysis and mapping of tar spot resistance in corn.

Tar spot, caused by Phyllachora maydis, is the most significant disease of corn in the north central United States. Elucidating an integrated management approach to protect grain yield is a high priority. The integration of tillage, hybrid, and fungicide application on tar spot severity, canopy greenness, and grain yield was assessed in Indiana in 2019, 2020, and 2021. A split-plot arrangement of two tillage treatments as main plots with factorial arrangement of subplots with three hybrids (tar spot susceptible and two partially resistant) and fungicide application (propiconazole +benzovindiflupyr +azoxystrobin) were evaluated. Further, a multistate study comparing two hybrids (susceptible and partially resistant) and fungicide application (propiconazole +benzovindiflupyr +azoxystrobin) was conducted in Indiana, Michigan, and Wisconsin in 2020. This research demonstrates that partially resistant hybrids with a sAUDPC of 2.5 and 3.0 had significantly less tar spot than a susceptible hybrid with a sAUDPC of 10.1 and increased canopy greenness rating of 48.1 and 51.5% which were significantly higher than the susceptible hybrid at 13.0% at maturity. Tar spot severity was further reduced and canopy greenness increased with a fungicide application in the susceptible hybrid. These results suggest that partial resistance alone may be used to manage tar spot. Yields did not differ significantly across hybrids with or without a fungicide application. However, a general trend of preserved yield potential was observed in the treatments with fungicide, indicating that knowledge of genetics and yield potential will be necessary to achieve the most benefit from partially resistant hybrids and judicious fungicide applications.

Soybean production in the Upper Midwest region of the United States is consistently limited by the disease Sclerotinia stem rot (SSR). To control SSR, multiple management practices have been studied and implemented to reduce SSR development and preserve yield. This study examined the effects of integrating soybean seeding rates and pesticide programs under nitrogen fertilizer applications in the form of urea (46-0-0) that may occur due to management for other crops, such as corn. From ten site-years between 2020 and 2021, low seeding rates decreased SSR development while also decreasing yields and partial profits. The effect of pesticide applications on SSR development was influenced by both seeding rates and nitrogen applications. Consistently, applications of the fungicide, Endura, reduced SSR to the lowest levels, while also maintaining the highest yields and partial profits. Soybeans grown with nitrogen applications experienced increased SSR development, and decreased yields and partial profits. Overall, this work suggests that using low seeding rates and fungicides improves the management of SSR, and using nitrogen fertilizer applications can result in greater SSR development and decreased economic returns, especially in fields with a history of SSR.

In September 2021, signs of black circular to oval shaped fungal structures (stromata) were observed on corn (Zea mays L.) leaves on a non-commercial inbred line in Todd County, Kentucky. Signs were only observed in a small pocket within the larger field, with disease levels ranging from 1- 5% incidence and 1-25% severity on individual leaves affected in the field. Corn leaves had senesced and only fungal structures were available to aid in diagnosis. Microscopic examination of stromata uncovered ascomata within the clypei/stromata. Further examination of ascomata revealed multiple asci containing eight hyaline, uniseriate, aseptate, oval to ovoid ascospores ranging in size from 8 to 12 µm x 5 to 7 µm. Observed signs were consistent with published reports of tar spot caused by Phyllachora maydis (Parbery 1967; Valle-Torres et al. 2020). For molecular confirmation of the causal agent, corn leaves were surface sterilized in diluted bleach (10%) for 30 seconds and stromata were excised from the leaves using a sterile scalpel. Five to seven stromata were placed into each microcentrifuge tube. Liquid nitrogen was added to the microcentrifuge tubes and the frozen stromata were ground using a sterilized pestle. The ground stromata tissue was used for DNA extraction using a Synergy 2.0 plant DNA extraction kit (OPS Diagnostics, Lebanon, NJ). A portion of the internal transcribed spacer (ITS) region was amplified by PCR utilizing ITS-4 and ITS-5 primers. Amplicons were subjected to Sanger sequencing to obtain a consensus sequence. Using the BLASTn algorithm the consensus sequence shared 100% similarity to three P. maydis Genbank accessions: MG881848.1, MG8814847.1, MG881846.1. A representative sequence was deposited in GenBank (accession no. OQ034699.1). Due to P. maydis being an obligate parasite, Koch’s postulates were not attempted.