First report of Phyllachora maydis causing tar spot on corn in Delaware.

In October 2023, lesions consistent with descriptions of tar spot (Phyllachora maydis) were observed on corn (Zea mays) in Kent and Sussex County, Delaware (DE). Black, raised stromata were observed on leaves of commercially grown corn hybrids. Plants were at physiological maturity and disease severity was low with symptoms present on 1 to 10% of plants. In collected tissue, individual leaf severities ranged from 1 to 3% of leaf area with lesions. Hyaline conidia measuring approximately 15.5 µm in length and 0.5 µm in width were observed microscopically (n=5). Stromata were excised and sterilized in a 0.825% sodium hypochlorite solution for 30 s, rinsed in sterile deionized water for 30 s, and dried on a sterile paper towel for 30 s. Tissues were ground in a 1.5 mL microcentrifuge tube with a sterile plastic pestle. DNA was extracted using a DNeasy Plant Mini Kit (Qiagen). DNA was amplified at the internal transcribed spacer (ITS) region with ITS4 and ITS5 primers using polymerase chain reaction (PCR). NCBI BLAST search results yielded 100% sequence homology and 100% query cover (350/515 bp) to P. maydis accession MG881848.1 (Moura et al. 2023). Koch's postulates could not be completed due to the obligate nature of P. maydis. Tarspot was initially discovered in the United States in 2016 in Indiana and Illinois (Ruhl et al. 2016).This is the first report of tar spot on corn in DE. Yield losses from P. maydis can range depending on time of infection, environmental factors, and hybrid susceptibility and have been recorded up to 100% (Rocco da Silva et al. 2021). Because the disease did not enter the area until the end of the season, no yield impact was observed for 2023. Monitoring for the progression of disease will be crucial for future seasons (Telenko et al. 2020). High humidity and moisture levels favor disease development. Approximately half of DE corn acreage is irrigated due to sandy soils, current irrigation timing strategies may need to be reevaluated. Fungicide efficacy trials for management of tar spot have been conducted in other regions, but continued research will be needed to assess management options and optimize application timing for farmers in DE and the Mid-Atlantic region.

Effect of Soil Temperature on Reproduction of Root-knot Nematodes in Flue-cured Tobacco with Homozygous Rk1 and/or Rk2 Resistance Genes.

Most commercial flue-cured tobacco cultivars contain the Rk1 resistance gene, which provides resistance to races 1 and 3 of Meloidogyne incognita and race 1 of M. arenaria. A number of cultivars now possess a second root-knot resistance gene, Rk2. High soil temperatures have been associated with a breakdown of root-knot resistance genes in a number of crops. Three greenhouse trials were performed from 2014 to 2015 investigate the effect of high soil temperature on the efficacy of Rk1 and/or Rk2 genes in reducing parasitism by a population of M. incognita race 3. Trials were arranged in randomized complete block design in open-top growth chambers set at 25°, 30°, and 35°C. Plants were inoculated with 3,000 eggs and data were collected 35 days post-inoculation. Galling, numbers of egg masses and eggs, and reproductive index were compared across cultivar entries. Nematode reproduction was reduced at 25°C and 30°C on entries possessing Rk1 and Rk1Rk2 compared to the susceptible entry and the entry possessing only Rk2. However, there were often no significant differences in reproduction at 35°C between entries with Rk1 and/or Rk2 compared to the susceptible control, indicating an increase of root-knot nematode parasitism on resistant entries at higher temperatures. Although seasonal differences in nematode reproduction were observed among experiments, relative differences among tobacco genotypes remained generally consistent.

Strain-Level Identification of Bacterial Tomato Pathogens Directly from Metagenomic Sequences.

Routine strain-level identification of plant pathogens directly from symptomatic tissue could significantly improve plant disease control and prevention. Here we tested the Oxford Nanopore Technologies (ONT) MinION sequencer for metagenomic sequencing of tomato plants either artificially inoculated with a known strain of the bacterial speck pathogen Pseudomonas syringae pv. tomato or collected in the field and showing bacterial spot symptoms caused by one of four Xanthomonas species. After species-level identification via ONT's WIMP software and the third-party tools Sourmash and MetaMaps, we used Sourmash and MetaMaps with a custom database of representative genomes of bacterial tomato pathogens to attempt strain-level identification. In parallel, each metagenome was assembled and the longest contigs were used as query with the genome-based microbial identification Web service LINbase. Both the read-based and assembly-based approaches correctly identified P. syringae pv. tomato strain T1 in the artificially inoculated samples. The pathogen strain in most field samples was identified as a member of Xanthomonas perforans group 2. This result was confirmed by whole genome sequencing of colonies isolated from one of the samples. Although in our case metagenome-based pathogen identification at the strain level was achieved, caution still must be exercised in interpreting strain-level results because of the challenges inherent to assigning reads to specific strains and the error rate of nanopore sequencing.

Reproduction of Meloidogyne incognita Race 3 on Flue-cured Tobacco Homozygous for Rk1 and/or Rk2 Resistance Genes.

Most commercial tobacco cultivars possess the Rk1 resistance gene to races 1 and 3 of Meloidogyne incognita and race 1 of Meloidogyne arenaria, which has caused a shift in population prevalence in Virginia tobacco fields toward other species and races. A number of cultivars now also possess the Rk2 gene for root-knot resistance. Experiments were conducted in 2013 to 2014 to examine whether possessing both Rk1 and Rk2 increases resistance to a variant of M. incognita race 3 compared to either gene alone. Greenhouse trials were arranged in a completely randomized design with Coker 371-Gold (C371G; susceptible), NC 95 and SC 72 (Rk1Rk1), T-15-1-1 (Rk2Rk2), and STNCB-2-28 and NOD 8 (Rk1Rk1 and Rk2Rk2). Each plant was inoculated with 5,000 root-knot nematode eggs; data were collected 60 d postinoculation. Percent galling and numbers of egg masses and eggs were counted, the latter being used to calculate the reproductive index on each host. Despite variability, entries with both Rk1 and Rk2 conferred greater resistance to a variant of M. incognita race 3 than plants with Rk1 or Rk2 alone. Entries with Rk1 alone were successful in reducing root galling and nematode reproduction compared to the susceptible control. Entry T-15-1-1 did not reduce galling compared to the susceptible control but often suppressed reproduction.