Frogeye leaf spot, caused by Cercospora sojina Hara, is a foliar disease affecting soybean (Glycine max (L.) Merr.), often managed by applications of quinone outside inhibitor (QoI) fungicides. In 2013 and 2014, symptomatic leaf samples were collected from Mississippi soybean fields leading to the collection of 634 mono-conidial C. sojina isolates. In vitro bioassays were performed to evaluate the sensitivity of 14 isolates plus a baseline. Resistant and sensitive isolates were characterized by determining the effective fungicide concentrations at which 50% of conidial germination was inhibited (EC50). Additionally, the molecular mechanism of resistance was determined for all 634 isolates. Greater than 93% of C. sojina isolates collected in Mississippi carried the G143A amino acid substitution indicating a shift to a QoI-resistant population throughout Mississippi soybean fields. Greenhouse studies confirmed that due to this amino acid substitution, symptoms caused by QoI-resistant isolates developed in spite of a QoI fungicide application.

Soybean is an economically important crop. It is a selfertilized species grown on vast contiguous acres. These facts predispose soybean to disease epidemics. Cercospora sojina, causal agent of frogeye leaf spot, has reduced United States soybean productivity 0.3 percent on average per year between 2008 and 2010. Several states have reported the pathogen developing resistance to the strobilurin class of fungicides. To date genetic host resistance has been identified as single dominant genes (Rcs1, Rcs2, and Rcs3). However, the lifespans of Rcs1 and Rcs2 were 10 and 16 years respectively. Currently, the Rcs3 locus has been utilized in all major soybean breeding programs of the US and has been for over 20 years. Seventyive accessions of soybean were found to exhibit resistance to multiple races of C. sojina while not exhibiting the Rcs3 haplotype. Twenty of these plant introductions (PIs) were screened by six races within the new race classification system of C. sojina representing all domestic variability of the pathogen. Two agronomically favorable PIs, PI398993 and PI399068, were found in this research to exhibit broad resistance to sources documented to contain most domestic variability of the pathogen. Two segregating populations were developed by crossing PI398993 x ‘Blackhawk’ and PI399068 x Blackhawk. Segregation ratios of F2 as well as F2:3 family seedling screens of both populations indicating single dominant gene action in both resistance sources. Single marker analysis indicated markers associated with the phenotype were indeed on chromosome 16 (MLG J), but possibly beyond Rcs3 in both sources. Interval mapping placed the highest probability of the resistance loci near SNP_171 and SNP_368, 72.86 and 72.48 cM respectively, but distal to the Rcs3 locus. Analysis of reaction ratings also indicated significant influence on phenotype was also associated with markers located at or beyond the published Rcs3 locus. The evidence in this research supports the hypothesis that both PIs may contain a resistance loci, potentially different than Davis, but within the same gene cluster. Equally as likely, the resistance could prove allelic to Davis.

Cercospora sojina, the causal agent of frogeye leaf spot, is an important pathogen of soybean that reduces soybean yield by an average of 9.1 million bushels each year in the United States. Management strategies include using resistant cultivars and fungicide applications. Hill plots were used to evaluate over 600 commercial and public varieties for resistance or susceptibility to C. sojina. Each hill plot was spaced 0.6 m apart, planted with 10 seeds from a single cultivar, and was replicated three times. At flowering, the plants were inoculated by spraying a spore solution across all plants in the field. Hill plots were rated for disease severity at 14, 28 and 35 days after inoculation. There were 259 varieties that were susceptible to FLS, with 181 varieties with intermediate resistance, and 161 resistant varieties. A greenhouse trial evaluated fungicides and their ability to hinder symptom production and the duration of this suppression. Spores of the pathogen were applied at 7, 14 and 21 days after application of fungicides. Disease ratings were collected at 5 weekly intervals following inoculation. The significantly different treatment with the lowest disease ratings was the fungicide Stratego YLD (a strobilurin and triazole mix). The fungicide Tilt was found to be significantly different from all other treatments in the number of lesions on a single tracked leaf, including the non-treated control. These results can be used to recommend to producers available varieties.

Soybean (Glycine max [L.] Merr.) is one of the most important crops in the world. The average annual yield losses due to soybean diseases and pests are estimated to be around 11% in the United States. Soybean yield losses due to sudden death syndrome (SDS), caused by the fungus Fusarium virguliforme O'Donnell & T. Aoki have been problematic in majority of the soybean producing states. In recent years, reniform nematode (RN, Rotylenchulus reniformis Linford and Oliveira) and frogeye leaf spot (FLS), caused by the fungus Cercospora sojina K. Hara have emerged as a major problem in the southern soybean producing states. Planting resistant cultivars is one of the most cost-efficient methods in managing SDS, RN, and FLS, therefore it would be critical to identify and map the quantitative trait loci (QTL) that underlie their resistances. Two soybean populations were evaluated in this study. The ‘Essex’ × ‘Forrest’ 77 near-isogenic lines were screened in the field to evaluate the disease index of SDS. The Essex × Forrest and ‘Flyer’ × ‘Hartwig’ recombinant inbred lines were screened in the greenhouse to assess the reproduction index of RN and the disease severity of FLS. The BARCSoySNP6k chip was used to genotype the two populations. Four QTL that underlie SDS resistances were mapped in the same region as Rfv06-01, Rfv06-02, Rfv13-01, and Rfv19-01. The Rfv06-02 interval in this study was smaller than the one previously reported. Rrr08-01, Rrr13-01, Rrr15-01, Rrr18-01, and Rrr18-02 were reported to confer resistances to RN. Rrr08-01, Rrr13-01 and Rrr15-01 were novel whereas Rrr18-01, and Rrr18-02 were mapped in previous studies. cqSCN-001 (soybean cyst nematode, Heterodera glycines Ichinohe) was identified in the same region as Rrr18-01, and Rrr18-02 whereas cqSCN-006 was identified in the same region as Rrr15-01. These findings provide further evidence that there are common sources of genetic resistances to RN and SCN. Rcs15-01 and Rcs15-02 were reported to confer resistances to FLS. Rcs15-01 was novel and Rcs15-02 was mapped at the same region as an Rcs mapped in a previous study. This indicated that Rcs15-02 has dual resistances to C. sojina races. Candidate genes were inferred in this study. The QTL mapped in this study could potentially be used in soybean breeding programs that aim to introgress genetic resources that confer resistances to SDS, RN, and FLS.

Frogeye leaf spot is a foliar disease of soybean caused by Cercospora sojina Hara, which until recently had been successfully managed by quinone outside inhibitor (QoI) fungicides. After widespread resistance to the QoI fungicides was reported throughout Mississippi, the next step in characterizing C. sojina was to study the fitness of selected isolates. Fitness measurements of resistance stability, colony growth, conidia production and germination, and virulence were assessed. A phylogenetic analysis was also conducted to assess the genetic similarity of the QoI-resistant and -sensitive C. sojina isolates. All isolates remained stable in terms of QoI resistance. Results of all fitness measurements indicated no significant differences between the QoI-resistant and -sensitive C. sojina isolates. The phylogenetic analysis supported these results revealing similarity between QoI-resistant and -sensitive C. sojina isolates. Based on these results no indication of a fitness cost is associated with QoI resistance in C. sojina isolates from Mississippi soybean.

Frogeye leaf spot (FLS), caused by Cercospora sojina Hara, is a foliar disease of soybean (Glycine max (L.) Merr.). FLS has re-emerged as an important disease due to the widespread QoI-resistant fungal population in Mississippi. Growers have transitioned from a stand-alone QoI fungicide to a multi-mode of action fungicide. One disadvantage of certain MOA’s is phytotoxicity. Phytotoxicity is a concern among soybean growers as to whether or not yield is lost. Disease, phytotoxicity, green stem, yield, plant morphology, and grain quality were analyzed to determine the impacts of fungicide phytotoxicity. Yield was significantly and negatively correlated as a result of phytotoxicity. Numerical differences in phytotoxicity severity were observed among all cultivars. Phytotoxicity severity were greatest in the Hornbeck 4950 LL with an average of 26.9%. Tank-mixing dodine with Manniplex B Moly, Megafol, and Veritas decreased phytotoxicity by 8.5, 2.7, 11.4% compared to dodine as a stand-alone treatment, respectively.

Soybeans (Glycine max L.) are an important crop globally for its food, feed, and oilpurposes. It is impacted by many diseases, including Cercospora sojina, the causal agent of Frogeye Leaf Spot (FLS). Chemical and cultural controls to this fungal pathogen are insufficient, so genetic resistance must be acquired for adequate control. To this end, two recombinant inbred populations were screened in a greenhouse setting for their relative resistance to FLS, and their genomes were analyzed for contributing quantitative trait loci (QTL). In the Essex ́ Forrest population, one QTL was discovered on chromosome 13, and in the Forrest ́ Williams 82 population, two QTL were identified on chromosomes 6 and 11, respectively. These populations were then also screened in a field setting for agronomic traits. These traits were analyzed to detect one superior line for both FLS resistance and advanced agronomic traits, F ́W 125. This line should be used in future breeding projects to increase FLS resistance and reduce linkage drag for other desired characteristics.

<p>Frogeye leaf spot (FLS) is a foliar disease in soybean (<i>Glycine max</i> (L.) Merr.) caused by the fungal pathogen <i>Cercospora sojina</i> Hara. FLS is commonly found in hot and humid regions of the southern United States but has become more common in the North Central states. Foliar application of quinone outside inhibitor (QoI) fungicides has been one of the major tools used in the management of this disease, but QoI-resistant <i>C. sojina</i> isolates have been already confirmed in 21 states, including Indiana. We hypothesized that resistant populations of <i>C. sojina</i> to QoIs fungicides are widespread in Indiana and that sexual reproduction is occurring within <i>C. sojina</i> populations, likely contributing to the dissemination of fungicide resistance. The main objectives of this research were to determine the distribution of QoI-resistant <i>C. sojina</i> isolates on soybean from Indiana and to evaluate <i>C. sojina </i>populations for potential sexual reproduction. In the summer of 2019 and 2020, 406 isolates of <i>C. sojina </i>were collected from 32 counties across Indiana and screened for QoI-fungicide resistance using a PCR-RFLP method. An i<i>n vitro </i>fungicide sensitivity test was performed on a subset of isolates to evaluate the sensitivity of <i>C. sojina</i> isolates to azoxystrobin, pyraclostrobin, picoxystrobin, and prothioconazole. A discriminatory dose of picoxystrobin (QoI) and prothioconazole (demethylation inhibitor- DMI) were established at 1 μg/ml and 10 μg/ml, respectively, to distinguish between QoI-resistant and sensitive isolates and to identify a reduction in sensitivity to DMI fungicides, respectively. Discriminatory doses were estimated by testing five concentrations (0.001, 0.01, 0.1, 1, and 10 µg/ml) of each fungicide. QoI-resistant isolates were found in 29 out of the 32 counties. Two hundred and fifty-one (251) out of the 406 isolates (61.8%) were confirmed as QoI-resistant. Partial nucleotide sequences of the cyt <i>b</i> gene from four resistant and four sensitive <i>C. sojina</i> isolates corroborated the presence and absence of the G143A mutation, respectively. Results from the sensitivity assays with azoxystrobin and pyraclostrobin discriminatory doses supported the findings from the PCR-RFLP assay as all QoI-resistant mutants were inhibited less than 50% when exposed to these doses. Results from this study indicated that QoI-resistant <i>C. sojina</i> isolates are spread throughout Indiana and that prothioconazole (DMI) could be a potential supplemental or alternative fungicide to control FLS. Additionally, mating type distribution was determined in 43 <i>C. sojina</i> populations for assessment of potential sexual reproduction. Fifteen (15) populations did not deviate significantly from the expected 1:1 ratio, suggesting potential for cryptic sexual reproduction in these populations, but further research on genetic diversity is required to verify these results. </p> <p> </p>

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