First report of Zymoseptoria tritici causing Septoria tritici blotch in wheat in Paraguay

Wheat (Triticum aestivum) is the third most cultivated field crop in Paraguay; it is grown on over 450,000 hectares with an annual production of 927,776 tons (fao.org/faostat). In 1952, Septoria tritici blotch (STB) was associated with the fungus Septoria tritici solely based on microscopic observation of conidia (Viedma and Delgado 1987). However, no morphometric or molecular studies have been performed in Paraguay up to date. Over the following decades, STB epidemic outbreaks were recorded, with a reduction in wheat production of up to 70% (Viedma and Delgado 1987). During winter 2021, leaf blotch symptoms were observed with an incidence above 50% in wheat fields in Capitán Miranda, Itapúa, Paraguay. Scattered, spherical, buried, and light brown necrotic spots with dark edges were observed on the leaves. Pycnidia with prominent central ostiole were observed. Leaves with symptoms were washed with 1% sodium hypochlorite for 1 min, rinsed with sterile distilled water, and incubated in wet chambers to induce sporulation of the fungus. Pycnidia produced greyish to white cirri. Isolated conidia were thin, elongated, and hyaline, ranging from 26.9-72.7 × 1.5-2.9 μm with one to three septa. Monosporic colonies on potato dextrose agar (PDA, ; Difco laboratories, Detroit, MI) media varied in color from white to pink, dark gray to black, or black with stroma-like structures. Based on morphology, the fungus was characterized as Zymoseptoria tritici (Hoorne et al. 2002; Gilchrist-Saavedra et al. 2005). Fungal DNA was extracted from mycelia, and the internal transcribed spacer (ITS), translation elongation factor 1-α (TEF1-α), 28S rRNA gene (LSU), actin gene (act), calmodulin (CaM) were amplified using ITS1/ITS4, EF1-728F/EF-2, LSU1Fd/ LR5, ACT-512F/ACT-783R, CAL-228F/CAL737R primers, respectively. PCR amplicons were sequenced at Macrogen (Seoul, Republic of Korea) and deposited in the NCBI GenBank database (ITS: OQ360718; TEF1-α: OQ999044, LSU: OQ996413, act: OQ999046, CaM: OQ999045). Sequences were aligned with several isolates of Septoria spp. previously reported (Verkley et al. 2013; Stukenbrock et al. 2012) using ClustalW. The alignments were concatenated with Bioedit (Hall 1999). The UPGMA method with 1,000 bootstrap replications, was used to construct the phylogenetic tree using MEGA11 with Readeriella mirabilis as the outgroup. The isolate from Paraguay grouped into the Zymoseptoria tritici clade with 96% bootstrap support. To confirm pathogenicity, ten wheat plants cv. Itapúa 80 were grown in pots for three weeks in growth chambers (22 ± 2°C; 16 h photoperiod). Subsequently, these plants were inoculated with 1×107 conidia ml-1 suspension, and ten non-inoculated plants served as control. Seven days after inoculation (DAI), symptoms were observed displaying oval necrotic lesions and approximately 14 DAI abundant pycnidia were observed on and around the lesions. Segments of symptomatic leaves were placed in moisture chambers overnight to enhance cirri development. Conidia were mounted on a slide and observed under the compound microscope. Individual cirrhus were transferred to plates containing PDA and produced colonies like those used in the inoculation (Hoorne et al. 2002). We confirmed that the causal agent of STB from wheat fields in Paraguay was Zymoseptoria tritici. This pathogen causes annual wheat disease epidemics in Paraguay; therefore, optimizing surveillance for early detection and understanding its distribution will improve integrated management.

Validation of Standard Area Diagrams to Estimate the Severity of Septoria Leaf Spot on Stevia in Paraguay, Mexico, and the United States

Septoria leaf spot (SLS) affects stevia leaves, reducing their quality. Estimates of SLS severity on different genotypes are made to identify resistance and as a basis to compare management approaches. The use of standard area diagrams (SADs) can improve the accuracy and reliability of severity estimates. In this study, we developed new SADs with six illustrations (0.5, 1, 10, 25, 40, and 75% severity). The SADs were validated by raters with and without experience in estimating SLS. Raters evaluated 40 leaf photos with SLS severities ranging from 0 to 100% without and with the SADs. Agreement (ρ_c), bias (C_b), precision (r), and intracluster correlation (ρ) coefficients were significantly closer to "true" severity values when the SADs was used by inexperienced (ρ_c = 0.89; C_b = 0.97; r = 0.90, ρ = 0.81) and experienced (ρ_c = 0.94; C_b = 0.99; r = 0.95, ρ = 0.91) raters. The SADs were tested under field conditions in Paraguay, Mexico, and the United States, with inexperienced raters assigned to two groups, one SADs trained and the other not trained, that estimated SLS severity three times: first, all raters without SADs and no time limit for the estimates; second, only the SADs-trained group used SADs and no time limit; and third, only the SADs-trained group used SADs, with a time limit of 10 s imposed per specimen assessment. Agreement and reliability of SLS severity estimates significantly improved when raters used the SADs without a time limit. The use of the new SADs improved the accuracy, precision, and reliability of SLS severity estimates, enhancing the uniformity in assessment across different stevia programs.