The Origin of The Problem: Characterization of Paraguayan Septoria steviae, Causal Agent of Septoria Leaf Spot of Stevia Based on Multi-Locus Sequence Analysis.

Septoria leaf spot is a significant disease affecting cultivated stevia, potentially reducing yields by > 50%. The disease is caused by Septoria steviae, first identified in 1978 in Japan as a new pathogen of stevia. Understanding the origin of S. steviae could clarify how it spread to new production areas. To investigate this, twelve isolates of Septoria sp. were obtained from stevia's native range in the Amambay forests and field plantings in Paraguay from 2018 to 2020. These isolates underwent colony morphology and molecular characterization of Actin, ß-Tubulin, Calmodulin, ITS, LSU, RPB2, and TEF1α loci. GenBank sequences from S. steviae isolates collected in France, Japan, and the United States (USA) were included. Multi-locus sequence phylogenetic analysis generated a maximum likelihood (ML) tree. The morphological characteristics of Paraguayan isolates were similar to previously reported S. steviae type cultures from Japan. The ML analysis showed that Paraguayan isolates formed a monophyletic group with S. steviae isolates from France, Japan, and the USA. During blotter tests, pycnidia and cirri of S. steviae were observed on multiple stevia seed surfaces from different sources. Further characterization confirmed viable pathogenic conidia of S. steviae. This observation suggests that S. steviae could be associated with stevia seed, possibly spreading from the center of origin to other countries. This research is the first to genetically characterize S. steviae from Paraguay and propose its potential spread mechanism from the center of origin to the rest of the world.

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 (ρc), bias (Cb), precision (r), and intracluster correlation (ρ) coefficients were significantly closer to "true" severity values when the SADs was used by inexperienced (ρc = 0.89; Cb = 0.97; r = 0.90, ρ = 0.81) and experienced (ρc = 0.94; Cb = 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.

First Report of Macrophomina euphorbiicola Causing Charcoal Rot of Stevia in Paraguay.

Stevia (Stevia rebaudiana [Bertoni] Bertoni) is a perennial plant originating in Paraguay. Stevia is primarily cultivated for the production of non-caloric sweeteners. In December 2018, wilted stevia cv. 'PC4' were recovered from two separate fields of 0.3 ha (24.66 S 56.46 W) and 0.5 ha (24.69 S 56.44 W), both with 3 years history of stevia production in San Estanislao County, San Pedro, Paraguay. The wilted plants were randomly distributed in beds covered with plastic mulch and a 30% disease incidence was recorded. Dark brown septate hyphae and microsclerotia were observed on stem bases and black necrotic roots of the wilted plants. Root and crown regions were washed, cut into 0.5 to 1.0 cm pieces, and then surface-disinfested with 0.6% NaOCl before placing them in Petri dishes containing acidified potato-dextrose-agar. Plates were incubated for one week at 25 ± 5°C under fluorescent light with a 12 h photoperiod yielding five isolates SP1PY, SP2PY, SP3PY, SP4PY and SP5PY with gray-black colonies without conidia but showing numerous microsclerotia. Twenty microsclerotia from pure cultures of five isolates were measured, with mean width 38.8 ± 4.7 µm and length 68.8 ± 15.5 µm. Fungal DNA was extracted from mycelia of five isolates for PCR amplification of the internal transcribed spacer (ITS) and translation elongation factor 1-alpha (TEF1-α) using ITS4/ITS5 and EF1-728F/EF-2 primers (Machado et al. 2019). The resultant amplicons were sequenced at Eton Bioscience (Research Triangle Park, NC) and deposited in the NCBI GenBank database (ITS: MT645815, OM956150, OM956151, OM956152, OM956153; and TEF1-α: MT659121, OM959505, OM959506, OM959507, OM959508). Sequences were aligned with several isolates of Macrophomina spp. previously reported (Huda-Shakirah et al. 2019; Machado et al. 2019; Santos et al. 2020; Poudel et al. 2021) using ClustalW. Alignments (ITS and TEF-1α) were concatenated to generate a maximum likelihood tree using MEGA7. The novel isolates grouped into the M. euphorbiicola clade with 95% of bootstrap support. Stevia plants cv. 'Katupyry' were grown in 10 cm-diameter nursery bags containing autoclaved sandy soil and kept under greenhouse conditions (28 ± 5°C; 16 h photoperiod). Fifteen plants per isolate (n=75) were inoculated by adding 20 g of rice infested with M. euphorbiicola to each plant. Infested grains were distributed around the crown of the plant at a depth of 0.5 cm; non-infested rice was added to four control plants. Lower-stem lesions and microsclerotia of M. euphorbiicola developed on all inoculated plants. No lesions or microsclerotia were observed on control plants. The M. euphoribiicola fungus was re-isolated from inoculated stevia plants but not from the non-infested rice treated plants. Koch's postulates were repeated twice with similar results. Previously, M. phaseolina was reported causing charcoal rot on stevia in Egypt (Hilal and Baiuomy 2000), and in North Carolina, USA (Koehler and Shew 2017). However, Paraguayan isolates grouped with isolates of M. euphorbiicola based on the combined sequences of the ITS and TEF-1α regions. Machado et al. (2019) reported M. euphorbiicola causing charcoal rot on castor bean (Ricinus communis) and bellyache bush (Jatropha gossypifolia) in Brazil, which borders northeast Paraguay, a major stevia production area. This pathogen has a significant impact on stevia production during hot, dry weather by reducing the number of harvestable plants and increasing replanting costs in perennial production systems.

Anaerobic Soil Disinfestation Reduces Viability of Sclerotinia sclerotiorum and S. minor Sclerotia and Root-Knot Nematodes in Muck Soils.

Experiments were conducted to evaluate potential functional and mechanistic differences in the suppression of Sclerotinia sclerotiorum and S. minor and root-knot nematodes in muck soils by anaerobic soil disinfestation (ASD) using different carbon source amendments. Volatile compounds produced during ASD in muck soil amended with molasses, wheat bran, or mustard greens at 20.2 Mg/ha or a 2% ethanol solution significantly reduced the mycelial growth and number of sclerotia produced by both Sclerotinia spp. compared with the anaerobic control. In amended soils, acetic and butyric acids were detected in concentrations that reduced the viability of sclerotia of both pathogens. Higher concentrations of carbon dioxide were observed in ASD-treated soils, regardless of the amendment, than in the nonamended anaerobic control. Only amendment with wheat bran did not increase the production of methane gas during ASD compared with the controls. Meloidogyne hapla survival was completely suppressed in soils treated with ASD regardless of carbon source. Field trials were conducted in Ohio muck soil to assess survival of sclerotia of both Sclerotinia spp. The viability of sclerotia of both Sclerotinia spp. was significantly reduced in soil subjected to ASD amended with wheat bran (20.2 Mg/ha), molasses (10.1 Mg/ha), or wheat bran (20.2 Mg/ha) plus molasses (10.1 Mg/ha) compared with the controls. A consistent negative correlation between soil reduction and viability of sclerotia of both pathogens was observed. Wheat bran and molasses are both widely available amendments that can be used as ASD carbon sources for the management of soilborne pathogens in muck soils.