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.

Stevia is a semi-perennial crop grown to obtain the diterpene glycosides in its leaves, which are processed to manufacture non-caloric sweeteners. Anaerobic soil disinfestation (ASD) and fungicide application were evaluated for the management of stevia stem rot (SSR) and Septoria leaf spot (SLS) in lab and field experiments. In 2019 and 2021, experiments using carbon sources for ASD were carried out in microplots at NCSU (Clayton, NC, USA). In 2020/21 and 2021/22 seasons, field experiments were conducted at CSAEGRO, Mexico (MX) and CEDIT, Paraguay (PY) using a 2 × 3 factorial design with two ASD treatments and three fungicide treatments. ASD treatments included soil amended with cornmeal (MX) or wheat bran (PY) at a rate of 20.2 Mg ha⁻¹, molasses at 10.1 Mg ha⁻¹, and non-amended controls. Fungicide applications included chemical (azoxystrobin), organic (pyroligneous acid, PA), and a non-treated control. ASD was effective in reducing sclerotia viability of Sclerotium rolfsii in laboratory assays (p < 0.0001) and microplot trials (p < 0.0001) in NC. During field trials, the viability of sclerotia was significantly reduced (p < 0.0001) in soils amended with cornmeal + molasses or wheat bran + molasses as carbon sources for ASD. While there was no significant effectiveness of ASD in reducing SLS in 2020 and 2021 or SSR in MX 2020 field trials (p = 0.83), it did exhibit efficacy on SSR in 2021 (p < 0.001). The application of fungicides was significantly effective in reducing SSR (p = 0.01) and SLS (p = 0.001), with azoxystrobin being the most consistent and PA not being statistically different from the control or azoxystrobin. The effects of ASD on fresh yield were inconsistent, exhibiting significant effects in Mexican fields in 2020 but not in 2021. During Paraguayan field trials, ASD only significantly interacted with fungicide applications in the dry yield in 2022. In the 2020/21 MX and 2020 PY field trials, fungicides were significantly effective in enhancing dry leaf yields, with azoxystrobin showing the highest consistency among treatments and PA variable control. In conclusion, utilizing ASD alongside organic fungicides can be a valuable tool for stevia farmers when the use of chemical fungicides is limited. Further research is required to enhance consistency and reduce the costs associated with these treatments under diverse edaphoclimatic conditions.

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.

La agricultura es un pilar importante para la economía del Paraguay y de muchos países en la región. Uno de los mayores desafíos con el que la agricultura se enfrenta actualmente es generar seguridad alimentaria, para un mundo con una población que no cesa de crecer. Una limitante en la producción de alimentos, muchas veces pasada por alto, es el daño al cultivo ocasionado por patógenos de plantas. Entre estos patógenos, los nematodos fitoparásitos son importantes organismos que causan daños significativos a nuestros cultivos agrícolas, amenazando la productividad y por consiguiente impactando negativamente la economía del productor. El presente manual de nematología agrícola es el resultado del trabajo en conjunto de un excelente equipo de investigadores. Decidimos crear algo que nos ayude y apoye a otros investigadores a capacitar más profesionales en el área de la nematología. Como equipo, pusimos mucha energía e ilusión en la producción de este manual porque estamos comprometidos con la formación de futuros nematólogos y la protección de nuestra producción agrícola. Con el presente manual ofrecemos las herramientas necesarias que permitirán abrir las puertas a un mundo de maravillas poco explorado aún en Paraguay, la nematología agrícola.

Wheat yellow (stripe) rust caused by Puccinia striiformis Westend. f. sp. tritici Eriks. (Pst) is an important disease worldwide (Chen 2005; Afzal et al., 2007; Hovmøller et al. 2011). In Latin Ame...