Damping‐off of soybean is a common problem in some fields in southern Brazil. Frequently, Phytophthora sojae has been pointed out as the main causal agent. However, questions were raised as to whether other oomycetes could also be present. Hence, the objectives of this study were to isolate, identify, and determine the pathogenicity and aggressiveness of oomycetes isolated from soybean seedlings. Thirty‐three isolates were obtained from 13 fields. Isolates were identified based on morphology and molecular methods. Nine species were recovered: Pythium conidiophorum (n = 14, 42.4%), Globisporangium rostratifingens (n = 7, 21.2%), G. irregulare (n = 2, 6.1%), G. spinosum (n = 1, 3.0%), G. ultimum var. sporangiiferum (n = 2, 6.1%), Pythium acanthicum (n = 2, 6.1%), P. deliense (n = 1, 3.0%), P. inflatum (n = 1, 3.0%), and P. torulosum (n = 3, 9.1%). Pathogenicity and aggressiveness were determined with two soybean cultivars (DM57i52 and NEO530) in a seed rot assay and a root rot assay. All isolates were pathogenic to soybean with variable levels of aggressiveness in both assays. The range of seed disease severity index varied from 7.5 to 100 for DM57i52 and from 23.1 to 100 for NEO530. The mean root disease severity index of the soybean cultivars ranged from 29.4 to 92.8. This is the first official report of G. irregulare, G. rostratifingens, G. spinosum, G. ultimum var. sporangiiferum, P. acanthicum, P. conidiophorum, P. deliense, P. inflatum, and P. torulosum causing damping‐off of soybean in the state of Rio Grande do Sul in southern Brazil.

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Rhizoctonia root rot (RRR) is a disease caused by the soilborne pathogen Rhizoctonia solani, which kills soybean plants in patches over croplands. Control is difficult because the pathogen survives in the soil and has a wide host range; therefore, cultural control methods might help to diminish disease pressure. The objective of this study was to evaluate liming as a strategy to reduce the incidence of RRR in soybean grown in four soil types: acrisol (Ac), cambisol Ta (CaTa), cambisol Tb (CaTb) and nitisol (Nit).The treatments were arranged in a factorial design of four doses of limestone (0, 3, 6 and 9 t/ha) at two sites within each field (inside and outside a Rhizoctonia patch).The incidence of RRR inside the patch was on average 40.3% whereas outside it was 1.2%. The reduction in disease incidence was 3.9%, 3.7%, 3.6% and 1.8% per 1000 kg of limestone applied for the Ac, CaTa, CaTb and Nit soils, respectively. The highest soybean yield was of 4713 and 3349 kg/ha for the Ac and Nit soils with application of 6.6 and 6.2 t/ha of limestone, respectively. Soybean yield in the cambisols increased linearly with liming, with an increase in yield of 121 and 172 kg/ha in CaTa and CaTb, respectively, for each 1000 kg of limestone applied. Thus, liming reduced the incidence of RRR in all soils and promoted better soybean nutrition, increasing soybean yield. Therefore, liming is a practice that contributes to both soil fertility and disease control.

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Brazil is the largest soybean producer in the world (USDA, 2022). The area cultivated with soybean in the country is divided into macroregions due to the differences in environmental conditions (Kaster & Farias, 2012), which require cultivars adapted to each macroregion.Damping-off of soybean is a common problem reported by several growers in Macroregion 1 (Costamilan et al., 2013;Molin et al., 2021a), which includes the southern region of Brazil (Kaster & Farias, 2012). Symptoms of the disease include seed decay and root rot (Broders et al., 2007;Rojas et al., 2017), which can compromise seed germination, seedling emergence and initial establishment of