First report of the identification of Diaporthe ueckerae from Soybean Seeds with Seed Decay in Argentina

During the 2023-2024 cropping season, soybean [Glycine max (L.) Merrill] plants from a breeding population (210 F2:3 lines) planted in the Villarino experimental field from the National University of Rosario, Zavalla, Santa Fe (33°01'S, 60°52'W) in the main core soybean producing zone in Argentina, exhibited stem canker (10% incidence) during the season and seed decay (25% of seed samples at harvesting). Stem canker was attributed mainly to Diaporthe caulivora. Samples of soybean seed showing symptoms of Diaporthe seed decay, typically moldy and chalky, were collected in the. Seeds from multiple lines were harvested and processed to isolate Diaporthe spp. (George et al. 2025). A total of 28 isolates were obtained mainly from the symptomatic but also from asymptomatic seeds. These isolates were cultured on potato dextrose agar (PDA), revealing morphological characteristics similar to those of Diaporthe spp. The E.Z.N.A. Plant & Fungal DNA Kit was used to extract fungal DNA from pure isolates according to the manufacturer’s instructions. The translation elongation factor 1-alpha gene (EF-1: EF1-728F and EF1-986R), B-tubulin (Btub, Bt2a and Bt2b) and Calmodulin (cal: CAL-228F and CAL737R) genes were amplified and sequenced following previous studies (Carbone and Kohn 1999; George et al. 2025; Glass and Donaldson 1995). Sequences were identified using Blastn, concatenated and aligned using MAFFT, followed by a maximum likelihood tree using IQ-Tree (Minh et al. 2020). The majority of the isolates were identified as Diaporthe longicolla, but one isolate was identified as Diaporthe ueckerae (ZAV24-164-1) . Isolate D. longicolla ZAV24-124-2 originated from a heavily symptomatic seed and ZAV24-164-1 was isolated from a symptomatic seed with less severe symptoms. All sequences were deposited in GenBank: (PZ418046-PZ418107). Diaporthe ueckerae exhibited aerial mycelia with a radiate growth pattern on PDA and displayed pigmentation with cream to pale brown at the center on the reverse side of the plates. Both isolates produced black and globose pycnidia. Alpha conidia were observed for both species. Pathogenicity tests were performed twice by inoculating a mycelial agar plug (5 mm diameter) of D. ueckerae ZAV24-164-1 and D. longicolla ZAV24-124-2 onto the wounded stem of ten susceptible soybean plants from cv. Williams82. Dark brown necrotic lesions were detected on soybean stems from the susceptible genotype at 11 days post-inoculation with D. ueckerae ZAV24-164-1. Similar necrotic lesions were observed for D. longicolla ZAV24-124-2. No symptoms developed on control stems, which were mock inoculated with a sterile agar plug. Successful re-isolations were achieved from all the inoculated stems. Confirmation of D. ueckerae was established by EF-1 sequencing. Argentina is one of the leading soybean producers worldwide, ranking third after the USA and Brazil, with an average of 48 million metric tons (Mt) in the last growing season (USDA, Foreign Agricultural Service 2026). The increasing prevalence of seed decay and stem canker in South America, exacerbated by recent regional epidemics, underscores the urgent need for integrated management strategies to minimize seed damage and mitigate significant yield losses.