Fusarium fabacearum causing dwarfism and chlorosis of lima bean plants in Brazil

Lima bean (Phaseolus lunatus L.) is among the main crops grown in the Northeast of Brazil. In October 2023, we observed various lima bean plants (variety Orelha de Vó, 2022-2023 crop) infested with Fusarium at CCA/UFPB laboratory and greenhouse (Areia - PB/Brazil, 6°58'12″ S; 35°42'15″ W). Seeds were collected from field in the municipality of Remígio, Paraíba, Brazil (6° 57' 53.2" S; 35° 47' 49.2" W). Approximately 200 plants were inspected during this survey. Seedlings from these seeds were also infected, with 19% of them showing dwarfism and chlorosis, which are symptoms associated with Fusarium infections. Fusarium strains were isolated from symptomatic plants and then grown in pure culture on potato-dextrose-agar (PDA) and synthetic-nutrient-poor-agar (SNA) media. Monosporic cultures were obtained by transferring a single germinated conidium to fresh PDA plates to ensure culture purification. Colonies on PDA were floccous and dense varying from white to pale pink. Colonies on SNA were hyaline, with macroconidia measuring 20.8-13.9 × 5.7-4.1 μm, slightly falcate with three to four septa. Oval microconidia were abundant and formed on monophyalides, measuring 8.1-4.1 × 3.6-1.9 μm. Chlamydospores absent. Morphological features were consistent with the F. oxysporum species complex (Leslie & Summerell, 2006). To ensure the identification, a total of 2 isolates were obtained, and the isolate F25 was chosen as a representative for further analysis. The elongation factor 1α region (EF1-α) was amplified and sequenced using primers EF1 and EF2 (O'Donnell et al., 1998) and the RNA polymerase II second largest subunit (RPB2) using primers RPB2-5F2 (Reeb et al., 2004) and RPB2-7cR (Liu et al., 1999). Sequences generated in this study were deposited in GenBank (PQ045254 and PQ063816 for EF1-α and RPB2, respectively), and compared to other Fusarium species found at Fusarium-ID and GenBank databases. A phylogenetic analysis was inferred using a maximum likelihood and Bayesian inference, including previously published sequences (Nicolli et al., 2020). The EF1-α and RPB2 sequences shared 99.84% and 100% sequence identity with F. fabacearum CPC25802 (GenBank: MH485030.1 and MH484939.1, respectively). For the pathogenicity test, the isolate was cultured on PDA at 25 ± 2 °C for seven days. A F. fabacearum 5.0 × 10⁵ conidia/mL conidial suspension was prepared and used to immerse 100 lima bean seeds (variety Orelha de Vó, 2022-2023 crop) for five minutes. Inoculated seeds were then sown on sterile substrate in a greenhouse. Negative control was represented by seeds immersed in sterilized water. Typical dwarfism and chlorosis symptoms were observed on 29% of the emerged plants when compared to negative controls (healthy plants). The pathogen was reisolated from the symptomatic plants and phenotypically compared with the isolate inoculated, fulfilling Koch's postulates. This test was performed twice. This species was previously isolated from Glycine max L. and recently described (Lombard et al., 2019). Infection of lima beans may cause plant vigor reduction, thus resulting in yield and economic losses for agricultural producers in Brazil (Farias et al., 2022). The findings underscore the need for disease monitoring programs and the development of integrated management strategies, while reinforcing the critical importance of using high-quality, pathogen-free seeds to maintain crop health and prevent potential yield losses. Accurate pathogen identification is essential for the implementation of effective control measures. To the best of our knowledge, this is the first report of F. fabacearum causing dwarfism and chlorosis in lima beans in Brazil.