First Report of Fusarium Oxysporum and F. longifundum Causing Dry Rot of Potato (Solanum tuberosum) in Lesotho

Dry rot is a postharvest disease of potatoes caused by various Fusarium species (Tiwari et al. 2020) and poses a major threat to potato production. During a disease survey conducted in Roma, Maseru district, Lesotho (29.4467°S, 27.7081°E) in 2023, potatoes with dry rot symptoms, characterised by sunken and wrinkled brown to black tissue patches on the tubers, were observed on 35% of the harvested tubers (Figure 1). To investigate the causal agent, fungi were isolated from the lesions by excising small sections from the margins of necrotic areas of the tuber surface. Tissue pieces were plated on potato dextrose agar and incubated at 25°C for 7 days. Five isolates (PDR1–PDR5) were obtained and characterised using morphological and molecular methods. Preliminary identification of the isolates was based on the general appearance of the colonies and morphology of conidia (Figure 2a,b). Mycelial growth pattern was smooth and flat in all isolates with white-purplish colonies for F. oxysporum (PDR1–PDR3) (Hafizi et al. 2014) and saffron to pale orange colonies for F. longifundum (PDR4 and PDR5) (Kim et al. 2024). Microscopic observations revealed oval to kidney-shaped macroconidia with 3 septa per conidium while microconidia were oval or elliptical with 0–2 septa for both Fusarium species (Figure 2c,d). DNA was extracted from the cultures using a Quick-DNA Fungal Microprep Kits (Zymo Research, USA) followed by PCR and sequencing of the ITS1-5.8S-ITS2 rDNA region using ITS1/ITS4 primers (White et al. 1990), and the partial translation elongation factor 1-alpha (telf1) and RNA polymerase II subunit B2 (rpb2) using EF1/EF2 and 5f2-7cr/7cf-11ar primers, respectively (O'Donnell et al. 2022). The resulting amplicons were sequenced and deposited in GenBank (Accession Nos: ITS [PV156104-PV156108], telf1 [PV640474-PV640478], and rpb2 [PV624693-PV624697]). The identity of the isolates was confirmed by BLASTn searches in the NCBI FUSARIUM-ID v.3.0 database and phylogenetic analyses. Isolates PDR1, PDR2 and PDR3 clustered in the same clade as F. oxysporum, while PDR4 and PDR5 clustered with F. longifundum (Figure 3). To test the pathogenicity of the isolates, disease-free mini tubers (cv. BP1) were inoculated with inoculum from each of the five isolates, prepared on autoclaved barley grains (10 g), and placed on top of seed tubers planted in 2-L pots filled with sandy loam soil. Control tubers received uninoculated barley grains. Plants were maintained at 22°C and 12 h day/light in a greenhouse for 90 days. Five plants were sampled for each isolate and assessed for dry rot symptoms on progeny tubers at harvest. Dry rot incidences on tubers from plants inoculated with the isolates ranged from 30% to 50%, with no symptoms observed on tubers of the control plants (Figure 4). Fusarium oxysporum and F. longifundum were re-isolated from symptomatic tubers and their identity was confirmed by morphological and molecular characterisation as previously described, fulfilling Koch's postulates. This is the first report of F. oxysporum and F. longifundum causing dry rot of potato in Lesotho. The authors would like to thank the National University of Lesotho for granting them the opportunity to undertake the study and the Government of Lesotho for student support, as well as Ms Marealeboha Mofube for her technical assistance in the Crop Science laboratory.