Watermelon (Citrullus lanatus) is cultivated in Chile across ca. 2,900 hectares, particularly concentrated in O'Higgins (49.1%), Maule (36.6%), and Metropolitan (9.2%) regions (ODEPA, 2025). Production currently yields 40-50 tons per hectare, mainly for local consumption. During the 2023 growing season, symptoms of leaf yellowing and wilting, stunting and plant death were observed on watermelon cv Santa Amelia in an experimental plantation located in Rengo, O'Higgins Region (34°23'34.8"S, 70°52'40.8"W). Uprooting affected plants revealed necrotic roots and vascular browning. The mortality of three-month-old watermelon plants was approximately 2%. To identify the causal agent, root and stem fragments were surface-disinfected with sodium hypochlorite (2% v/v, 1 min), rinsed with sterile water, and then dried. The samples were cultured on Potato Dextrose Agar (PDA, Difco) at 25ºC (±1ºC) in darkness for 7 days. A total of three isolates (Fc-R.1-3) were obtained. The colonies exhibited cottony white aerial mycelium that turned salmon-coloured over time, with septate hyphae producing terminal or intercalary chlamydospores. Macroconidia (n = 50) were hyaline, curved with 1-3 septa, measuring 22.8-34.0 µm long (avg. 27.8 µm) and 3.7-7.0 µm wide (avg. 5.4 µm). Microconidia (n = 50) were hyaline, fusoid, aseptate, 5.7-12.1 µm long (avg. 7.9 µm) and 2.5-6.7 µm wide (avg. 3.9 µm). These characteristics were consistent with Fusarium species. DNA was extracted from the isolated representative “Fc-R.1” using the DNeasy Plant Mini Kit (i.e. Qiagen), following the manufacturer’s protocol. PCR amplifications and sequencing of calmodulin (cmdA; CAL-228F/CAL-2Rd) (Carbone EF1/EF2) (O’Donnell et al., 1998) were performed. BLASTn analysis exhibited 99.8% (GenBank no. GQ505509) and 100% (GenBank no. GQ505597) identity with Fusarium clavum based on the partial cmdA and tef-1α sequences, respectively. Furthermore, maximum-likelihood analysis of a concatenated dataset (cmdA and tef-1α), conducted in Geneious Prime v2025.2 with GTR model and 1,000 bootstrap replicates, confirmed the identity of our isolate as Fusarium clavum (Xia et al., 2019). The sequences were deposited in GenBank under the accession number PX396090 (cmdA) and PX396089 (tef-1α), respectively. Pathogenicity test was conducted in triplicated on two-month-old watermelon plants (cv Santa Amelia). Plants were maintained in a vermiculite/perlite (2:1) substrate in 5 L containers under semi-controlled conditions (10/14 h light/dark, 10-20ºC, 40-70% RH). A wound was made in the plant crown using a sterile scalpel. A 0.25 cm2 mycelial plug (four-day-old), grown on PDA, was then inserted into the lesion and covered with wet cotton and Parafilm. After 30-days, watermelon plants exhibited chlorosis, stem necrosis, and wilting. F. clavum was successfully re-isolated from symptomatic tissues and its identity confirmed through morphological techniques, thereby fulfilling Koch’s postulates. The negative control, inoculated solely with PDA, remained symptomless throughout the test. In reference to this first report of Fusarium clavum causing wilt symptoms on watermelon in Chile, a comprehensive research into novel integrated control strategies is imperative to mitigate potential agricultural losses.
Fuentealba et al. (2026) studied this question.