RESUMO
Sweet corn (Zea mays L.) is one of the most popular crops grown in Jordan. Fusarium verticillioides (Sacc.) is a major pathogen of corn and a producer of mycotoxin fumonisins (Blacutt et al. 2018). During September and October 2019, ear rot symptoms were observed on Ì´30% of the sweet corn variety Mis Dolce grown in the Jordan Valley. The disease caused substantial losses, including damage to greater than 50% of the kernels within 15-20 days after harvesting. A total of 350-corn kernels were randomly taken from 70 plants distributed in five fields with a total area of 2 ha. About 35% of the samples showed typical symptoms of the disease. Discolored corn kernels were surface sterilized with 5% NaOCl solution for 1 min, then rinsed three times with sterilized distilled water (SDW), plated on potato dextrose agar (PDA) at 25°C, and incubated in the dark for 7 days. Twelve putative isolates of the genus Fusarium were hyphal-tipped on new PDA plates. Isolates were cultured on synthetic low-nutrient agar (SNA) with a ca. 1 × 2-cm strip of sterile filter paper on the agar surface (Nirenberg 1976). Cultures were incubated for 10 to 14 days at 20°C in dark conditions. When sporulation was observed, agar blocks were mounted on a microscopic slide with a drop of lactophenol cotton blue and examined under the microscope at 400x. Colonies grew rapidly with abundant pink to violet aerial hyphae. Sporodochia formed on the agar, and the aerial conidiophores branched sparsely, often alternately or oppositely, terminating with up to three verticillate phialides. Macroconidia were abundant, falcate to straight, three- to five-septate, with a distinct foot cell, 27 to 73 × 3.1 to 5.6 µm. Microconidia produced on polyphialides and aggregating in heads, were unicellular, ovoidal, or ellipsoidal, 4.4 to 17 × 1.5 to 4.5 µm (Fig. 1A, B, C, D, E, and F). Based on morphological characteristics, isolates were tentatively identified as F. verticillioides (Al-Hatmi et al. 2016; Guarro 2013). Two representative isolates were DNA extracted and the translation elongation factor 1-α gene (TEF1) was amplified (O'Donnell et al. 1998), and sequenced from both directions at Macrogen Inc, South Korea. The consensus sequences of the two isolates Fvcorn2021JO-03 (OK040159) and Fvcorn2021JO-04 (OK040160) were used as BLASTn query on the NCBI website and were 100% and 99% similar with F. verticilloides JF740717 and JF740737 accessions, respectively. Similarly, the two isolates were 100% and 99.85% similar with F. verticilloides reference sequences MH582332 and MH582327 on the Fusarium MLST database, respectively. The pathogenicity of the two isolates was tested on 15 cobs by injecting 2 ml of a 2.5 × 105 conidia/ml suspension into the silk channel and into kernel wounds of the primary ear (three replicates) for each treatment (Reid and Hamilton 1996). Inoculated kernels were incubated at 25°C for 2 weeks in plastic boxes. The healthy kernels were injected with 2 ml of SDW as a negative control. Grains started to rot after 2 weeks, in the form of a thick, cottony, crimson-looking growth between the ear and its covers, with only some grains or a group of a adjacent grains rotting, and then white lines appear on the outer skin of the grain, yielding symptoms similar to those in the field (Fig. 2A, B, C, and D). The fungus was re-isolated from the inoculated kernels and was morphologically identified as F. verticilloides thus fulfilling Koch's postulates. The fumonisins-producing potential of the isolated F. verticillioides was confirmed using the AgroQuant Total Fumonisins Assay (Romer Labs, Singapore). To our knowledge, this is the first report of F. verticillioides causing Fusarium ear rot on corn in Jordan. Further investigation is needed to gain a better understanding of the spatio-temporal dynamics of this novel pathogen.
