First Report of Alternaria alternata Causing Leaf Spot on Broccoli in Ecuador

Abstract

In Ecuador, broccoli (Brassica oleracea var. italica) production is located in the Andean region, specifically Cotopaxi-Ecuador (INEC, 2019). A leaf pathogen has been constantly observed in this area, showing brown circular necrosis surrounded by yellowish halo-like spots causing leaf death (Fig. 1a). This pathogen was believed to be Alternaria sp.; however, the species was not determined either using classical or molecular tools. In 2021, ten leaves showing similar symptoms were collected in Cotopaxi and sent for pathogen identification. Here, leaf explants (0.25 cm2) showing disease symptoms were surface sterilized with 2% sodium hypochlorite (NaClO) and 70% ethanol (C2H6O), rinsed with sterile water, and transferred to Potato Dextrose Agar (PDA) media. Petri dishes were incubated in darkness at 25°C for five days. The single hyphal tip method was used to purify the cultures on PDA. Fifteen pure isolates were obtained after incubating for 14 days. Isolates were incubated under blacklight for two days to induce fungal sporulation. All isolates presented early white cotton-like mycelium that later became dark green (Fig 1b). Under the microscope, we observed straight primary conidia in simple or branched chains. Also, the conidia were obclavate, long ellipsoids, moderate in size (19.5-43.9 μm in length, 7.1-17.2 μm in width), and septate with few longitudinal septa. Lastly, the conidium body can narrow itself into a secondary conidia (Fig 1c) (Woudenberg et al., 2013). According to colony and conidia morphology, isolates were identified as Alternaria sp. (Woudenberg et al., 2013). Five isolates were randomly selected for DNA extraction and sequencing of ITS (internal transcribed spacer; Chou, H.H. and Wu, W.S. 2002), TEF (translation elongation factor; O'Donnell et al., 1998), and RPB2 (RNA polymerase II second largest subunit; Liu et al., 1999) gene regions. DNA sequences obtained from each marker were identical for all isolates. Consensus sequences and alignment were built using ClustalX in MEGA X (Kumar et al., 2018). Consensus sequences were deposited in GenBank with the following accession numbers: ITS, ON982232; TEF, ON983964; RPB2, ON983963. A multilocus Bayesian inference phylogenetic tree was constructed in Beast software (version 1.8.4) using the concatenated sequences (Drummond et al., 2012; Maharachchikumbura et al., 2014). The isolates in our study clustered with isolates of Alternaria alternata, confirming their identity (Figure 2). For Koch's postulates, healthy broccoli plants were grown in sterile soil for six weeks. The fungal conidia were suspended in sterile distilled water (1×106 conidia/ml), and the leaves were inoculated by spraying the spore solution. The control treatment was sprayed with sterile distilled water alone. Plants were maintained at 28°C and had more than 85% relative humidity (Sigillo et al., 2020). Seven days after inoculation, plants showed chlorosis and necrosis. Ten days later, 100% of the treated leaves presented brown circular necrosis (Fig. 1d). Control plants showed no disease symptoms. Re-isolation of the pathogen from the diseased leaf tissue was performed as previously described. The isolates presented the exact morphology of pure cultures obtained from field-diseased leaves. The pathogenicity test was performed twice. To our knowledge, this is the first report on A. alternata being the causal agent of leaf spot on broccoli in Ecuador. Disease diagnosis contributes to providing strategies against this pathogen. Further investigations are needed to find biological/chemical techniques or cultivar resistance to control this pathogen in broccoli.

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