First Report of Leaf Blight of Coleus scutellarioides Caused by Alternaria alternata in China

Coleus scutellarioides (L.) Benth, also called coleus, is a well-known ornamental plant that has colorful foliage in different cultivars (Zhu et al. 2015). It is used as a garden plant worldwide and as a medicinal plant in some countries, including India, Indonesia, and Mexico (Natasa et al. 2002). In August 2018, leaf blight was observed on coleus (cv. Wizard Golden) at two commercial flower nurseries located about 5 km apart in Harbin, Heilongjiang Province, China. No serious occurrence of the disease was found in other varieties of coleus in this area. Disease symptoms were observed on approximately 40% of the plants, and 30% leaves were infected. The symptoms began with small brown spots on the leaves or a dust-color lesion under the leaf tip, which expanded and became V-shaped blight on the leaf tip or irregular brown necrotic spots on the leaves. Affected leaf tissues (5 × 5 mm) were collected and surface sterilized with 75% ethanol for 5 s and 0.1% HgCl₂ for 2 min and rinsed three times with sterile water. The tissues were then incubated on potato dextrose agar (PDA) at 26°C in darkness. After 3 days, fungal cultures were isolated from diseased tissues, and single-spore isolates were generated from the cultures. Six single-spore isolates were grown on PDA and incubated at 26°C for 5 days, and morphological characteristics were observed. Colonies with white-to-dark gray mycelia were formed on PDA with black pigmentation. Conidia were pale brown to light brown, straight or flexuous, obclavate to obpyriform or ellipsoid, with smooth or verruculose surface and longitudinal and transverse septa tapering to a prominent beak attached in chains on a simple and short conidiophore. The spores were 4.5 to 11.5 × 9.3 to 29.1 μm in size with zero to five transepta and one to five longisepta. According to these morphological characteristics, the fungus was identified as Alternaria alternata (Garibaldi et al. 2019). For molecular identification, genomic DNA was extracted from a representative isolate (WCS1-5), and translation elongation factor 1-alpha gene (TEF-1a) was amplified using primers EF1-728F/EF1-986R (Carbone and Kohn 1999). The PCR products were sequenced and deposited in GenBank (accession no. MK791316). MegaBLAST analysis showed that it had 99% similarity with A. alternata isolate JYHB-1 (accession no. MH708309). To determine pathogenicity of isolate WCS1-5, coleus plants (cv. Wizard Golden) were grown in 15-cm pots containing a commercial potting mix (one plant/pot). After 2 months of growing, three plants were inoculated by spraying conidial suspension (10⁶ spores/ml) of the isolate. Three plants treated with sterile distilled water served as a control. All plants were placed in a humidity chamber (>95% relative humidity) for 48 h before they were put back in the greenhouse at 23 ± 5°C under a light/dark cycle (12 h/12 h) with watering once every 3 days. Inoculated plants showed symptoms 8 days after inoculation that were identical to those observed in the nurseries, whereas no symptoms developed on the control plants. The fungus was reisolated from the diseased leaves and confirmed to be A. alternata based on morphological characteristics and sequence analysis. To our knowledge, this is the first report of A. alternata causing leaf spot on coleus in China. Coleus is a common ornamental and medicinal crop in China, and occurrence of this disease needs to be monitored and properly managed in coleus production.