First Report of Colletotrichum truncatum Causing Anthracnose on Oxalis corniculata in China.

Abstract

Oxalis corniculata L., which belongs to the family Oxalidaceae R. Br., is a very common perennial herb. It is usually planted on bare land or under the forest as landscaping plants, and the whole plant can be used for its medicinal values of clearing heat, detoxification and detumescence. In August 2019, typical symptoms of anthracnose on O. corniculata leaves were observed in the green belt on the campus of Shandong University of Technology (36.81°N, 117.99°E), Shandong Province, China. The disease incidence was above 40% by investigating more than 300 m2 of planting area. Most of O. corniculata are planted under the forest where the disease is found, mainly in the environment with high relative humidity and less ventilation. The infected leaves appeared initially as tawny oval or irregular spots, and then the lesions enlarged gradually until the leaves became dieback or wholly withered, which greatly reduced the landscape effect of O. corniculata. Diseased leaves were collected by cutting into small pieces and sterilized with 75% ethanol for 30 s and 2% sodium hypochlorite (NaClO) for 60 s, rinsed with sterile deionized water for three times. Each air-dried tissue segment was cultured on potato dextrose agar (PDA) and incubated at 25℃ for 5 to 7 days in the dark (Zhu et al. 2013). Fifteen isolates were obtained from 20 symptomatic leaves and the cultures were initially gray white, subsequently became grayish to dark green after 7 days, with copious gray aerial mycelium and black microsclerotia. Three isolates were verified by the amplification of DNA sequences of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), actin (ACT), histone H3 (H3) and chitin synthase (CHS1) genes, using the primer pairs GDF1/GDR1, ACT-512F/ACT-783R, CYLH3F/CYLH3R, and CHS-79F/CHS-234R (Damn et al. 2019, Fu et al. 2019, Liu et al. 2013), respectively. The sequenced genes (GenBank accession no. OK017473, OK159078, OK159076, OK159077) shared 99.62 to 100.00% nucleotide identity with the corresponding genes of Colletotrichum truncatum strain UASB-Cc-10 (GenBank accession no. KF322064.1, KF322055.1, KF322073.1, KF319059.1), respectively, which was consistent with the morphological identification (Sawant et al. 2012). Pathogenicity test was performed with six healthy O. corniculata plants infected with mycelial plugs (about 3 mm in diameter) of three C. truncatum isolates from a 5-day-old culture, while the negative controls on the same leaves were inoculated with sterile PDA plugs. All plants were placed in a greenhouse at 25 to 30℃ with 90% relative humidity. The experiment was conducted three times. Five days later, all inoculated leaves appeared brown sunken spots, whereas no symptoms appeared on negative controls. The same pathogens, C. truncatum, were identified from the inoculated leaves on the basis of morphological and molecular characteristics as described above, confirming Koch's postulates. To our knowledge, anthracnose caused by C. truncatum on O. corniculata is the first report in China. The discovery of this new disease is beneficial to the application and protection of O. corniculata, a popular landscape and medicinal plant.