First Report of Anthracnose Caused by Colletotrichum ciggaro on Lonicera macranthoides Hand.-Mazz. in China.

Abstract

Lonicera macranthoides Hand.-Mazz. is a traditional medicinal plant that is cultivated in Hunan, Yunnan, and Guizhou Provinces in China. In June 2020, a new leaf spot disease was observed on this plant in Longhui County, Shaoyang City, Hunan Province, China, where 14,000 hm2 of L. macranthoides had been planted. About 20% of the total cultivated area exhibited symptoms. Brown spots appeared on the leaves during the early stage and gradually expanded into irregular lesions, which became necrotic and dry. The whole plant withered and died in severe cases. To isolate the pathogen, the infected leaves were collected from different fields and washed with flowing sterile water. The small lesions were then cut and surface sterilized with 75% alcohol for 45 s followed by a 3 min treatment in 3% sodium hypochlorite. The lesions were rinsed five times in sterile water, incubated on potato dextrose agar (PDA) plates and cultured for 3-5 d at 28℃. In total, eleven isolates were obtained, and eight of them were Colletotrichum (isolation frequency 73%). Three representative isolates (JYH1, JYH2, and JYH5) were selected for further study. The fungus grew as circular white colonies, which then became grey. The older colonies looked like cotton and had dense aerial hyphae. The conidia were aseptate, transparent, cylindric, and thin walled, which measured 11.54 to 22.64 × 3.55 to 4.75 μm (n=100). Six genetic regions were amplified and sequenced to further confirm the identity of fungus. They included β-tubulin (TUB2), the internal transcribed spacer (ITS), actin (ACT), chitin synthase (CHS), calmodulin (CAL) and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). The sequences were submitted to GenBank (ITS: OQ746331; ON954583; OQ746334; TUB2: OQ772278; ON960155; OQ772279; CHS: OQ772280; ON960156; OQ772281; ACT: OQ772282; ON960157; OQ772283; CAL: OQ772284; ON960158; OQ772285; GAPDH: OQ772286; ON960159; OQ772287). The construction of a 6-gene joint phylogenetic tree analysis showed that the three isolates unambiguously clustered with Colletotrichum kahawae subsp. ciggaro strains C022-1 (GenBank: KJ001120.1, KJ001124.1, KJ001109.1, KJ001102.1, KJ001106.1, KJ001113.1) and R019 (GenBank: JN715847.1, KC860023.1, KC859980.1, KC859954.1, KC859972.1, KC859997.1), which was recently reclassified as C. cigarro (Cabral et al. 2020). Three representative isolates were used for the pathogenicity test on the young leaves of the whole plant. A sterile pin was used to prick the leaf epidermis, and 6 × 6 mm mycelial blocks that had been cultured on PDA for 7 d were placed on the leaf wounds. The controls were treated in the same manner except that sterile blocks of PDA were used. There were three replicates per treatment. All the plants used in the experiment were maintained at 28°C in a climate chamber. There was a 12 h photoperiod, and the chamber was kept at 80% relative humidity. Dark brown spots appeared at the sites of inoculation on the plants after 5 days. All the strains that were re-isolated from the lesions shared the same morphological characteristics and had the same type of colonies as the pathogen Colletotrichum ciggaro. Thus, Koch's postulates were fulfilled. C. ciggaro had been shown to cause anthracnose on Olea europaea L. (Weir et al. 2012), Mangifera indica L. (Ismail et al. 2015), Citrus reticulata L. (Perrone et al. 2016) and Areca catechu L. (Zhang et al. 2020). To our knowledge, this is the first report of C. ciggaro causing anthracnose on L. macranthoides in China and worldwide. This research provides a basis for further research to control epidemics of this disease.