First Report of Anthracnose on Passiflora edulis Caused by Colletotrichum siamense in Guangdong Province in China.

Abstract

Passion fruit (Passiflora edulis Sims) is an important tropical and subtropical species with high commercial value due to its wide uses in juice production and fresh fruit consumption. A disease affecting the leaves was observed on passion fruit vines in the South Subtropical Botanical Garden in Zhanjiang city in April and September 2020 with an incidence of 30 to 60%. Affected leaves usually developed small circular reddish-brown spots, which enlarged to oval spots and large irregular spots. The spots were brown and surrounded by a yellow halo with sunken cavities, which are typical symptoms of anthracnose. Diseased cultivars HuangjinBaiXiangGuo were collected in Zhanjiang city (21.2N110.3E), Suixi city (21.4N110.2E) and Leizhou city (20.9N110.1E) in Guangdong province. Leaf samples were disinfested with 1% NaOCl for 5 min, and cultured on potato dextrose agar (PDA) at 28 °C for one week. Eight single-spore isolates were recovered from PSA medium. Colonies developing on PSA were grayish white with a regular border. Conidia weretraight, hyaline with rounded ends, 5.3 to 6.8×15.2 to 21.3μm (n = 50 conidia of each isolate). Fungal mycelia were hyaline, septate, and branched. Conidia were born on a long conidiogenous cell, appressoria were dark, smooth-walled, oval, 15.5 to 21.3×4.0 to 5.8 μm (n=50).The isolates were morphologically identified as Colletotrichum sp. (Weir et al. 2012). Tests of pathogenicity were performed according to Koch's postulates using three isolates. Five plants with 2-3 vines were inoculated with each isolate separately and 3-4 leaves of each plant were inoculated. Fresh wounds were made with a sterile needle on the healthy surface of leaves of P. edulis HuangjinBaiXiangGuo and each leaf was covered with a piece of cotton drenched with 200 μL of conidial suspension (106 conidia/ml) from each isolate. Control seedlings were inoculated identiclly except sterile water was used to drench the cotton. Inoculated plants were placed in a moisturizing light incubator at 25℃ and 80% humidity under a 12-h light/dark cycle for 14 days and examined daily to monitor disease symptom development. Small round brown spots were observed at the inoculation sites 3 days after the inoculation and eventually became large brown lesions. There were no symptoms observed in the water-inoculated plants. A Colletotrichum sp. strain based on morphology was consistently reisolated from leaves lesions fulfilling Koch's postulates. For molecular identification, the internal transcribed spacer (ITS) region of ribosomal DNA, calmodulin (Cal), tubulin (Tub) and Apmat loci of three isolates were amplified using primer pairs of ITS4/ITS5, CL1C/CL2C, T1/T2 and AM-F/AM-R (Sharma et al. 2015). A phylogenetic tree derived from a neighbor-joining analysis of a concatenated alignment of ITS, CAL, Tub and ApMAT sequences was created. The accession numbers of three isolates BXGZJ 1-1, BXGSX 2-2 and BXGLZ 3-2 used in this study were MZ605383- MZ605385 for ITS, MZ605389- MZ605391 for CAL, MZ605392- MZ605394 for Tub and MZ605386- MZ605388 for ApMAT. The sequences of the three isolates were aligned with related species of Colletotrichum (Sharma et al. 2015). Analyses based on concatenated data sets of four genes showed that the sequences of the 3 isolates had the highest identity of 99% to those of the type strain C. siamense ICMP18578. According to both morphological and sequence analyses, the P. edulia pathogen was identified as C. siamense. There is a report of foliar diseases on P. edulia caused by C. brevisporum in Hainan province in China (Qiu et al., 2021). To our knowledge, this is the first report of anthracnose on P. edulia caused by C. siamense in China. Identification of pathogens could provide valuable information for disease diagnosis and effective chemicals to control the disease, so as to reduce yield loss and improve fruit quality.