First report of leaf spot caused by Colletotrichum siamense on Salix matsudana in China.

Abstract

Salix matsudana Koidz. (Chinese willow) is an important landscaping tree species widely grown in China (Zhang et al. 2017). In October 2019, a characteristic leaf spot disease of S. matsudana was found on the campus of Nanjing Forestry University. Most 25-year-old S. matsudana trees (13 out of 21, approximately 62%) on campus showed the leaf spot disease. On average, 70% of the leaves per individual tree were affected by this disease. Foliar symptoms began as dark brown, irregular spots and the centers were gray-white, gradually enlarging with time. Leaf spot symptomatic leaves were collected from three infected S. matsudana trees (10 leaves/tree), and small infected tissues (3-4 mm2) were surface-sterilized in 75% ethanol for 30 s, 1% NaClO for 90 s, rinsed in ddH2O, dried on sterilized filter paper, and plated on potato dextrose agar (PDA), and then incubated at 25°C. Three isolates (NHY1-1, NHY1-2, and NHY1-3) of the same fungus were obtained in 85% of the samples and deposited in China's Forestry Culture Collection Center (NHY1-1: cfcc55354, NHY1-2: cfcc55355, NHY1-3: cfcc55359). The colonies of three isolates were white, but the reverse side was grayish-white. The conidia of NHY1-1 were one-celled, straight, subcylindrical, hyaline, 14.4 ± 0.9 × 5.4 ± 0.4 µm (n = 50), with a rounded end. Conidiophores were hyaline to pale brown, septate, and branched. Appressoria were one-celled, ellipsoidal, brown or dark brown, thick-walled, 8.0 ± 0.9 × 5.9 ± 0.5 µm (n = 50). The conidia and appressoria of the other two isolates weralmost identical to NHY1-1. The morphological characters of the three isolates were matched with those of the Colletotrichum gloeosporioides complex (Weir et al. 2012). For accurate identification, the DNA of the three isolates was extracted. The internal transcribed spacer region (ITS), actin (ACT), calmodulin (CAL), chitin synthase (CHS-1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), superoxide dismutase (SOD2), and β-tubulin 2 (TUB2) genes were amplified using the primer pairs ITS1/ITS4, ACT-512F/ACT-783R, CL1C/CL2C, CHS-79F/CHS-345R, GDF1/GDR1, SODglo2-F/SODglo2-R, and Bt2a/Bt2b, respectively (Weir et al. 2012). The sequences were deposited in GenBank [Accession Nos. MW784679 and MW808959 to MW808964 for NHY1-1; MW784726 and MW808965 to MW808970 for NHY1-2; MW784729 and MW808971 to MW808976 for NHY1-3]. A BLAST search of GenBank showed that ITS, ACT, CAL, GAPDH, SOD2, and TUB2 sequences of the three isolates were identical to Colletotrichum siamense at a high level (>99%), and CHS-1 sequences of three isolates were consistent with Colletotrichum fructicola at a high level (>99%). A maximum likelihood and Bayesian posterior probability analyses using IQtree v. 1.6.8 and Mr. Bayes v. 3.2.6 with the concatenated sequences (ITS, ACT, CAL, CHS-1, GAPDH, SOD2, and TUB2) placed NHY1-1, NHY1-2, and NHY1-3 in the clade of C. siamense with high bootstrap support values (ML/BI = 93/1). The pathogenicity of three isolates were tested on potted 2-yr-old seedlings (50-cm tall) of S. matsudana, which were grown in a greenhouse. Healthy leaves were wounded with a sterile needle and then inoculated with 10 µL of conidial suspension (106 conidia/mL). Controls were treated with ddH2O (Zhu et al. 2019). In total, 12 seedlings were inoculated including controls. Three seedlings/isolate and 10 leaves/seedling were used for each treatment. The plants were covered with plastic bags after inoculation and sterilized H2O was sprayed into the bags twice/day to maintain humidity and kept in a greenhouse at the day/night temperatures at 25 ± 2 / 16 ± 2°C. Within 7 days, all the inoculated points showed lesions similar to those observed in field, whereas controls were asymptomatic. The infection rate of each of the three isolates is 100%. C. siamense was re-isolated from the lesions, whereas no fungus was isolated from control leaves. The diseases caused by C. siamense often occur in tropical and subtropical regions of China, with a wide range of hosts, such as Hevea brasiliensis and Coffea arabica, etc. (Cao et al. 2019; Liu et al. 2018). This is the first report of C. siamense causing leaf spot of S. matsudana in China and the world. These data will help to develop effective strategies for managing this newly emerging disease.