Brown leaf spot of Prunus serrulata Caused by Colletotrichum fioriniae in Sichuan, China.

Abstract

Prunus serrulate Lindl is widely cultivated in urban areas of China. It is mainly used for wood cultivation and urban landscaping. In May 2021, new leaf spot disease was observed in Chengdu city (30°42' to 30°45'N, 103°51' to 104°7'E), with 69.3% disease incidence, which could inhibit leaf growth and reduce their biomass. A planting area of more than 1000 square meters was investigated. The diseased leaves were mostly concentrated in the lower position of plants, where the humidity was higher. The disease infected P. serrulata leaves and occurred in the field from March to October, with the highest incidence in early May. The typical symptoms initially appeared as brown necrotic lesions on the margin of the leaves. The lesions then enlarged gradually and developed into reddish brown spots, eventually coalescing into large irregular, necrotic lesions with dark brown margins. Finally, the diseased leaves withered and died. Conidiomata were not formed on the diseased tissue. Ten symptomatic leaves were collected from 5 different trees in the planting area. Infected tissues from ten samples were cut into small pieces of 3 × 3 mm. The infected tissues were surface-sterilized by 3% sodium hypochlorite and 75% ethanol respectively for 30s and 60s, and rinsed three times in sterile water. Then they were blot-dried with autoclaved paper towels and cultured on potato dextrose agar (PDA) amended with streptomycin sulfate (50 μg/mL), and incubated at 25°C for 4 to 8 days. After culturing for 8 days at 25℃ and 12 h/12 h light/dark on PDA, the colony diameter reached 67.5 to 78.6 mm. The colonies were initially white, cottony, then became light pink to misty rose at the center, and the reverse side of the colony turned dark red to red and had pale yellowish borders. The conidia were straight, smooth-walled, colorless, fusiform with acute ends, measuring 8.2 to 16.7 × 3.1 to 5.9 μm in size (n = 100). For molecular identification, the genomic DNA of the representative isolate RBWY202105 was extracted using a fungal genomic DNA extraction kit (Solarbio, Beijing). The internal transcribed spacer (ITS) [ITS1/ITS4 (White et al., 1990)], histone3 (HIS3) [CYLH3F/CYLH3R (Crous et al. 2004)], chitin synthase (CHS-1) [CHS-79F/CHS-345R (Carbone & Kohn, 1999)], actin (ACT) [ACT512F/ACT (Carbone & Kohn, 1999)], β-tubulin (TUB2) [BT2A/BT2B (O'Donnell et al., 1997)], and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) [GDF/GDR (Templeton et al. 1992)] were amplified. Sequences were deposited in GenBank (ITS: ON000436, HIS3: ON014581, CHS-1: ON014579, ACT: ON014583, TUB2: ON014582, and GAPDH: ON014580). BLAST results indicated that the ITS, HIS3, CHS-1, ACT, TUB2 and GAPDH sequences showed >99% identity with Colletotrichum fioriniae (Marcelino & Gouli) R.G. Shivas & Y.P sequences at NCBI (GenBank MW497230 (561/582), MT740312 (415/415), KU736865 (258/258), MK680659 (246/246), MK967342 (757/757), and MW656269 (263/263)). The conidial suspension (1 × 106 conidia/ml) was used for inoculation by spraying leaves of ten 4-year-old P. serrulata plants for pathogenicity test. Fifteen leaves of each plant were inoculated with spore suspensions on the leaves (600 μl per leaf). The same amount of control leaves was sprayed with sterilized distilled water as a control. Finally, all potted plants were placed in a greenhouse at 25°C under a 16 h/8 h photoperiod and 67 to 78% relative humidity. After ten days, the symptoms observed on the inoculated plants were similar to those of the original diseased plants, but the controls remained asymptomatic. Colletotrichum fioriniae was re-isolated from the infected leaves and identified by both morphological characteristics and DNA sequence analysis (The ITS, HIS3, TUB2, CHS-1, GAPDH and ACT genes). The pathogenicity test was repeated thrice, which showed similar results, confirming Koch's postulates. To our knowledge, this is the first report of brown leaf spot on P. serrulata caused by C. fioriniae in China. The identification of C. fioriniae could provide relevant information for taking management strategies and further research on the Prunus serrulata disease.