Fusarium acuminatum Associated with Root Rot of Ophiopogon japonicus (Linn. f.) in China.

Abstract

Ophiopogon japonicus (Linn. f.) is a perennial evergreen in the Liliaceae family that is cultivated in many provinces of China due to its high medicinal and economic value . In April 2019, an unknown root rot disease was observed on the rhizomes of O. japonicus in a commercial production field in Xiangyang City (30.83° N, 112.53° E), Hubei Province. Disease incidence was approximately 10-20%. Symptoms included chlorosis, drooping and rolling of the leaves followed by rapid death of entire plant. Infected roots appeared to be softened, necrotic, and shriveled with reddish fungal growth. Infected tissues were disinfested on surface with 75% ethanol for 30 s and 0.1% HgCl2 for 1 min, rinsed with sterile distilled water, and dried. Small pieces (2 mm × 2 mm) were then excised from disinfested tissue and incubated on potato dextrose agar (PDA) medium at 25 ℃ in the dark. After 3 days of incubation, six isolates with 75% of isolation rate and same colony morphology were sub-cultured and purified by hyphal tip isolation. Purified cultures grew rapidly and media plates (70×70 mm ) were covered with hyphae after 3 to 4 days. Cultures were initially white and became pink or red over 5 days. Microconidia were not observed. Macroconidia were produced from monophialides on branched conidiophores, which were slender, equilaterally curved, and measured 32.5 to 53.5 μm in length and 3.5 to 5.1 μm in width, with three to five septa. All strains were preliminarily identified as Fusarium acuminatum (Eslie and Summerell 2006) on the basis of morphology. To confirm the identity of the pathogen, molecular identification was performed with strain MD1. Following DNA extraction, PCR was performed using the TSINGKE 2×T5 Direct PCR Mix kit. Target areas of amplification were internal transcribed spacer (ITS), RNA polymerase second largest subunit (RPB2) and beta-tubulin gene (TUB2) regions of rDNA, using ITS1,4 (Yin et al. 1990) , RPB2-5f2/7cr (O'Donnell et al. 2010)and Btu-F-F01, Btu-F-R01 primers(Wang et al. 2014), respectively. Nucleotide sequences were deposited in NCBI (GenBank MT525360.1; MW164629; MT588110.1). BLAST analysis of the ITS sequence had 100% similarity to a 517 bp portion of F. acuminatum sequence in GenBank (MK764994.1) ；RPB2 sequence had 100% similarity to a 687 bp portion of F. acuminatum sequence in GenBank (HM068330.1) and TUB2 sequence had 99% similarity to a 964 bp portion of F. acuminatum sequence in GenBank (KT965741.1). A pathogenicity test was performed in laboratory on O. japonicus roots with isolate MD1. Mycelial plugs (5 mm) were excised from the margin of colony cultured for 5 days, and placed on three-years-old tuberous roots covered with wet sterile cotton and kept at 25℃, under 80% relative humidity. Controls were inoculated with non-colonized PDA plugs (5 mm). All treatments had three replicate plants. On incolated plants, white hyphae covered on O. japonicus roots 3 DPI became pink and by 5 DPI, roots had rot symptoms. By comparision, the control plants had no symptoms. The pathogen was reisolated from the inoculated roots and exhibited same morphological characteristics and ITS sequence as those of F. acuminatum. F. acuminatum was reported to cause fruit rot on postharvest pumpkin and Vaccinium corymbosum in China (Li et al. 2019; Wang et al. 2016).To our knowledge, this is the first report of root rot caused by F. acuminatum on O. japonicus in China.