First report of root rot on Dendrobium officinale caused by Fusarium incarnatum-equiseti species complex in Zhejiang Province, China.

Abstract

Dendrobium officinale Kimura et Migo is a rare and valuable Chinese herb cultivated in Zhejiang and Yunnan Provinces, China, which is known for its functions as an anti-neoplastic and for lowering the blood sugar (Cheng et al., 2019). In September and October of 2018 and 2019, symptoms of root rot on D. officinale were observed with an incidence of 15-20% in Wuyi County, Zhejiang Province, China. The pathogen mainly infected roots causing severe root rot, which resulted in significant economic losses. At the early stage of this disease, the stalk turned brown, then the whole plant rotted from bottom to top within a few days. Symptomatic roots were cut into small pieces (1.0 cm × 1.0 cm) and disinfected successively by submersion in 75% ethanol for 30 s and 1% NaClO for 30 s under aseptic conditions. After rinsing with sterile water three times and air drying, segments were placed on potato dextrose agar (PDA). After incubation at 25 °C for 5 d in the dark, white to pale cream colored colonies were produced. The average mycelial growth rate was 15.2-18.5 mm day-1 at 25 ℃. Macroconidia were falciform with three to five septa and (18.0-32.0)×(3.0-5.0) μm in size. Microconidia were fusiform with two to three septa (7.0-10.0)×(2.1-3.0) μm. Based on morphological characteristics of macroconidia, and microconidia, isolates were identified as Fusarium incarnatum-equiseti species complex (span style="font-family:'Times New Roman'; font-size:12pt">FIESC) (Avila et al., 2019). The internal transcribed spacer (ITS) region, translation elongation factor (EF-1α), RNA polymerase largest subunit (RPB1), and RNA polymerase second largest subunit (RPB2) gene were amplified and sequenced respectively using ITS1/ITS4, EF1/EF2, Fa/G2R and 5f2/7cr primers (O'Donnell et al., 2010). BLASTN analysis of FUSARIUM-ID using ITS (Accession NO. MW172977), EF-1α (Accession NO. MW172978, RPB1(Accession NO. MW172979), and RPB2(Accession NO. MW172980) showed 99.8%, 100%, 99.74%, and 98.63% identity to FIESC isolates NRRL43619, NRRL34059, NRRL32864, and NRRL32175, respectively. To verify pathogenicity, ten 1-year-old healthy D. officinale plants were used for inoculation tests. One milliliter of a conidial suspension (106 conidia ml-1) was pipetted onto the soil around the base of D. officinale plants per pot. Ten plants, which were treated with sterile water, were used as the control. All plants were maintained in a climatic chamber (26 ± 1 ℃, 70-80% relative humidity and a photoperiod of 16:8 [L: D] h). Seven days later, all inoculated plants showed typical symptoms of root rot identical to those observed in the fields. Control plants remained symptomless and healthy. The pathogenicity analysis was repeated three times. Pathogens re-isolated from symptomatic plants were identified as FIESC species by morphology observation and sequence analysis. To our knowledge, this is the first report of root rot caused by FIESC species on D. officinale in Zhejiang, China.