First Report of Fusarium equiseti Causing Fusarium Wilt on Potato (Solanum tuberosum) in China.

Abstract

Potato (Solanum tuberosum) is one of the most economically important crops in China, containing carbohydrates, protein, fiber, numerous vitamins and minerals, and is a heart healthy food (Raidl, 2020). Potato infected by Fusarium spp. exhibits quality and yield decline, and even death. In infected plants, the upper leaves exhibit chlorosis, the lower leaves wither and the vascular bundles of stems and tubers turn yellow, and then tan to brown. In August 2018, symptomatic potato stems and roots were collected from Zhangye city, Gansu province, China. Diseased stem tissues were surface sterilized with 75% alcohol for 30 s, and then rinsed in sterile water. The tissue pieces were placed on potato dextrose agar (PDA) and incubated at 25°C in darkness. Fusarium-like colonies were consistently isolated and three monoconidial isolates were obtained. Isolate 3SMJ-2 was selected as a representative for morphological characterization, molecular analysis, and pathogenicity tests. 3SMJ-2 was inoculated in PDA liquid medium, grown on a shaker for 7 days at 25℃ to obtain a mix suspension of hypha fragments and spores (107 spores/mL). Healthy potato plants, named "Xin Daping" and were planted in pots (17 cm diameter by 12 cm) filled with 2L of sterile soil per pot. After 8 weeks, the plants were inoculated with the inoculum or distilled water. Then they were incubated in growth chambers at 25°C under a 12-h/12-h day/night potato period with 90% relative humidity for 24 h. For each treatment, 3 pots were inoculated. After 50 days, 100% of the inoculated potato plants exhibited wilt symptoms similar to those in the field but the control plants were symptomless. A Fusarium identical to strain 3SMJ-2 was re-isolated from symptomatic potato plants to fulfilling Koch's postulates. Morphological characteristics of the re-isolated strain were identical to the original isolate, which confirmed pathogenicity of strain 3SMJ-2 originally isolated from the potatoes. Colonies of 3SMJ-2 were white with short conidiophores, a few microconidia and sickle-shaped macroconidia (25.2 to 42.9× 3.1 to 4.6 µm) (n = 60) with 4~7 septa, and mostly 5 septa, after cultivated on PDA in an incubator at 25℃ for 14 days. Spherical terminal or intercalary chlamydospores were observed on the mycelium. Strain 3SMJ-2 was identified preliminarily as Fusarium sp. based on morphological characteristics (Leslie et al., 2006). Genomic DNA was extracted from 3SMJ-2 using the OMEGA Fungal DNA kit according to the manufacturer's protocol. The internal transcribed spacer (ITS), translation elongation factor 1-alpha (TEF) and RNA polymerase II second largest subunit (RPB2) were amplified using ITS1/ITS4 (White et al., 1990), Ef728M/Tef1R (Stępień et al., 2012) and 5F2 /7cR (O'Donnell et al., 2007), respectively. After sequencing by Beijing TSINGKE Biological Technology Co., Ltd., 3 fragments of approximately 519 bp, 587 bp and 1059 bp from the strain 3SMJ-2 were deposited in GenBank as MN420681, MW561963 and MW561964. The ITS, TEF and RPB2 sequences were 100%, 100% and 99.8% identical to those of F.equiseti (KY365589, KF499577, and MH582110). Based on the pathogenicity tests, morphological characteristics and molecular analyses, we identified the strain 3SMJ-2 as F. equiseti, the pathogen causing Fusarium wilt on potato in Zhangye City. Although, F. equiseti has been reported to cause root rot of cowpea (Li et al., 2017) and sugar beet (Cao et al., 2018) in China. To our knowledge, this is the first report confirming F. equiseti causing potato wilt in China. Potato is an economically important crop in Gansu and the occurrence of the new disease caused by F. equiseti on potato needs to be properly managed to reduce yield loss.