First Report of Fusarium xylarioides Causing Root and Stem Rot on Aloe vera in China.

Abstract

Aloe vera (L.) Burm f. is a perennial herb belonging to the family liliaceae. It is widely grown for medicinal, cosmetic and vegetable use. In 2018 and 2019, a root rot disease occurred on potted A. vera plants in a nursery in the Hunan Province of China. Symptoms of the disease include water soaking lesions, brown spots on taproot or basal part of the stem. The plants were easy to pull out when the taproot is rotten or necrotic. As the disease progressed upward, leaves in the basal part of stems became red-brown and gradually fell off. In severe cases, the whole plants became rotten and wilted. For isolation purposes, diseased tissues were excised from the lesion margins, surface disinfested with 70% ethanol for 10 s, 0.1% HgCl2 for 2 min, rinsed with sterile water thrice, and then placed on potato dextrose agar (PDA) and incubated at 26°C for 3 days in the dark. When cultured on PDA, fungal strains with similar morphology were consistently isolated and purified by single spore isolation. Colonies showed thick, pink aerial mycelium with a growth rate of 1.3 cm /day. The pigmentation was more intense in the colony center and became pale orange and white at the edge of colony. When cultured on SNA (Spezieller Nährstoffarmer agar), the fungus showed less pigmentation and thinner hyphae. Microconidia were abundantly produced, clavate and oval to kidney shaped, 7.1 to 15.2 μm × 2.5 to 5.1 μm, with 0 to 1 transverse septa. Macroconidia were sickle shaped, slender, slightly incurved in apical cell and foot-shaped in the basal cell, measured 27.9 to 53.2 μm × 2.5 to 3.5 μm, with 3 to 5 septa. These morphological characteristics were similar with those of Fusarium spp. (Booth 1971). For molecular identification, genomic DNA of the fungus was extracted by cetyl trimethyl ammonium bromide method. A portion of EF-1α (translation elongation factor 1-α) and RPB1 (the largest subunit of RNA polymerase) genes were amplified and directly sequenced using the EF-1/EF-2 and Fa/G2R primers (O'Donnell et al. 2010). The EF-1α and RPB1 were deposited in the GenBank with accession numbers MT755386 and MT755387. The EF-1α and RPB1 had 97.14% (ID FD_01334) and 99.62% identity (FD_03853), respectively, to F. xylarioides strains in the Fusarium-ID database (Geiser et al. 2004). In addition, the EF1-a showed 96.825% identity to the F. lateritium CBS 119871(AM295281) (a synonym of F. xylarioides), and the RPB1 showed 99.623% identity to the F. xylarioides NRRL 25486 (JX171517.1). Accordingly, the fungus was putatively identified to be F. xylarioides. For pathogenicity assay, A.vera seedlings were pot planted using sterilized nursery soil and inoculated with conidia suspension (1 × 105 conidia/ml), which were eluted from 7-day-old PDA cultures with sterilized water, according to the method described previously (Vakalounakis et al. 2015). The collar of each potted plant was poured with 20 ml of conidia suspensions. Plants mock inoculated with sterile water were used as control. All the inoculated plants were placed in a growth chamber at 25°C under 12/12 h light/dark cycle. The inoculation assays were carried out twice, with each one had three replicated plants. After 30 days, rot symptoms seen from the roots and basal part of stems were observed on the inoculated plants, but no visible symptoms were observed on control plants. The fungus was re-isolated from the inoculated plants and identified to be F. xylarioides by morphological and molecular characteristics, thus confirming Koch's postulates. As we know, many Fusarium species have been reported to cause root and stem rot disease in A.vera such as the F. oxysporum (Ji et al. 2007) and F. solani (Vakalounakis et al. 2015). However, to the best of our knowledge, this is the first report of F. xylarioides causing root and stem rot disease of A.vera in China. The identification of the pathogen fungus might provide a foundation for taking appropriate control strategies to this disease.