First Report of Fusarium oxysporum Causing Stalk Rot and Root Rot of Portulaca molokiniensis in China

Portulaca molokiniensis, known also as ihi, is a succulent plant endemic to Hawaii. This plant is federally listed as an endangered species in Hawaii. In China, it is planted as a landscape plant. In 2017, three samples of P. molokiniensis were collected from a flower production field in Hebei, China, and displayed dead branches and root and stem rot. The roots and stems of the sampled plants were sterilized in 75% ethanol for 30 s and 0.1% HgCl₂ for 1 min, after which the samples were washed three times in sterile distilled water and cultured on potato dextrose agar (PDA). After 3 or 4 days, four isolates were purified by the single-spore technique and recultured on PDA in the dark at 25°C. The initial color of the colony was white, and it turned pale violet after being cultured on PDA for 5 to 7 days. Macroconidia were formed in carnation leaf agar, which were almost straight, thin-walled, and usually three-septate. Microconidia were typically zero-septate, oval, elliptical, or reniform (kidney-shaped), and formed abundantly in false heads on short monophialides. Fungal colonies displaying morphological characteristics of Fusarium spp. (Leslie and Summerell 2006; Leslie et al. 2005) were purified by the single-spore technique. To identify the isolates to species, universal primers ITS1/ITS4 (White et al. 1990) coupled with EF-1α primers EF1/EF2 were used for polymerase chain reaction–based molecular identification. The amplification region was sequenced by Sangon Biotech (Shanghai, China) and submitted to GenBank (accession nos. MF679142 and MF679141). BLAST analysis showed that they were all 99% identical to Fusarium oxysporum (GenBank accession nos. KY090783 and LT841203) (Nelson et al. 1983). Pathogenicity tests were conducted three times by drop inoculating 20 ml of conidia suspension (10⁶ spores/ml) from four isolates of F. oxysporum on the roots and stems of 2- to 3-year-old healthy potted P. molokiniensis; each isolate was tested on four to five plants. Controls were treated with distilled water. The inoculated plants were then placed in a greenhouse under 28°C, 65% relative humidity, and a 16/8-h light/darkness photoperiod. After 30 days, the infected plants showed dead branches and root and stem rot, the same symptoms as observed on the original plants. The control plants showed no symptoms. The fungus was reisolated from infected roots and stems and showed the same characteristics as described above and was identical in appearance to the isolates used to inoculate the plants. On the basis of the above, this fungus was identified as F. oxysporum. To our knowledge, this is the first report of F. oxysporum infecting P. molokiniensis in China. This disease may become a significant problem for P. molokiniensis, and preliminary management practices are needed for reducing the cost of production.