Neocosmospora rubicola Causing Stem Rot of Pitaya (Hylocereus costaricensis) in China

Pitaya (Hylocereus costaricensis) is a tropical and subtropical plant of the family Cactaceae. In June 2016, a stem rot disease was observed affecting over 15% of a pitaya planting in Dongfang (Hainan Province). Symptomatic plants initially showed yellowing of stem base, and lesions developed that were shriveled, curling, and soft. The vascular bundles of stems changed internally from pale yellow to reddish brown. As the disease developed, it sometimes developed into root rot. Pitaya stems with typical symptoms were washed with tap water. Small pieces of tissue (4 × 4 mm) were cut from the margin of a stem lesion and were disinfected in alcohol (75%) for 30 s and HgCl₂ (0.1%) for 1 min, followed by washing three times in sterile water. The disinfected tissues were placed on potato dextrose agar (PDA) and incubated at 28°C. Twenty-one isolates were obtained, and all had similar cultural and morphological characteristics; two of these (HLG0112 and HLG0117) were selected for further characterization. Seven days after incubation, colonies were fast growing with white aerial mycelia and a canary yellow color on the reverse side. Two types of spores were produced on PDA: microconidia were oval to kidney shaped, single celled, 7.0 to 13.5 × 2.2 to 3.4 μm. Macroconidia (4.9 to 6.3 × 31.5 to 45.7 µm) had two to five septa, sickle shaped, slightly curved. Based on morphology, the fungus was identified as Fusarium-like. Four gene regions of isolate HLG0112 were sequenced, including the ribosomal internal transcribed spacer (ITS), calmodulin (cmdA), 28S large subunit nrDNA (LSU), and histone H3 (his3) regions. The gene regions were amplified and sequenced using the primers ITS1/ITS4, CYL-228F/CYL2Rd, LR0R/LR7, and CYLH3F/CYLH3R (Lombard et al. 2015). The ITS, LSU, cmdA, and his3 sequences (MH277488, MH277489, MH332387, and MH297490) all had 99% identity to Neocosmospora rubicola (CBS 320.73; GenBank accession nos. KM231799, KM231666, KM231371, and KM231523, respectively). Based on morphological and molecular characteristics, the fungus was identified as N. rubicola L. Lombard & Crous (Lombard et al. 2015). Pathogenicity tests were conducted by placing 5-mm-diameter mycelial plugs obtained from 7-day-old cultures on the stem base. Controls were treated with PDA plugs. Ten plants were placed into each group, which were maintained in a growth chamber at 28°C and covered with a plastic bag to maintain a high relative humidity. After 5 to 10 days, symptoms identical to those of field samples developed on all the inoculated plants, whereas the control plants remained asymptomatic. The pathogenicity tests were conducted three times. The same fungus was reisolated from the infected plants, and the identity was confirmed by cultural and morphological characters, confirming Koch’s postulates. Several Neocosmospora species have recently been reported from diseases of other tropical and subtropical plants (Sandoval-Denis et al. 2018). Fusarium fujikuroi has been reported to cause stem rot lesion of H. polyrhizus in Malaysia (Masratul Hawa et al. 2017) and Sclerotium rolfsii in China (Zheng et al. 2018). To our knowledge, this is the first report of N. rubicola causing stem rot of pitaya in China.