First Report of Basella alba Naturally Infected with Basella Rugose Mosaic Virus in Hawaii

Basella alba, also known as Malabar spinach, is a plant belonging to the family Basellaceae. It is native to tropical Asia and Africa and introduced to Hawaii, where it is grown as a vegetable crop (Wester 1992). Basella rugose mosaic virus (BaRMV) is a potyvirus that was first isolated from B. alba and reported in December 2006 in Taiwan (Zheng et al. 2008). In December 2018, B. alba plants with leaves exhibiting mosaic and rugose symptoms were found growing in a community garden consisting of about 150 small gardens 10 m² in size on Oahu, Hawaii (21°17′05″ N, 157°49′40″ W). Five leaf samples with severe mosaic and rugose symptoms from five different B. alba plants in five different small gardens in the same community garden were tested by ELISA using potyvirus-specific antibodies (Agdia, Elkhart, IN) (Wang et al. 2019). All symptomatic leaf samples tested positive for potyvirus infection by the ELISA test. Total RNAs were extracted from these samples and tested by reverse transcription PCR (RT-PCR) using the universal primer pair CIFor and CIRev designed to amplify the cylindrical inclusion protein (CI) gene from the genomes of potyviruses (Ha et al. 2008). To identify the specific potyvirus infecting the B. alba samples, amplicons of approximately 600 bp corresponding to the targeted potyviral CI region were directly sequenced (MN075828). BLAST analysis of the 564-bp sequences showed that this virus isolate respectively shared 97 and 95% nucleotide and amino acid sequence identities with the homologous sequences of a peace lily mosaic virus (PeLMV) isolate from Vietnam (DQ851494/ABI34613). PeLMV was first reported infecting peace lily (Spathiphyllum patinii, family Araceae) in Vietnam in 2008 (Ha et al. 2008). Because the Vietnam isolate of PeLMV (DQ851494/ABI34613) respectively shared 83 and 92% nucleotide and amino acid sequence identities to an isolate of BaRMV (DQ821939/ABH10135), PeLMV is considered a strain of BaRMV (Wylie et al. 2017). However, our Hawaii isolate (MN075828) of BaRMV only shared 83 and 94% nucleotide and amino acid sequence identities, respectively, with the isolate of BaRMV (DQ821939/ABH10135). We then designed new BaRMV CI gene-specific primers B-CIF (5′-TCAGTTTTTGGTTCGACGCC-3′) and B-CIR (5′-GATTTCCGTTGCACCCGACT-3′) from the BaRMV-specific sequence (MN075828) to further characterize the viral isolate from B. alba. BLASTn analysis of the 389-bp sequence of the CI fragment amplicon (MN075829) indicated that it shared 98% nucleotide sequence identity with the same isolate from Vietnam (DQ851494), whereas BLASTx analysis showed that the CI amplicon shared 96% amino acid sequence identity with the corresponding amino acid region of the same isolate from Vietnam (ABI34613). The CI fragment amplicon (MN075829) also revealed 83 and 95% nucleotide and amino acid sequence identities, respectively, to the Taiwan isolate of BaRMV (DQ821939/ABH10135). Our sequence comparison results indicated that our Hawaii isolate was more closely related to the Vietnamese BaRMV isolate initially identified as PeLMV than to the Taiwan isolate of BaRMV. The primer set B-CIF and B-CIR was further used in RT-PCR reactions to determine infection by BaRMV in nine additional symptomatic samples of B. alba based on amplification of DNA fragments of the expected size. We have also used RT-PCR to test 20 more samples from this community garden; our results show that two asymptomatic samples were negative and 18 symptomatic samples were positive. Therefore, the disease incidence of BaRMV on B. alba was approximately 92%. To our knowledge, this is the first report of BaRMV infecting B. alba plants in the United States. Further studies to obtain information on the full-length genome sequence, insect vector(s), and geographic distribution of this BaRMV isolate are necessary to help control BaRMV infection in B. alba and other vegetables among Hawaii’s community and family gardens.