First Report of Bean Yellow Mosaic Virus Infecting Nasturtium (Tropaeolum majus) in Hawaii

Garden nasturtium (Tropaeolum majus L.) is an herbaceous annual plant commonly used as a garden ornamental, culinary herb, or a medicinal plant. It has become naturalized in many locations and is currently considered an alien invader in New Zealand, Lord Howe Island, and Hawaii (U.S. Forest Service 2012). Bean yellow mosaic virus (BYMV) is an aphid-transmitted nonpersistent potyvirus with a wide host range (Parrella and Lanave 2009). In February 2018, we observed nasturtium plants exhibiting severe chlorotic mosaic symptoms at the Kula Botanical Gardens on Maui, HI (20°44′40.01″ N, 156°19′03.00″ W). We tested six samples that had severe yellow mosaic and leaf distortion symptoms and three leaves from nonsymptomatic plants using potyvirus-specific ELISA (Agdia, Elkhart, IN) and universal potyvirus-specific reverse transcription polymerase chain reaction (RT-PCR) with potyvirus nuclear inclusion body (NIb) primers NIb2F and NIb3R (Zheng et al. 2010). All of the symptomatic nasturtium samples tested positive for potyvirus infection in both tests, whereas three nonsymptomatic nasturtium leaves tested negative in both tests. To identify the specific potyvirus involved, amplicons (309 bp) of the NIb region that were generated by RT-PCR using universal potyvirus-specific primers were sequenced (accession MH422546). BLASTn analysis of the sequences showed that this virus shared 93% nucleotide identity with a BYMV isolate from Australia (HG970862), and BLASTx analysis showed it shared 93% amino acid identity with a polyprotein of BYMV isolate from Australia (CDO67686). Two additional sets of PCR primers were used to further characterize the isolate: BYMV NIb-specific primers BYMV-NIb-3 (5′-CCACTGTTATTCCCTTTAAAC-3′) and BYMV-NIb-5 (5′-AATAAGTTCTACAGTTTGCACC-3′), and BYMV coat protein (CP)-specific primers BYMV-CP-3 and BYMV-CP-5 (Wang et al. 2017). BLASTn analysis of the 257-bp NIb amplicon (accession MH422547) shared 94% identity to an Australian isolate of BYMV (HG970847), and the 595-bp CP amplicon (accession MH422548) shared 98% identity to another Australian isolate of BYMV (HG970860). BLASTx analysis of the 257-bp NIb amplicon shared 93% identity to an Australian isolate polyprotein of BYMV (AHQ95554), and the 595-bp CP amplicon shared 98% identity to another Japan isolate polyprotein of BYMV (BAA93683). All six symptomatic samples also tested positive for BYMV by triple-antibody sandwich ELISA using a BYMV-specific antibody (DSMZ, Braunschweig, Germany). From March to June 2018, 24 additional nasturtium samples were collected from two different locations on Maui, and all tested positive for BYMV infection by RT-PCR and ELISA. All of the symptomatic nasturtium samples tested positive for BYMV infection in both tests, while three nonsymptomatic nasturtium leaves from other plants tested negative in both tests. To our knowledge, this is the first report of BYMV infecting nasturtium plants in Hawaii. Nasturtium is a widely grown herbaceous ornamental in Hawaii. It could be harboring BYMV in the off-season and moving it to cultivated legume plants. Therefore, further study is needed to determine whether legume aphids can transmit BYMV from nasturtium to healthy legume plants. In addition, further studies on the distribution of BYMV-infected nasturtium and identification of other BYMV-infected plants in the Hawaiian Islands are needed.