Banana bunchy top disease (BBTD), caused by the banana bunchy top virus (BBTV), stands as the most economically important viral disease affecting bananas (Musa spp.). First recorded in Fiji in 1880, the virus was later studied in detail in the 1920s and 1930s, revealing banana aphids as vectors (Magee, 1927). BBTV has historically devastated banana cropping areas in countries like Australia, Malawi, Pakistan, and India, and its spread persists, as evidenced by recent cases in East Africa (Shimwela et al., 2022; Kesavamoorth, 1980; Thomas et al., 1994a; Ocimati et al., 2021). In the Philippines, BBTV previously devastated banana cultivar ‘Lakatan’ (AAA) bananas in Luzon (Molina et al., 2009) and now threatens banana cv. ‘Saba’ (BBB/ABB) in CALABARZON region. In 2021, cv. Saba farms in Batangas were affected by BBTD (Mendoza et al., 2021), which is particularly concerning given cv. Saba’s known tolerance to the virus. The virus is a member of the species Babuvirus musae, genus Babuvirus in the family Nanoviridae, and has a genome comprising six circular ssDNA components, each separately encapsidated in a 17-19 nm diameter isometric virion (Thomas et al., 2021). These components comprise DNA-R (master Rep), DNA-S (coat protein), DNA-C (cell cycle link protein), DNA-M (movement protein). DNA-N (nuclear shuttle protein) and DNA-U3 encoding a protein of unknown function. Some isolates also contain autonomously replicating, independently, encapsidated alphasatellites, which may act by depressing viral replication and also possibly diverting antiviral RNAi components away from the host virus (Guyot et al., 2022).

Symptoms of BBTD in bananas include leaf marginal chlorosis, dark green streaks on the petiole, distinctive dark green dot and dash flecks on leaves, and a ‘bunchy appearance’ marked by narrow leaves and erect growth as the disease progresses (Thomas et al., 1994b; Magnaye and Valmayor, 1995; Hooks et al., 2008). The exclusive vectors of BBTV are two aphid species: Pentalonia nigronervosa (banana aphid) and P. caladii (cardamom aphid) (Watanabe et al., 2013; Thomas, 2008; Foottit et al., 2010). Although the banana aphid can carry the virus throughout its lifespan, it cannot transmit it to its offspring (Magee, 1940; Hu et al., 1996).

Reported hosts of BBTV mainly belong to the Musaceae family and a few related species, including M. balbisiana, M. jackeyi, M. acuminata, M. textilis, M. coccinea, M. velutina, and Ensete ventricosum (Wardlaw, 1961; Magee, 1927; Magee, 1948; Thomas and Dietzgen, 1991; Espino et al., 1993; Eusebio and Bajet, 1994; Furuya et al., 2003). In closely related families in the Zingiberales, Alpinia zerumbet, A. galangal, C. longa (Zingiberaceae), C. indica (Cannaceae), and Heliconia spp. (Heliconiaceae) (Pinili et al., 2013; Hamim et al., 2017; Rahayuniati et al., 2021) are reported as alternative hosts of BBTV. Additionally, in the order Alismetales, C. esculenta (family Aracaceae) is identified as an alternative host of BBTV (Pinili et al., 2013). Recognizing alternative hosts is crucial, as they can act as reservoirs and sources of inoculum for the virus, necessitating their inclusion in mitigation efforts against BBTD. Neglecting these hosts may lead to long-distance spread through the unrestricted transfer of infected planting material. Managing BBTV spread is challenging, as infected banana plants typically do not recover. Besides insect transmission, no other modes of transmission for BBTV have been reported, and mechanical transmission using the virus sap is deemed not possible (Thomas et al., 1994b). This study specifically aimed to investigate the potential of ornamental Musa and related species in the Philippines to serve as alternative hosts for BBTV.

Alternative hosts aid in the spread and survival of pathogens. Reported alternative hosts of BBTV belong to the Zingiberales families Musaceae, Cannaceae, Zingiberaceae, and Heliconiaceae and also the family Araceae in the Alismetales (Thomas, 2024). These families have a diverse set of species that are yet to be tested. Hence, this study evaluated common ornamentals in the Philippines predominantly from the order Zingiberales and also from the order Alismetales.

The study confirmed the susceptibility of M. coccinea to BBTV as reported by Thomas (2019) and also recently in Indonesia of M. velutina (Thomas and Dietzgen, 1991; Rahayuniati et al., 2021). On the other hand, this is the first report of M. laterita (bronze banana) as a new alternative host of BBTV. The presence of BBTV in C. indica and C. longa test species corroborates the findings of Pinili et al (Pinili et al., 2013; Su et al., 1993). and Rahayuniati et al (Rahayuniati et al., 2021), respectively. C. longa is a root crop producing rhizomes commonly known as turmeric. Canna indica is a known alternative host of BBTV (Su et al., 1993; Eusebio and Bajet, 1994) and has common names including wild canna lily, Bandera Espanola, edible canna, and Indian shot. It is a perennial herb propagated using seeds and rhizomes. Ornamentals M. velutina, M. coccinea, M. laterita, C. indica, and C. longa are widely cultivated in the Philippines as landscape plants. These plants are purchased both online and in physical markets and their distribution could result in the long-distance spread of BBTV-infected planting materials within the Philippines through unregulated transfer. The successful aphid transmission of BBTV from infected C. indica to banana cv. Lakatan proves that these plants may act as sources of inoculum for banana in the presence of the banana aphid. This report of alternative hosts will benefit the mitigation efforts in this country against BBTV.

In the present work, Etlingera elatior, Alpinia purpurata, Adelmeria gigantifolia, Alpinia luteocarpa (Zingiberaceae), Caladium bicolor (Araneae), Heliconia sp. (Heliconiaceae), Canna flacida, Canna compacta (Cannaceae), and Costus afer (Costaceae) appeared to be non-hosts of BBTV in the Philippines. In a report by Su et al (Su et al., 1993), H. coronarium is also claimed as a non-host of BBTV. In other reports, BBTV was detected in Alpinia species including A. zerumbet (Pinili et al., 2013), A. purpurata (Wong et al., 2019) and A. galangal (Rahayuniati et al., 2021). Natural infection by BBTV in Heliconia sp. collected around banana-growing areas was recently reported in Hawaii (Hamim et al., 2017). However, samples of A. purpurata and Heliconia sp. in this study were negative after multiple rounds of screening. The reason for these discrepancies is not clear, but it seems that the research indicating the individual species as host or non-host was soundly conducted. This situation may result from the particular variety of plant used, the particular isolate of the virus used, or differences in the efficiency of transmission and host preferences of the two known aphid vectors, P. caladii and P. nigronervosa. Many earlier studies were conducted before these aphids were recognized as two distinct species, so it is unclear which of the two were used.

This wide range of alternative hosts seems to be encompassed within the host ranges of its oligophagous vectors, P. nigronervosa and P. caladii including mainly members of the Musaceae, Cannaceae, Araceae, Zingiberaceae, and Heliconiaceae (Foottit et al., 2010; Bagariang et al., 2019). Generally, P. nigronervosa favors Musa hosts and P. caladii the non-Musa hosts though there is some overlap (Foottit et al., 2010; Bagariang et al., 2019). This situation opens the possibility for BBTV to be transmitted between these hosts, depending in part on feeding preferences of the two aphid species and the range of co-located plant species in an area.

The BBTV genome itself may play a role in the virus’ ability to infect a variety of hosts, though no specific sequence differences have yet been identified. Several mechanisms may contribute to host adaptability and overall viral fitness. BBTV possesses a multipartite genome, with each of its genome components encapsidated in a separate particle. These components can be expressed independently. This genome organization may provide a selective advantage allowing the virus to rapidly adapt to new hosts and environmental conditions (Foottit et al., 2010; Bagariang et al., 2019; Bashir et al., 2022). Additionally, BBTV has the ability to recombine both inter- and intra-component, enhancing its genetic diversity and evolutionary potential (Fu et al., 2009; Stainton et al., 2015).

Seed transmission of BBTV was an unexpected discovery. There are currently no records of seed transmission of BBTV or indeed any other members of the Nanoviridae. It has long been considered that phloem-limited viruses are not seed transmitted, owing to the lack of a direct vascular connection to the seed embryo, whereas some mechanically transmissible viruses capable of non-vascular cell-to-cell movement may be seed transmitted (Bennett, 1940). Recent work with begomoviruses, some of which are considered to be phloem-limited, has brought this general assumption into doubt. However, some individual isolates of these viruses, for example tomato leaf curl New Delhi virus, are mechanically transmissible, and in mixed infections with non-transmissible isolates can confer mechanically transmissibility to the latter (Renukadevi et al., 2022). It is unclear how an apparently exclusively phloem-limited virus such as BBTV could reach the seed embryo. The outer integument of the seed, containing the seed’s maternal phloem is isolated from the inner integument, endosperm and embryo, with no symplastic connection. The embryonic suspensor cells are thought to be the route for virus invasion of the embryo, but there is no vascular connection and these symplastic connections are broken prior to fertilization when the suspensor cells undergo programmed cell death (Renukadevi et al., 2022). C. indica is partially self-fertile (autogamous), partially cross-pollinated (Sibiya, 2018), leaving open the possibility that infected seeds could also arise through cross-pollination via pollen from a BBTV-infected male parent. However, as with ovule infection, the mechanism that would allow the infection of male gametes with a phloem-limited virus is unclear.

BBTV was detected in the seed coat + endosperm and in the embryo. However, an infected embryo does not always guarantee a virus-infected seedling (Sastry, 2013). Thus, in our work, embryo infection was also supported by PCR detection of BBTV in 28% and 34% of the seedlings from the two seed samples from BBTV-infected C. indica, and further by back inoculation of BBTV from infected C. indica seedlings to cv. Lakatan banana.

It is probable that seed transmission of BBTV has not been studied or noted before as edible bananas produce parthenocarpic (seedless) fruit. While wild bananas produce seeds, BBTV-infected bananas rarely produce a bunch. Wild bananas have also not been extensively studied with respect to BBTV infection.

This study highlights two alternative means of dispersal of BBTV which may be significant factors in the geographic spread and the maintenance of reservoirs of BBTV: non-Musa hosts and seed transmission. BBTV-susceptibility and seed transmission of BBTV in C. indica are important concerns to the banana industry, as this plant provides a source of infection through direct aphid transmission and also through vegetative propagation via infected rhizomes or seed.

There are now numerous examples of susceptibility to BBTV in alternative hosts, especially within the order Zingiberales. Thus, exploration of other species in the group may reveal additional hosts and advance epidemiology and mitigation studies for BBTV. The findings of this study are significant with the growing demand for ornamentals in the Philippines today. Many growers now propagate ornamental plants at home to avoid a series of quarantine restrictions. Since the wider community is generally not familiar with BBTD, uncontrolled movement of these ornamentals poses a threat to the banana industry. Awareness of the need for responsible propagation and sale of reported alternative host plants must be promoted. Elimination of these species around banana cropping areas may also aid BBTV mitigation measures.

M. velutina, M. coccinea, M. laterita, C. indica, and C. longa are alternative hosts of BBTV in the Philippines. These plants may act as reservoirs and sources of the virus in banana growing areas and thus may lessen the efficiency of mitigation practices if they are neglected. Furthermore, BBTV was shown to be seed-borne in the ornamental plant, Canna indica. Seed and grow-out seedling samples of naturally and artificially infected plants were positive for BBTV based on PCR assays. BBTV was also transmitted by the aphid vector from seed-transmitted C. indica seedlings to banana, demonstrating that C. indica is not a “dead-end” host of BBTV and could play a role in the epidemiology of BBTV. The discovery of a new mode of transmission of BBTV is advantageous to mitigation efforts against BBTV. Consideration should be given to the inclusion of the identified alternative hosts in the formulation of quarantine policies regarding the safe movement of the plant hosts. It would be valuable to further explore species belonging to order Zingiberales as potential alternative hosts to BBTV, especially crop species grown in banana cropping areas.