Introduction

Sugarcane mosaic disease has historically been attributed to a single potyvirus known as Sugarcane Mosaic Virus (SCMV), which was believed to include multiple strains or even represent a complex of closely related potyviruses (Koike & Gillaspie, 1989; Shukla et al., 1994). Early classification of these strains relied mainly on symptom expression in differential host plants and serological behavior.

Evolution of Potyvirus Classification in Sugarcane

Recent advances in molecular virology have redefined the taxonomy of sugarcane-infecting potyviruses. These viruses are now grouped into a SCMV subgroup, which includes four distinct but related species:

  • Sugarcane Mosaic Virus (SCMV)
  • Sorghum Mosaic Virus (SrMV)
  • Maize Dwarf Mosaic Virus (MDMV)
  • Johnsongrass Mosaic Virus (JGMV)

A related virus, Zeam Mosaic Virus (ZeMV), has also been suggested as a potential member of this subgroup based on genomic similarities (Seifers et al., 2000).

Among these, only SCMV and SrMV are naturally infectious to sugarcane, and both are recognized as primary causal agents of sugarcane mosaic disease (Grisham, 2000).

SCMV Strain Diversity and Genetic Variation

Multiple SCMV strains have been identified across different host plants and geographical regions, including:

  • Sugarcane-derived strains: A, B, D, E, SC, Isis, Brisbane
  • Grass-associated strains: IK, Bundaberg, Sabi
  • Maize-associated strains: MDB (formerly MDMV-B)

Phylogenetic analyses based on coat protein gene sequences revealed that sugarcane and maize isolates form two distinct monophyletic groups (Alegria et al., 2003).

SrMV, another important member of this subgroup, includes strains such as SCH, SCI, and SCM.

Despite these classifications, sugarcane mosaic has been reported in more than 70 countries, suggesting that the actual diversity of SCMV-related viruses is significantly underestimated (Grisham, 2000). Several strains, including SCMV-C, F, G, K, L (USA) and SCMV-N (India), remain poorly characterized.

Emergence of Sugarcane Streak Mosaic Virus (SCSMV)

Initially, isolates such as SCMV-F were classified as SCMV based on symptom expression in indicator plants. However, molecular studies later demonstrated that these isolates were genetically distinct and should not be classified as SCMV.

This led to the identification of a new virus species:

Sugarcane Streak Mosaic Virus (SCSMV) (Hall et al., 1998)

Subsequent studies confirmed another isolate in India (SCSMV-AP), showing high sequence similarity (93.6%) to the Pakistani isolate, further validating SCSMV as a distinct viral species (Hema et al., 1999a).

These findings demonstrated that sugarcane mosaic symptoms in Asia are caused by at least two distinct virus species:

  • SCMV
  • SCSMV

The main objective of this research was to identify the causal agents responsible for mosaic symptoms in sugarcane samples collected from:

  • Bangladesh
  • India
  • Pakistan
  • Sri Lanka
  • Thailand
  • Vietnam

The study aimed to determine whether SCMV, SCSMV, or mixed infections were present in these regions.

Materials and Methods

Virus Isolates and Plant Material

Field samples of symptomatic sugarcane leaves were collected from multiple Asian countries and maintained under controlled conditions at −80°C.

Virus Purification and Electron Microscopy

Partially purified viral extracts were prepared from infected plant tissue following standard protocols . Samples were observed under electron microscopy using leaf-dip preparation techniques.

RNA Extraction and RT-PCR Detection

Total RNA was extracted using the RNeasy Plant  Kit. Virus detection was performed using RT-PCR with specific primer sets targetin.

Amplified products were analyzed using agarose gel electrophoresis.

Cloning and Sequencing

Selected RT-PCR fragments were purified, cloned into pGEM-T Easy vectors, and sequenced using automated capillary sequencing systems. Sequences were analyzed using BLAST for molecular identification and comparison with GenBank references.

Mechanical Inoculation

Virus transmission experiments were performed on:

  • Sugarcane cultivars
  • Sorghum
  • Maize

Plants were mechanically inoculated using carborundum abrasion and maintained in controlled greenhouse conditions.

Results

Symptom Expression in Host Plants

Inoculated plants developed mosaic symptoms within 1–3 weeks depending on virus isolate. Symptom severity varied significantly:

  • SCMV CAM6/1 and SrMV isolates → severe mosaic
  • SCSMV-related isolates → mild to moderate mosaic patterns

Electron Microscopy Observations

All isolates produced flexuous filamentous particles typical of the Potyviridae family. However, differences were observed in particle abundance and length:

  • SCMV/SrMV: ~750 nm, abundant particles
  • SCSMV isolates: variable length up to ~810 nm, less abundant

RT-PCR Detection Patterns

Molecular analysis revealed clear differentiation:

  • SCMV-specific primers → positive only for SCMV isolates
  • SrMV-specific primers → positive only for SrMV isolates
  • SCSMV primers → positive for most Asian field isolates

Notably, one Pakistani sample showed mixed infection (SCMV + SCSMV).

Sequence Analysis

Sequencing of five SCSMV isolates showed:

  • 97–99% identity with Pakistani SCSMV isolate
  • 93–95% identity with Indian SCSMV-AP isolate

This confirms the presence of distinct SCSMV strains across Asia.

Discussion

Reevaluation of Sugarcane Mosaic Etiology

The study confirms that sugarcane mosaic disease in Asia is not caused solely by SCMV. Instead, SCSMV is a major and widespread causal agent, often misidentified in earlier studies.

Geographical Distribution

SCSMV was detected in all examined countries and is reported here for the first time in Vietnam, indicating a much wider distribution than previously known.

Mixed Infections

The detection of SCMV and SCSMV co-infections suggests complex viral interactions that may influence disease severity and epidemiology.

Taxonomic Implications

Phylogenetic evidence indicates that SCSMV may represent a distinct evolutionary lineage within the Potyviridae family, potentially even a new genus.

Conclusion

This study demonstrates that:

  • Sugarcane streak mosaic virus (SCSMV) is widely distributed in Asia
  • It is a major causal agent of sugarcane mosaic disease
  • Mixed infections with SCMV occur in some regions
  • SCSMV exhibits significant genetic diversity across countries

The disease formerly classified as sugarcane mosaic should now be more accurately described as:

Sugarcane streak mosaic disease (SSMD) when caused by SCSMV.

Further research is required to fully characterize the genetic diversity, epidemiology, and global distribution of SCSMV and related strains.