Ramu stunt, an important new sugarcane disease in Papua New Guinea

Planthopper Transmission of Ramu Stunt Virus, a Tenuivirus Causing the Sugarcane Disease Ramu Stunt, and its Distribution in Papua New Guinea

Ramu stunt is a serious disease of sugarcane, currently only reported from Papua New Guinea. It is found in both commercial sugarcane grown on the Ramu Agri Industries Limited (RAIL) estate and in chewing canes (Saccharum officinarum L.) grown in village gardens. The vector of Ramu stunt disease is the island sugarcane planthopper, Eumetopina flavipes Muir. Here we report on the successful transmission of Ramu stunt using E. flavipes and verify that the disease is caused by Ramu stunt virus, a virus with homology to the genus Tenuivirus. Diagnostic reverse transcription PCR screening, with partial genome sequencing and viral protein characterization, was used for confirmation. Disease surveys were undertaken on the RAIL estate, along roadsides, and in village gardens in parts of Papua New Guinea. When the disease was identified, partial genome sequencing of the virus was performed to assess the extent of genome variability among isolates. The disease was less common than predicted from early surveys based on symptoms alone, and genotypic variation was associated with geographic location.

Detection by next generation sequencing of a multi-segmented viral genome from sugarcane associated with Ramu stunt disease

Ramu stunt disease of sugarcane was first reported in Papua New Guinea in the mid 1980s. The disease can reduce sugarcane yields significantly and causes severe stunting and mortality in highly susceptible cultivars. The causal agent of Ramu stunt has been investigated but its characterization has not been completed. Sugarcane cv. Ragnar from Papua New Guinea with symptoms of Ramu stunt was analyzed by next generation sequencing. Total RNA was extracted and whole transcriptome shotgun sequencing was performed using an Illumina platform. Over thirty-seven million reads with an average length of 100 nucleotides were obtained. More than eighteen thousand contigs were assembled and subjected to BLASTX analysis. Twenty-one contigs were virus related and six were associated with plant viruses. The BLAST algorithms revealed sequence similarity to Tenuivirus and Phlebovirus, genera of viruses whose members contain genomes consisting of multiple RNA segments. The six contigs derived from the RNA sequencing data correspond to six RNAs that compose the Ramu stunt virus genome. Primers were designed for each of the six RNAs and RT-PCR amplicons were obtained only from the symptomatic sugarcane. There was concordance between the sequence data of the contigs obtained from the NGS and that of the amplicons obtained by RT-PCR. The NGS approach allowed us to determine the complete genomic sequence of Ramu stunt virus. It is likely that this virus is the causal agent of Ramu stunt disease.

The effect of host structure on the distribution and abundance of the island sugarcane planthopper, Eumetopina flavipes Muir, vector of Ramu stunt disease of sugarcane

The island sugarcane planthopper, Eumetopina flavipes Muir, is the only known vector for Ramu stunt disease of sugarcane. This study examined the relationship between host plant distribution and abundance, and E. flavipes distribution, abundance and levels of population connectivity in Papua New Guinea (PNG), the Torres Strait (TS) and northern peninsula area (NPA) of Australia, as a first step in establishing E. flavipes invasion potential through the TS/NPA. Results show that E. flavipes utilises a wide range of Saccharum host species in PNG and that the occupation rates and abundances differed significantly among host types. For hosts in common, the proportion of plants occupied in PNG was significantly greater than in TS/NPA. This is likely the result of greater overall host density and connectivity in PNG. E. flavipes abundance per plant did not differ significantly between the two regions suggesting a possible plant-specific abundance and/or dispersal threshold independent of location. Whilst E. flavipes presence and persistence was highly variable at some TS/NPA locations, large and stable infestations occurred down the western edge of the TS archipelago. These populations appear to link PNG to the NPA and offer a potential incursion route for Ramu stunt disease. The stability of these populations appears to be associated with the availability and persistence of host material, which in turn is significantly affected by variation in cultivation practices. In the TS/NPA, implementation of pre-emptive management of E. flavipes via cultivation techniques, such as simultaneous tip-pruning, may be an effective means of control for the pest, and would be simpler and preferable to the direct management of Ramu stunt disease should it be detected in the TS/NPA.