Evaluation of the impact of Dasheen mosaic virus on and other viruses on taro yield

Dasheen mosaic virus (DsMV) commonly affects taro throughout the Pacific region though can be observed more readily in some countries compared to others. Despite its widespread occurrence there have been no studies carried out to determine the impact of this virus on yield in the Pacific. Studies elsewhere on the impact of viruses on crop yields have indicated that yield gains can be achieved when plants are free of viruses. The aim of this project was to determine whether any impact on yield occurred when taro was infected with DsMV.
Five taro viruses are known for which sensitive diagnostic techniques are now available, as a result of five years of research during the AusAID-funded Taro Genetic Resources: Conservation and Utilization (TaroGen). The taro viruses are from four taxonomic groups. Dasheen mosaic virus (DsMV) is found everywhere; Taro bacilliform virus (TaBV) occurs throughout the region, often without producing symptoms; two rhabdoviruses occur, with Taro vein chlorosis virus (TaVCV) in both north and south of the Pacific, and Colocasia bobone disease virus (CBDV) confined to Papua New Guinea and Solomon Islands, associated with different diseases depending upon the variety; and Taro reovirus (TaRV), found in New Caledonia, Papua New Guinea, Solomon Islands and Vanuatu. Some of the viruses occur as latent infections, and all can be found in various combinations, making it difficult to ascribe symptoms of infection to a specific project.
As the project participating countries were Fiji and Samoa, taro plants, infected with DsMV (showing visible symptoms), and plants free of DsMV (no visible symptoms) of three accessions from each country were collected. In Samoa locating taro plants with visible symptoms of DsMV was not as easy as expected and as a result, the cultivars which had been identified as desirable to work with, were not available. In Fiji problems were incurred in locating plants that only contained DsMV. Sampling was carried out on two separate occasions and on both occasions, two of the varieties were infected with TaBV. As TaBV is one of the most widespread of the five taro viruses, the project team agreed that further collecting would not be worthwhile. Attempts could be made to remove the virus, but this would require meristem culture and would not necessarily be successful. After consulting the biometrician at the University of the South Pacific, Alafua Campus, as to the complications that might arise with analysing the experiment, it was agreed that the project would proceed with the material that had been collected. The presence of TaBV does add another dimension to the project and could result in more information being available about taro and the impact of viruses on yield.
The six different cultivars from Fiji and Samoa have been screened for the viruses at the tissue culture stage. The Centre for Pacific Crops and Trees implements the same virus testing protocol as used by Queensland University of Technology. As such, all plants are screened at the tissue culture stage for the presence of viruses and then planted in the post-entry quarantine facility, where they are usually re-tested at three month and six month stages. As these plants will be transferred back to the country of origin, and the virus testing is being carried out merely to ensure these viruses are present, virus testing will only be conducted at the three month stage, so each accession will have been tested twice for viruses. While this process is ongoing, the cultivars are also being multiplied in vitro so that when the virus testing has been completed there will be sufficient numbers of plants to start the field experiments. Funds have been transferred to the Ministry of Agriculture and Primary Industries (MAPI) for modification to the screenhouse for the next stage of the project, and MAPI staff appointed to work on the project. Training in virus testing methodologies has also been conducted for the same MAPI staff. Similarly discussions have been held with USP, Alafua Campus, screenhouse modification designs drawn up and staff appointed. The project has suffered significant delays, due to the departure of one of the key SPC staff, namely Dr Richard Davis, the problems in collecting samples in Fiji, and delays in obtaining virus testing reagents from overseas.