Taro virus survey Tonga 2-6 July 2002 Introduction This brief report by Peter Revill and Grahame Jackson summarises fieldwork in Tonga on taro viruses. The Queensland University of Technology is carrying out a regional survey in support of research commissioned by ACIAR to detect, isolate and develop indexing methods for taro viruses to facilitate the international transfer of germplasm. The project works in collaboration with TaroGen, a project funded by AusAID to conserve taro in Pacific Island countries and breed for resistance to leaf blight. Taro appears to have had a renaissance in Tonga in the wet years since the last El Nino and much is now grown. The dominant variety is Lau ila, (Dalo ni Tonga in Fiji). This variety is very susceptible to taro leaf blight so there is need to consider how the industry can be safeguarded from this disease, other than by quarantine regulations prohibiting the unrestricted movement of planting material from countries where the fungus is present. Farmers appear to have little knowledge of the disease or the devastation is caused in Samoa following its introduction in 1993. Tongatapu The only virus-like symptoms seen on Colocasia were those of DsMV, which were invariably present, although not common, and 1-2 mm chlorotic spots on breeders lines at Vaini Research Station of unknown cause. Typically, plants with DsMV had feather-like patterns along veins, but no distortions. Similar symptoms were found on young plants of an eddo (sato imo type) introduced by Afeaki from Japan and growing under irrigation near Vaini. Some of these plants also showed a faint mottle. Symptoms of DsMV were also found on Alocasia, mostly as indistinct patches of chlorosis, more common in the central region than the margins (see Fig. 3). Fig. 1 Symptoms of DsMV associated with marginal distortions, near Vaini Fig. 2 Xanthosoma with diffuse feather-like patterns along main and secondary veins, Tatakamotonga, Tongatapu Symptoms possibly caused by DsMV were more varied in Xanthosoma than in the other species. They ranged from typical feather patterns, with or without marginal distortions (Fig. 1 & 2), to more unusual ring spots (seen also in Vanuatu). Some
plants showed little leaf symptoms, but there was no evidence that these were associated with DsMV, and may have been caused by herbicide. Vava u Taro were inspected at the MAF station Fatai and in the following villages: Mataika, Leimatu a, Maiva, Tefisi and Longomapu. DsMV was found in Colocasia, commonly in Alocasia, but more rarely in Xanthosoma. Interestingly, Alocasia invariably showed symptoms of DsMV, in contrast to the other species, with indistinct patterns along the main veins, especially in the centre of the leaves (Fig. 3). In rare instances, bright feather-like patterns were seen on stunted, unfurled, twisted leaves (Fig. 5). However, it appeared that the leaves expanded normally, but remained short. Leaves emerging later appeared healthy. However, Soane Taula (Research Officer, MAF) said that these plants invariably died at maturity. At Leimatu a, leaves of Alocasia also showed large irregular blotch patterns, more obvious on leaves facing the sun and viewed from the underside (Fig. 5). This is the first time that such symptoms have been reported from Alocasia in Tonga or elsewhere. Fig. 3 Typical DsMV symptoms in Alocasia: diffuse patterns at the centre of the leaf Fig. 4 Distorted, stunted Alocasia leaf, with DsMV feathering, Leimatu a, Vava u Fig.5 Underside of Alocasia leaf with chlorotic patterns, Leimatu a, Vava u Fig. 6 Kava showing collapse of terminal shoot (arrowed), Vaini, Tongatapu
Other diseases Kava wilt Kava dieback was seen at two locations, near Vaini Research Station, Tongatapu and at MAF station, Fatai. Plants at Vaini showed a conspicuous stem break some 30 cm from the shoot tip (Fig. 6), accompanied by internal necrosis. Symptoms on Vava u were like those seen on kava of a similar variety in Fiji. Stems appeared to be defoliating and dying back, but not breaking as at Vaini. However, in both cases, mosaics, mottles and vein clearing could be found on the younger leaves. Marginal necrosis Xanthosoma, Tongatapu Near Vaini, Xanthosoma were seen with a severe marginal chlorosis that extended several cms to the centre of the leaf (Fig. 7). A similar problem has been noted on Colocasia in New Caledonia thought to be caused by Cladosporium. It is more likely to be an infection cause by Xanthomonas. Attempts to isolate the bacterium should be made in Tonga. Fig. 7 Marginal necrosis in Xanthosoma, possibly caused by bacterial infection, Vaini district Banana wilt Leimatu a (Sine Nafe) Several rows of plantains about 2-3-months-old were wilting (Fig. 8 & 9). They were taken from an adjacent block of older plants, some of which were showing similar symptoms. On one, the emerging leaf was still furled, torn and appeared to have stopped growing. Four of the younger plants were dug up and examined. There were no internal symptoms, except for weevil (Cosmopolites) infection in one plant. The lateral roots were decayed, but the cause of the problem was not obvious, and isolations are needed. One of the plants had symptoms similar to Radopholus on the outside of the bole, but the wilt is not likely to be caused by this nematode. Another possibility is Marasmiellus inoderma, but this seems unlikely too, especially as it has been dry in Vava u in recent months.
Fig. 6 A row of 2-3 month-old wilted banana plants (plantain variety) at Leimatu a, Vava u Fig. 7 Banana wilt at Leimatu a, Vava u showing wilt of lower leaves Pratylenchus in Dioscorea alata Farmers may not recognise symptoms of Pratylenchus coffeae on yam planting setts and are unwittingly spreading the nematode. They need information on the importance of the nematode, and the need to remove infections prior to planting. In one field inspected at the time of planting (owned by a member of MAF), infected setts were common (Fig 8). Fig. 8 Pratylenchus infection (arrowed) in planting setts of Dioscorea alata Acknowledgements We would like to thank the following for their assistance during the mission: we are particularly grateful for the assistance of Pita Taufatofua, Deputy Director, MAF, for arrangements, and to those who accompanied us on Tongatapu and Vava u. Pita Taufatofua, Deputy Director and Head of Research-Extension Division Lampetti Havea, Principle Agricultural Officer, Eastern District, Nulu alofa To ifalefehi Moala, Senior Agricultural Officer, Western District, Nulu alofa Paelata Vi Nai, Senior Agricultural Officer, Nulu alofa Sela Tupouniua, Senior Plant Pathologist, Nulu alofa Luseane Taufa, Senior Agricultural Officer, Nulu alofa Leody Vainikolo, Officer in charge, Vava u
Soane Taula, Senior Agricultural Officer, Vava u Hema Tu uheava, Extension Officer, Vava u Afeaki, Executive Chairman, Lau Lava Ltd, Nulu alofa