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February 2001. Samoa asked about identification of mealybugs, those associated with taro in particular; this was needed for the MSc study into the transmission of taro small bacilliform virus, a putative badnavirus, which if proven is likely to be transmitted by mealybugs.
In EcoPort it is pointed out that “Transmission of CBV has been demonstrated to occur by mealybugs, Pseudococcus longispinus (one occasion, Solomon Islands) and Planococcus citri (six of 50 tests, Rothamstead Experimental Station, UK) (Gollifer et al., 1977). Acquisition feeds were of 6-72 hours, which suggests that they are inefficient vectors.”
Regarding mealy bugs identification, the best way would be to place them in alcohol and have someone handcarry them (to New Zealand). Apparently, it is illegal to mail alcohol. For identification purposes use:
Williams DJ, Watson GW (1988) The scale insects of the tropical South Pacific Region. Part 2. The mealybugs (Pseudococcidae). CAB International, Wallingford. 260 pp.
Williams DJ, Watson GW (1990) The scale insects of the tropical South Pacific Region. Part 3. The soft scales (Coccidae) and other families. CAB International, Wallingford. 267 pp.
Other comments on the transmission of this virus (and the others involved in the alomae/bobone complex), by those involved in the work, mentioned that there were some occasions when transmission of the small bacilliform virus occurred, but the irregularity of transmission suggested, perhaps, other vectors were involved. The problem at the time – at Dala Research Station in the 1970s – was that the plants were not free from virus. Having disease-free insects was also a challenge. Tarophagus, for instance, were collected from Guadalcanal where the taro virus diseases were not known to occur. At that time, there was no access to seedlings or indexed tissue cultured plants.
However, apart from the mealy bug transmissions there was evidence of either: a) latent infection of the small bacilliform or b) Tarophagus was transmitting two viruses: the small bacilliform and also a rhabdovirus. The situation is complex:
1) Taking Tarophagus from plants where only the rhabdovirus was present and transferring them to apparently healthy plants resulted in diseased plants, in which only the rhabdovirus was present (bobone).
2) But taking Tarophagus from plants where both particles were present and transferring them to apparently healthy taro resulted in diseased plants with both viruses (alomae).
These tests were done dozens of times always with the same results; there were many interpretations!
Samoa is unique in that it is now easy to find the small bacilliform particle. This was not the case in 1979 when a survey was done there. It was found only rarely in taro and Alocasia. Its likely that as farmers have introduced taro from elsewhere in their search for taro blight resistant varieties or some of the taro presently grown are more susceptible to infection. The virus seems particularly noticeable in the Philippine variety, which may not have been adequately indexed before introduction.
In recent years, there has been a revision of Tarophagus species on taro. Asche and Wilson (Bull. En. Res. 79:285-298) revised the genus Tarophagus in 1989.Three species are recognised: T. colocasiae (Matsumara), T. persephone (Kirkaldy) and T. proserpina (Kirkaldy). The distribution of these is given in detail. T colocasiae is found from Taiwan, Palau, Guam, Vietnam, Thailand, the Philippines, Malaysia, Indonesia, PNG, Solomon Is, Micronesia, Marshall Islands and Hawaii.
Reference on seedborne viruses:
Maury Y, Duby C, Khetarpal RK (1998) Seed certification for viruses. In: A Hadidi, R K Khetarpal, H Koganaezawa (Eds.). Plant Virus Disease Control, pp. 237-248. APS Press, St. Paul, Minnesota, U.S.A.
There is information on the on TSBV as well as references on the transmission of plant viruses at www.ecoport.org.
Neergaard P (1979) Seed pathology Volumes I and II. The Macmillan Press.
Richardson MJ (1979) An annotated list of seed-borne diseases. Commonwealth Agricultural Bureaux
In a later question from a member, it was asked if any of the viruses known in taro are transmitted by thrips. The answer is that there are none.
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