Photo 1. The dark green distorted area on the leaf is typical of CBDV in the "male" taro, the common type of taro in the Pacific islands. This is not a serious disease as only 1-2 leaves are affected.
Photo 2. Bobone on the "female" taro variety Oga showing stunted distorted leaves (Malaita, Solomon Islands) after infection by CBDV.
Photo 3. Leaf infected with TaVCV showing the yellowing is along the smaller veins giving a feather-like symptom.
Photo 4. Leaf with symptoms of TaVCV. Note the yellow feather patterns are starting to decay as the leaf ages; this does not happen with Dasheen mosaic virus.
Photo 5. Feather like pattern on a leaf infected with TaVCV. Note the insects on the leaf are Tarophagus sp., which are likely to spread this virus.
Photo 6. Rod-shaped virus particles of TaVCV in a taro leaf. The virus particles can be seen lengthways and end on.
Bobone and an unnamed disease
Colocasia bobone rhabdovirus (CBDV) and Taro vein chlorosis virus (TaVCV).
Narrow. CBDV occurs in Papua New Guinea and Solomon Islands. TaVCV occurs more widely. It has been recorded from Federated States of Micronesia, Fiji, New Caledonia, Papua New Guinea, the Philippines, Samoa, Solomon Islands and Vanuatu. There is evidence that TaVCV in Fiji and Samoa are different strains.
These viruses have only been found in taro.
CBDV. This virus causes different symptoms depending on the variety of taro. In most varieties (so-called 'male' taro, which are susceptible to alomae, a lethal disease in Papua New Guinea and Solomon Islands) it causes small dark green distorted patches on one or two leaves and then healthy leaves are produced (Photo 1). In so-called 'female' taro (those that have resistance to alomae) the virus causes bobone (Photo 2). In this case, the plants are grossly distorted with short, thickened, twisted leaves that stay green. Gradually plants recover and leaves are produced that look healthy. (See Fact Sheet no. 1.)
TaVCV. Symptoms of this virus are seen in all varieties of taro, in all the countries where it is present. The virus produces a yellow feather-like pattern similar to Dasheen mosaic virus (see Fact Sheet no. 88), but in the case of TaVCV, the yellowing is much brighter (Photos 3-5). Also, as the leaf ages the yellow lines turn brown (Photo 4). Symptoms of TaVCV occur on plants with bobone and also with alomae, but it is unlikely that this virus is involved in either of these diseases.
Spread of CBDV and TaVCV is by planthoppers, Tarophagus species (Photo 7). The planthoppers suck up the viruses when they feed on sap. The viruses multiply inside them, and after 3-4 weeks are able to spread the viruses as they feed.
CBDV and TaVCV are also spread in other ways:
These viruses are relatively unimportant alone, but it is their association with other viruses that is of interest and concern. It is very likely that CBDV is involved with other viruses in alomae. Knowing what causes alomae is important if the disease is to be managed properly
Little is known about the impact of CBDV or TaVCV on the yield of "male" taro. Usually, only one or two leaves are affected before healthy-looking leaves develop, so it is likely that any impact is small. By contrast, the impact of CBDV on "female" taro is known. It reduces the yield of individual plants by about 25%. Plants usually start to produce healthy leaves after 4-6 weeks, and then appear normal. However, the disease only occurs in a few varieties of taro, and not all the plants develop bobone in any one planting, so the effect on yield is probably quite small, less than 10%.
Look for leaves showing irregular, thickened patches which are dark green on "male" varieties, or show more extensive, stunted green twisted leaves on "female" varieties (CBDV). Look for leaves showing bright yellow, feather-like symptoms along the main veins, and produce healthy leaves after one or two with symptoms (TaVCV). Under the electron microscope the virus particles are rod shaped (Photo 6).
Cyrtorhinus fulvus, a bug that feeds on the eggs of Tarophagus species (Photo 8), reduces the population of the plant hopper, but experience shows that it is not enough to stop the spread of alomae when Tarophagus populations are high (Photo 9).
At present, the cause of alomae is unknown; it can only be managed as follows:
AUTHORS Helen Tsatsia & Grahame Jackson
Photo 6 Rothamsted Research, Harpenden, UK.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
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