Photo 1. First signs of alomae on the mother plant and suckers (Malaita, Solomon Islands). Note the stunted, tightly rolled, yellow leaves.
Photo 2. Stunting on the mother plant and suckers probably caused by alomae (Madang, Papua New Guinea). In this case the plant has stayed green.
Photo 4. Plant destroyed by alomae: one live shoot and many dead leaves remaining (Madang, Papua New Guinea).
Photo 5. An outbreak of alomae. Note the collapse of the older leaves, and the young ones stay rolled. It is similar to a wilt. These symptoms are typical of an alomae epidemic on plants that had been growing rapidly.
Photo 6. Possibly bobone disease (the plant seems to be recovering) on a sucker (Madang, Papua New Guinea). Note the galls on the petiole, or leaf stalk.
Photo 7. Bobone on the "female" taro variety Oga showing stunted distorted leaves (Malaita, Solomon Islands).
Photo 8. Typical symptoms of bobone with stunted, twisted green leaves (Madang, Papua New Guinea). The plant will recover from these symptoms producing leaves that look healthy, but the plant will remain infected by the virus.
Photo 9. Galls on the leaf stalk, petiole, of a plant that is probably in the early stages of alomae (Madang, Papua New Guinea). Early stages of alomae and bobone are similar, and unless the variety is known it is not possible to tell which disease is present.
Photo 10. Taro vein chlorosis virus in taro from (Tanafoli, Vanuatu). It is common to find leaves with infections on part of the leaf with edges rolled down. The symptom is very similar to that of Taro badnavirus, except that the colour of the veins is brighter.
Photo 11. Taro badnavirus showing a vein chlorosis symptom on a plant in (Safaatoa, Samoa). Compare with the symptoms of Taro vein chlorosis virus (Photo 8).
Photo 12. Symptom of Dasheen mosaic virus in taro; notice the pale green feather-like pattern between the leaf veins. Often these patterns show along the main veins.
Photo 15. Nymphs, winged and wingless adults of Tarophagus sp., the planthopper that spreads Colocasia bobone disease rhabdovirus, and most probably Taro vein chlorosis virus, another rhabdovirus.
Alomae and bobone
Several viruses have been identified in plants with these diseases, but it is not yet certain which ones are the cause. The viruses associated with these diseases are: Colocasia bobone disease rhabdovirus (CBDV); Taro vein chlorosis rhabdovirus (TaVCV); Taro badnvirus (TaBV); Dasheen mosaic potyvirusvirus (DsMV). It recent years a tenuivirus has been detected, as well as genome sequences of Colocasia bobone disease-associated virus (CBDaV), but it is not yet clear if this is the same as CBDV.
Narrow. Alomae and bobone occur in Papua New Guinea and Solomon Islands. The distribution of the viruses is as follows: i) Colocasia bobone disease rhabdovirus occurs only in Solomon Islands and Papua New Guinea; ii) Taro vein chlorosis virus is recorded in Fiji, Federated States of Micronesia, New Caledonia, Palau, the Philippines, Samoa, Solomon Islands, and Vanuatu; and Taro badnavirus and Dasheen mosaic virus are present worldwide. The undescribed tenuivirus has been recorded from Papua New Guinea and Solomon Islands.
Note, Taro vein chlorosis virus in Vanuatu is similar to that of Samoa, but differs from that recorded in Fiji. The relationship of Taro vein chlorosis virus elsewhere to these two strains in not known.
Colocasia bobone disease virus and Taro vein chlorosis virus have only been recorded from taro (Colocasia esculenta); Taro badnavirus has been recorded from taro, Xanthosoma sagittifolium (tannier) and Alocasia macrorrhizos (giant taro). Dasheen mosaic virus occurs in all the edible aroids (taro, giant taro, giant swamp taro, Xanthosoma), and in many ornamental aroids.
Plants with alomae show a variety of symptoms. (Photos 1-5&9). The leaves are shorter than normal with distorted leaf blades, but still green, or they can show stunted young leaves which remain rolled, twisted and pale yellow. Galls may be present on the petioles of the distorted leaves. Soon after the initial symptoms, plants stop producing leaves and gradually die as the plants rot.
Plants with bobone are similar to alomae at first: the leaves remain green, but are stunted, thickened, curled or partly curled, and there are galls, but plants gradually recover (Photos 6-8).
Plants that die from alomae are called 'male' on Malaita, Solomon Islands, and those that are resistant, but show bobone are called 'female' (perhaps 5 or 6 varieties).
Insects spread the viruses that are found in alomae and bobone:
The insects suck up the viruses as they feed on diseased plants, and spread the viruses as they move to healthy plants and feed.
The viruses are also spread in planting material. Often, the viruses do not show symptoms; in this case they are said to be 'latent'. For instance, most plants examined contain Taro badnavirus and Dasheen mosaic virus without showing symptoms, or only rarely showing symptoms. However, the viruses inside the plant pass from mother plants to their suckers.
In the same way, it is likely that all 'female' taro are infected with the Colocasia bobone disease rhabdovirus. Sometimes bobone occurs after planting, when plant hoppers are absent. Possibility, stress at planting causes the virus to multiply and then symptoms occur.
Alomae is also spread in planting material. If infection occurs near harvest, plants may be used for planting the next crop as they still look healthy.
Alomae is a lethal disease that affects nearly all varieties of taro. On Malaita, Solomon Islands, it is very difficult to grow "male" taro in the lowlands. In recent years, the disease has spread to other islands in Solomon Islands with devastating results, such as the Weather Coasts of Guadalcanal and Makira. It is not unusual for entire gardens to be destroyed by alomae. Farmers want to grow "male" taro for their size and also there are many varieties so they have a choice of tastes and textures.
Bobone does not kill taro; it reduces the yield by about 25 per cent. Plants usually start to produce healthy leaves after 4-6 weeks, and then appear normal. The disease only occurs in a few varieties of taro. So where alomae exists, farmers have to grow "female" taro which are lower yielding.
Symptoms of other taro viruses on their own are often mild, and do little damage: see, (i) Taro vein chlorosis virus (Photo 10), (ii) Taro badnavirus (Photo 11), and (iii) Dasheen mosaic virus (Photos 12&13).
It is important to inspect taro gardens regularly. Look at the youngest leaves. If a plant has alomae or bobone, the youngest leaf will be stunted, slightly curled under, and crinkled. Sometimes, the young leaves with alomae are paler than older leaves, but not always so. Usually, the young leaves with bobone are green, thickened and distorted, perhaps with prominent galls on the petioles. However, it is difficult to tell alomae and bobone apart at an early stage.
Later, if plants have alomae, they will stopped producing leaves and start to die, whereas plants with bobone will produce 4-6 distorted leaves, and then leaves which look healthy.
Note, that there are many countries in the Pacific vulnerable to these diseases, so where they are not yet present (everywhere except Papua New Guinea and Solomon Islands), biosecurity authorities should be alert to potential pathways for their introduction. If countries want to introduce germplasm for testing, they should follow the FAO/IBPGR Technical Guidelines for the Safe Movement of Edible Aroid Germplasm (http://www.bioversityinternational.org/e-library/publications/detail/edible-aroid/). This stipulates that introductions should be as virus-tested plants growing in tissue culture.
Cyrtorhinus fulvus, a bug that feeds on the eggs of Tarophagus species (Photo 14), 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 (Photos 15&16).
Alomae is best controlled by cultural methods. It is very important that farmers:
Farmers should do the following:
Regular spraying with a synthetic pyrethroid insecticides will kill Tarophagus and reduce alomae. It will not prevent bobone as plants are already infected.
AUTHORS Helen Tsatsia & Grahame Jackson
Photo 14 Mike Furlong, University of Queensland, Australia.
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.
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