4.1 The Taro Beetle
4.2 Taro Leaf Blight
4.3 The Alomae/Bobone Virus Disease Complex
4.4 Dasheen Mosaic Virus Disease (DMV)
4.5 Other Diseases and Pests
Taro production in the Asia/Pacific region is currently under the stranglehold of one pest (the taro beetle) and one disease (the taro leaf blight), both of which are proving to be extremely menacing to the taro industry. Some countries such as Fiji have only the beetle; others such as Samoa have only the leaf blight; others such Papua New Guinea have both; while yet others such as Tonga have so far escaped either of these two afflictions. In most places where they have occurred, the beetle and the blight have posed serious problems for the taro industry. Their presence has resulted in quarantine isolation for some of the affected countries, and their resultant exclusion from the export taro trade. The most dramatic recent example has been Samoa, where the appearance of the taro leaf blight since 1993 has not only wiped out the lucrative taro export trade, but also seriously destabilised the internal food supply.
A third, but slightly less menacing, affliction of taro is the alomae/bobone virus disease complex. The characteristics of this and other diseases and pests of taro will be described in this chapter. Their presence and impact on the taro industry in various countries will be taken up in the sections where taro production is discussed for each country.
The taro beetles of economic importance are several species belonging to the genus Papuana (Coleoptera: Scarabaeidae). These include Papuana woodlarkiana, Papuana biroi, Papuana huebneri, and Papuana trinodosa. The adult beetle is black, shiny, and 15-20 mm in length. Many species have a horn on the head.
The adult beetles fly from the breeding sites to the taro field and tunnel into the soil just at the base of the taro corm. They then proceed to feed on the growing corm, leaving large holes that degrade the eventual market quality of the corm. Also the wounds that they create while feeding promote the attack of rot-causing organisms. The feeding activity can cause wilting and even death of the affected plants. After feeding for about two months, the female beetle flies to neighbouring bushes to lay eggs. The eggs are laid 5-15 cm beneath the soil close to a host plant, (Jackson, 1980). The eggs are cylindrical and brown or white in colour. A wide range of plants have been found to be hosts for taro beetle breeding (Sar et al., 1997). These include Johnson grass (Sorghum verticilliflorum), Elephant grass (Pennisetum purpureum), Kunai (Imperata cylindrica) and pitpit (Phragmites karka). Larvae hatch from the eggs in 11-16 days. The larvae feed on plant roots and dead organic matter at the base of the host plants. The larva moults about three times in its 3-4 months of life, and then pupates. After about two weeks, the adults develop from the pupa and fly to neighbouring taro plots to cause another cycle of damage. The adult lives for 4-8 months.
Not only does the taro beetle have a wide host range for breeding, but it also has a wide host range of crops that the adult feeds on and disfigures. Crops that are attacked include tannia, sugarcane, banana, sweet potato, yams, etc. This versatility of hosts makes the taro beetle additionally destructive, and its control much more difficult. The taro beetle is a pest in taro production in a wide sweep of territory from Indonesia through Papua New Guinea, Solomon Islands, Vanuatu to Fiji and New Caledonia; in short, virtually all of Melanesia and beyond. It was first reported in Fiji in 1984.
Numerous efforts have been made to develop effective control measures for the taro beetle. Mulching with polythene, coconut husk or grass has only been partially effective. The earlier recommendation of lindane for taro beetle control in Papua New Guinea has proved to be environmentally unsustainable. Other insecticides have proved not to be effective; nor has the use of physical barriers such as fly wire or shade cloth spread over the soil. The most recent research efforts are now concentrating on finding an effective biological control. Certain pathogens of the beetle have been identified. These include a fungus (Metarhizium anisopliae), a bacterium (Bacillus popilliae) and the protozoa Vavraia. Much of this research is taking place in Papua New Guinea and Solomon Islands, supported by the Pacific Regional Agricultural Programme (PRAP). Hopefully, a biological control measure for the taro beetle will become available before long.
Taro leaf blight (Figure 3) is caused by the fungus Phytophthora colocasiae. It was first reported in Java about a century ago, and has since spread to various parts of Asia and the Pacific. The list of countries where it has been reported include Indonesia, Papua New Guinea, Solomon Islands, Hawaii, Samoa, American Samoa, Thailand and the Philippines.
Figure 3. The Taro Leaf Blight Disease
The disease begins as purple-brown water-soaked lesions on the leaf. A clear yellow liquid oozes from the lesions. These lesions then enlarge, join together and eventually destroy the entire lamina in 10-20 days. Free water collecting on older leaves, as well as high temperature and high humidity are conducive to onset and spread of the disease and germination of the spores. The disease can be spread from plant to plant by wind and splashing rain. Spores survive in planting material for three or more weeks. Thus, infected planting material is one common means of spreading the disease over long distances and from season to season. The disease also attacks Alocasia macrorrhiza, a common aroid crop in the Pacific region, but the symptoms and yield losses are less severe.
The disease can cause yield losses of 30-50%, and results in lowering of the quality of the reduced harvest. Also taro leaves for human consumption are rare in affected areas. Most countries where the disease has been reported are under strict quarantine isolation.
Various approaches have been used to try to control the taro leaf blight. Agronomic methods that have given partial success include careful choice of planting material, planting at high density, intercropping taro with other crops rather than growing it as a sole crop, and crop rotation. Field removal of infected leaves has also been useful, but it is extremely laborious. In Samoa, control has been achieved by an intensive spraying programme with Ridomil or Manzate, and more recently with phosphorous acid (Foschek). Chemical control is extremely tedious, expensive, and not totally effective. An integrated control approach combining cultural and chemical methods seems to be the best at present. The ultimate solution must lie in the breeding and release of resistant cultivars. The taro breeding programme in Bubia, Papua New Guinea, has already identified several promising lines in this regard.
In some countries/territories, the taro leaf blight is present but causes relatively minor economic damage. This is true of the Philippines, Thailand and Hawaii. In other cases such as Samoa and American Samoa, the disease can be devastating. This situation has led to conjectures about the possibility that various strains of Phytophthora colocasiae may exist, and that in south-east Asia in particular, some of these strains may have evolved along with the taro crop (Lebot, personal communication) and may be less virulent. This factor is in addition to differences in the genetic make-up and genetic diversity of the taro crop in each country.
The alomae virus disease is caused by a complex of two or more viruses acting together. The two viruses that are definitely involved are the taro large bacilliform virus (TLBV) which is transmitted by the plant hopper Tarophagus proserpina, and the taro small bacilliform virus (TSBV) which is transmitted by the mealybug Planococus citri (Rodoni 1995). Neither virus is transmissible by mechanical contact, and their host range seems limited to aroids only. The full-blown alomae disease occurs when these two viruses (and possibly others) are present. Presence of only TLBV alone results in bobone, a milder form of the disease.
Alomae first starts as a feathery mosaic on the leaves. Lamina and veins become thick. The young leaves are crinkly and do not unfold normally. The petiole is short and manifests irregular outgrowths on its surface. The entire plant is stunted and ultimately dies. The symptoms of bobone are similar, but the leaves are more stunted and the lamina is curled up and twisted. With bobone, complete death of the entire plant does not usually occur.
Severe cases of alomae can result in total crop loss, while bobone can cause up to 25% yield loss. However, in many instances, only isolated plants in taro fields seem to be affected by either disease, and in the case of bobone infected plants may recover from the symptoms. The alomae/bobone disease complex has been reported in Papua New Guinea and Solomon Islands. The disease is controlled by pulling out diseased plants in the field, and by careful selection to ensure disease-free planting material. Ultimately, control will have to rely on breeding and disseminating resistant cultivars. Some tolerant cultivars bred through recurrent selection, have been released in Solomon Islands since 1992 (Gunua & Kokoa, 1995).
While the alomae/bobone disease is mainly confined to the Pacific, the dasheen mosaic virus disease occurs world-wide. Most taro-producing countries in the Asia/Pacific region have the disease. DMV is caused by a stylet-borne, flexuous, rod-shaped virus that is spread by aphids. It is characterized by chlorotic and feathery mosaic patterns on the leaf, distortion of leaves, and stunted plant growth. The disease is not lethal, but yield is depressed. Control is through the use of DMV-free planting material, field sanitation, and quarantine measures.
Other diseases and pests of taro include:
a) Corm and root rots caused by the fungi Pythium spp and Phytophthora.While these diseases and pests may be considered minor, they can become quite severe in certain locations or at certain times during the cropping season.
c) The taro planthopper, Tarophagus proserpina which not only transmits virus diseases, but can cause wilting and death of the plant in heavy infestations.
e) Taro hornworm which defoliates the plant.
f) Armyworms or cluster caterpillars which can also do extensive damage to the leaves.