Taro (Colocasia esculenta) is a major food crop in the Pacific Island Countries and Territories (PICTs) with wide cultural, economic and food security importance for nearly all PICTs. Annual regional production is more than 360,000 tonnes per year, but taro production is subject to significant losses from pests and diseases. Most Pacific taro pests have restricted distributions, making effective quarantine critical to their containment and management. Identifying pests already in a country is an ongoing requirement for growers, extension officers and those responsible for trying to gain international market access for the crop. However, limited access to relevant information and user-friendly diagnostic tools has been a major problem, hampering the development of effective quarantine, pest management and pest surveillance systems. During the 2003 ACIAR regional consultations, the lack of readily available information on taro pests was identified as a matter of concern and the development of a pest and disease tool kit for taro was subsequently agreed on as a regional priority. To work towards effective management of pests and diseases and to help make available diagnostic pest management information on taro, the Australian Centre for International Agricultural Research (ACIAR) and its partners invested heavily over recent years in funding the development of a pest and disease toolkit for taro.
TaroPest is a collaborative project involving the Secretariat of the Pacific Community (SPC) Plant Protection Service, National Agricultural Quarantine and Inspection Authority (NAQIA) of Papua New Guinea and the Queensland University of Technology in Australia. The technical content of the diagnostic tool were provided by regional research and extension officers and plant pest and disease specialists from around the world. TaroPest has been developed as a guide to the pests and diseases of taro in the Pacific. Its aim is to be a one-stop shop with keys, fact sheets, photographs, and consists of a field guide with an interactive CD-ROM that provides additional information. TaroPest captures all current information on the identification and management of taro pests and diseases in the South Pacific region. It also highlights the fact that for most of these pests and diseases, little is known about their economic or biological impact, differences in the susceptibility of various taro cultivars, and control methods.
Phytophthora colocasiae is primarily a foliar pathogen, but it also affects petioles and corms. The first symptoms on taro (Colocasia esculenta) are small, dark brown flecks or light brown spots on the upper leaf surface (Figure 2). These early spots often occur at the tips and edges of leaves where water accumulates. They enlarge rapidly, becoming circular, zonate, and purplish-brown to brown in color (Figure 3). On the lower leaf surface, spots have a water-soaked, or dry gray appearance (Figure 4). As spots increase in size they coalesce and quickly destroy the leaf (Figure 5). In dry weather, or on some resistant cultivars, the centers of lesions become papery and fall out, producing a “shot-hole” appearance. Dead leaves often hang on their long petioles like flags (Figure 6).
Taro Leaf Blight
Taro leaf blight 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.
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. 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 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/Bobone Virus Disease Complex
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.
Dasheen Mosaic Virus Disease (DMV)
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.
G. K. PARRIS
HAWAII AGRICULTURAL EXPERIMENT STATION of the UNIVERSITY OF HAWAII
Circular No. 18
In Hawaii taro is grown either submerged (wetland taro) on muck or heavy clay soil, or nonsubmerged (upland taro) on clay loam or sandy loam soil. Wetland taro is produced in valleys which extend from sea level to the base of the mountains, with irrigation water obtained from the uplands; upland taro grows at the higher levels, usually in regions of 70 inches or more of rain per annum. Most of the taro in Hawaii is produced under wetland culture. The plant, Colocasia esculenta Schott, adequately described by Whitney et al. (10).1 is subject to a number of diseases which attack either the conn and root or the leaf, and cause appreciab1e losses. Many of these diseases are best controlled by a proper understanding and appreciation of the culture and care of the plant. Wetland taro is particularly subject to conn and root diseases, but upland taro is singularly free from subterranean attacks. Of the two serious leaf spots, one is apparently limited to upland taro, the other is severe on wetland taro but occasionally attacks upland taro in rainy areas or seasons.
Taro diseases (7.3mb)
Jeri J. Ooka, Department of Plant Pathology, University of Hawaii - Kauai Branch Station
Although taro is susceptible to attack by at least twenty-three pathogens, only a few cause serious reduction in growth and production. Phytophthora blight (Phytophthora colocasiae) and Pythium root and corm rot (Pythium spp.) are the most serious fungal diseases of taro. Phytophthora blight is not yet found in Samoa, the Marquesas, the Society and Cook Islands. Dithane-M45 is available for control of Phytophthora blight. Pythium root and corm rot is found where ever taro is grown. Five Pythium spp. have been implicated as causal agents of the disease. Captan provides good control of the disease. Data to apply for metalaxyl registration on taro for control of Pythium root and corm rots are being collected. Phyllosticta leaf spot (Phyllosticta colocasiophila), Sclerotium blight (Sclerotium rolfsii), Black rot (Ceratocystis fimbriata), Rhizopus rot (Rhizopus stolonifer), Phytophthora root rot (P. palmivora) and Fusarium dry rot (Fusarium solanD are other fungal diseases which may be locally important. Hard rot (unknown etiology) is a major problem in wetland taro culture where it can cause substantial losses. Erwinia spp. may cause bacterial soft rots. Root-knot nematodes (Meloidogyne spp.) cause root galls and corm formations.
The root rot of taro (4.5mb)
T. F. SEDGWICK, (1902)
Agriculturist, Hawaii Agricultural Experiment Station.
History of taro, Usual method of culture, Taro Rot, Experiments on the control of taro rot, Record of the investigations, Suggested improvements in method of culture, Irrigation and preparation of land for planting, Fertilizers, Summary, Conclusions
Foto Gejala Hawar Daun Talas