Phytophthora colocasiae (taro leaf blight)
- Taxonomic Tree
- Distribution Table
- Risk of Introduction
- Host Plants and Other Plants Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Natural enemies
- Plant Trade
- Detection and Inspection
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Phytophthora colocasiae Racib.
Preferred Common Name
- taro leaf blight
International Common Names
- English: blight of dasheen; leaf blight of Colocasia spp.; leaf blight of Gabi; Phytophthora leaf blight
- French: flétrissure des feuilles de taro
Local Common Names
- China: yu yi ping
- PHYTOO (Phytophthora colocasiae)
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Chromista
- Phylum: Oomycota
- Class: Oomycetes
- Order: Peronosporales
- Family: Peronosporaceae
- Genus: Phytophthora
- Species: Phytophthora colocasiae
DescriptionTop of page Deciduous sporangia with apical papilla are produced on slender sporangiophores which branch irregularly or sympodially with a swelling at the point of branching. Sporangia are ovoid to ellipsoid, mostly 45-50 x 23 µm with a length-to-width ratio of 1:1.6. Chlamydospores are thick-walled, usually 26-30 µm diameter. Oospores averaging 29 µm diameter are produced in oogonia with amphigynous antheridia attached (Waterhouse, 1963; Stamps et al., 1990). Sex organs of individual isolates can be produced on polycarbonate membranes stimulated by sex hormones produced by the opposite mating type of P. colocasiae or a different species of Phytophthora (Ko, 1988).
DistributionTop of page
P. colocasiae occurs in South-East Asia, its probable area of origin, and has spread from there to many Pacific territories and parts of Oceania. It occurs in Indonesia (Raciborski, 1900), China (Sawada, 1911; Dai, 1927), India (Butlen et al., 1913), the Philippines (Reinking, 1919; Gomez, 1925), Malaysia (Thompson, 1939), Hawaii (Parris, 1941), Papua New Guinea (Shaw, 1963), British Solomon Islands (Jackson et al., 1975) and the Trust Territories of the Pacific (Plucknett et al., 1970; Trujillo, 1971). The report for Equatorial Guinea refers to Bioko Island (Fernando Po) (CABI/EPPO, 2014).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
Risk of IntroductionTop of page P. colocasiae has been distributed over long distances by means of vegetatively propagated material and probably by soil. Thus, where there is international, national or regional trade in plants and corms, there is a case for the prohibition of movement from diseased to disease-free regions. Where importation is from a region where P. colocasiae is known to occur, planting material may be treated with sterilizing chemicals such as metalaxyl.
Host Plants and Other Plants AffectedTop of page
Growth StagesTop of page Post-harvest, Vegetative growing stage
SymptomsTop of page Affected leaves initially show small dark spots which enlarge rapidly and turn purplish brown with yellowish margins. The lesions frequently form concentric zones and exude drops of yellowish liquid. Some of the diseased tissues may be covered with a whitish fuzz consisting of sporangia. As the disease progresses, the lesions (mostly along the leaf margin) continue to expand and frequently coalesce. Diseased tissues disintegrate, forming holes of irregular size and shape on the affected leaves. Occasionally the pathogen may cause water-soaked lesion on the petioles. Infected leaves collapse within 20 days of unfurling, compared to 40 days for healthy leaves. The normal 6-7 leaves per plant was reduced to 3-4 leaves per plant by severe disease incidence.
After harvest, grey-brown to dark-blue lesions occur on undamaged corms. These lesions enlarge rapidly and coalesce. The boundary between the healthy and diseased tissues is usually indistinct and soft. Affected corms are almost completely decayed at 8 days after harvest in wet conditions.
List of Symptoms/SignsTop of page
|Leaves / abnormal colours|
|Leaves / fungal growth|
|Leaves / necrotic areas|
|Stems / mould growth on lesion|
|Vegetative organs / soft rot|
|Vegetative organs / surface lesions or discoloration|
Biology and EcologyTop of page Life Cycle
Hyphae of the fungus generally survive longer in sterilized soil (30 days) than in natural soil (5 days). At >20°C and >55% soil moisture the hyphae disappeared with 5 days of burial in natural soil (Sitansan Pan et al., 1994). Survival of the fungus between crops is less clearly understood. Neither chlamydospores nor oospores have been reported under field conditions although they form readily in agar culture. Thus it is assumed that where the crop is seasonal the fungus survives as mycelium within stored corms used as propagating material for the next season's planting. Oospores may also survive in the corm and leaf tissue left in the field after harvest. In the Philippines sporangia on the leaves were found capable of germination after remaining under field conditions for 3 months (Gomez, 1925).
Free water is needed for sporangial germination and zoospore mobility. Close to 100% RH is needed for infection to occur. The period of leaf wetness, therefore, has a large effect on infection by P. colocasiae. At optimal temperatures of 24-27°C, sporangial germination, release of zoospores and penetration occur after 6-8 hours. The fungus enters the plant through the cuticle and a latent period requires 2-4 days at optimal temperatures of 27-30°C. In wet weather the lesions of infected leaves or petioles may produce many sporangia and zoospores are disseminated by rain splash.
Oospores occur infrequently in nature, and taro leaf blight is thus spread almost exclusively by sporangia from the anamorph. Dissemination via rain splash is the most common dispersal mechanism. Spread of the fungus within a taro planting occurs when sporangia and zoospores are splashed from infected to healthy leaves. The infection of new planting occurs by spores blown in wind-driven rain from adjacent diseased fields or from infected wild taro. Also the fungus has been distributed by means of vegetatively propagated material and probably by soil.
P. colocasiae occurs under conditions of high temperature and humidity, in wet areas and densely planted fields. Epidemics occur frequently between July and September in Hainan, China. Primary leaf infection has been observed following tropical storms.
Natural enemiesTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|Bulbs/Tubers/Corms/Rhizomes||hyphae; spores||Yes||Yes||Pest or symptoms usually visible to the naked eye|
|Leaves||hyphae; spores||Yes||Yes||Pest or symptoms usually visible to the naked eye|
|Stems (above ground)/Shoots/Trunks/Branches||hyphae; spores||Yes||Yes||Pest or symptoms usually visible to the naked eye|
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
|Growing medium accompanying plants|
|True seeds (inc. grain)|
ImpactTop of page This disease can lead to a 30-40% crop loss in heavily infected taro fields (Jackson et al., 1975). The fungus is widespread in South-East Asia and parts of Oceania, where it causes severe leaf damage and considerable loss of corm yield. For example, in the British Solomon Islands, it has been reported to be a limiting factor on taro production (Barrau, 1958; Plucknett et al., 1970). In the Philippines, yield reductions ranged from 24.4% in resistant to 36.5% in susceptible cultivars (Vasguez, 1990). The fungus is capable of infecting undamaged corm tissues under conditions of high humidity resulting in severe corm decay in the storage stage.
DiagnosisTop of page Diseased tissues (ca 5 x 5 mm) taken from advancing margins of lesions on leaves or petioles are placed between clean paper towels to remove free water, plated on a selective medium (per litre: 50 ml V-8 juice, 50 mg mycostatin, 100 mg ampicillin, 10 mg pentachloronitrobenzene, 20 g agar) (Ko et al., 1979), and incubated at 24-28°C. Mycelia growing from the diseased tissues are transferred to 10% V-8 agar (Ko, 1979; Aun et al., 1986).
Detection and InspectionTop of page Disease symptoms are easily visible in the field (see Symptoms for description). When lesions are unclear or where confirmation is needed, the lesions should be incubated to produce sporangia for identification (see Diagnostic Methods).
Prevention and ControlTop of page
Cultivars that are resistant to leaf blight have been the most important method of disease control. In Bangladesh, among 50 lines tested by artificial inoculation in the field, two were highly resistant to P. colocasiae, five resistant, 12 moderately resistant and the rest moderately to highly susceptible (Goswami, 1993). Of 270 Colocasia esculenta lines screened for natural resistance to leaf blight in the field at Trivandrum, India, 119 lines were resistant (Santha-Pillai et al., 1993). In tests carried out in Arunachal Pradesh, India, 23 varieties of taro were screened for resistance to P. colocasiae, five varieties were immune and one was moderately resistant (Chaudhary et al., 1988). Of 11 cultivars screened under natural epiphytotics, Burdwar local was the best for commercial cultivation in west Bengal, India (Ghosh et al., 1991). In the British Solomon Islands, none of the 181 local cultivars tested were highly resistant to the fungus (Gollifer et al., 1974). More than 200 local varieties have been screened for resistance to the fungus and of these only Abrueme has shown promise (Jackson et al., 1975).
Resistance to P. colocasiae was found in a wild taro (Colocasia esculenta) accession introduced from Thailand and designated Bangkok. Data from crosses between Bangkok and local cultivars indicated that resistance is controlled by a single dominant gene (Patel et al., 1984).
Cultural practices towards disease control include minimizing the source of inoculum, use of disease-free plant material, roguing infected leaves, and avoiding excessive levels of moisture.
Fungicidal control is largely practised against P. colocasiae in taro cultivation. Currently widely used products are systemic (metalaxyl) and non-systemic fungicides (copper oxychloride, mancozeb, zineb) applied as foliar sprays. In India spraying metalaxyl at intervals of 15 days was effective in controlling the disease under field conditions and gave maximum net financial return (Ghosh et al., 1991). Good control was obtained with metalaxyl and fair control with copper oxychloride (Aggarwal et al., 1987). Sahu et al. (1989) report that four sprays of zineb at 15-day intervals reduced the incidence of P. colocasiae and increased the yield. In Papua New Guinea five applications of metalaxyl at 3-week intervals resulted in an increase of almost 50% corm yields (Cox et al., 1990). Applications of mancozeb at 7-day intervals gave substantial disease control and increased yields in Hawaii (Bergquist, 1974). But in the Solomon Islands mancozeb did not control the disease or increase corm yields, while mist-blower application of copper oxychloride gave effective control of P. colocasiae and increased corm yield (Jackson et al., 1980).
ReferencesTop of page
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Distribution MapsTop of page
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