Brooks, F.E. 2005. Taro leaf blight. The Plant Health Instructor. DOI:10.1094/PHI-I-2005-0531-01
DISEASE: Taro leaf blight
PATHOGEN: Phytophthora colocasiae
HOSTS: Limited, mainly aroids (Araceae), including Colocasia esculenta (taro, kalo, dasheen) and Alocasia macrorrhiza (giant taro)
AuthorFred BrooksAmerican Samoa Community College Land Grant Program
Taro leaf blight disease on a resistant hybrid.
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).
Bright orange or reddish-brown plant exudate oozing from infection sites is another symptom of leaf blight disease in taro (Figure 7). The presence of yellow tissue around lesions (Figure 8) is not well understood, but could be a cultivar specific reaction or a response to dry weather. Infected corm tissue is brown, firm, and develops rapidly after harvest. A prominent sign of P. colocasiae is the white ring of sporangia around the edge of lesions (Figure 9).
The division Oomycota includes important plant pathogens, such as the downy mildews and the water molds, Phytophthora and Pythium. Though traditionally taught along with fungi in plant pathology courses, recent studies indicate these organisms are closely related to golden brown algae and are now placed in a different kingdom.
Important differences exist between the oomycetes and true fungi. For example, oomycetes have cell walls composed of β-glucans and cellulose rather than chitin. Oomycetes also are unable to produce sterols. These differences can affect isolation, culture, and management of oomycete pathogens. The mycelium of Phytophthora species is composed of tube-like hyphae with few or no cross-walls (coenocytic), in contrast to the septate hyphae of fungi. Their nuclei are diploid (2n), compared to haploid (n), or dikaryotic (n + n) nuclei in the mycelia of most fungi.
Asexual reproduction by Phytophthora colocasiae, an aerial species similar to P. infestans, occurs during wet weather. Sporangia are formed at the end of short, unbranched or sparingly branched sporangiophores at the edge of lesions. They are ovoid to ellipsoid with a distinct narrow apical plug (semi-papillate), average 40-50 x 23 µm, and have a length-to-width ratio of 1.6:1 (Figure 10). Sporangia are usually separated from sporangiophores by rain (caducous), leaving a stalk (pedicel) 3-10 µm in length attached to their base (Figure 11). During wet weather, sporangia germinate on the upper surface of leaves. When temperatures are near 20°C (68°F) and humidity is high (90-100%), most germination is indirect (Figure 12), producing zoospores that swim for a few minutes, encyst, and form germ tubes (Figure 13). This process can occur in two hours or less. Sporangia germinate directly (Figure 14) between 20-28° C (68-82°F), but may account for only a small percentage of total germination. The incubation period (time from germ tube penetration to development of symptoms) is 2-4 days at optimal temperatures of 24-27°C (75-80°F).
Sexual reproduction of this heterothallic species depends on the presence of both A1 and A2 mating types. Hormones produced by one mating type stimulate production of antheridia (male) and oogonia (female) in the opposite mating type. Each antheridium attaches to the base of an oogonium, surrounding its stalk-like attachment (amphigynous) (Figure 15). The nuclei in these organs undergo meiosis and the haploid nucleus from the antheridium unites with a haploid nucleus in the oogonium, forming a diploid oospore 18-30 µm (average 23 µm) in diameter (Figure 15).
Phytophthora colocasiae is a warm-weather pathogen, growing most rapidly at temperatures between 27-30°C (80-86°F). Minimum and maximum temperatures for growth are 10°C (50°F) and 35°C (95°F), respectively. Chlamydospores (thick-walled asexual spores) are sometimes produced in culture and develop either at the end of (terminal), or along (intercalary) hyphae. They measure 17-38 µm (average 27 µm) in diameter, with walls 2-3 µm thick.
Sporangia are detached from sporangiophores at the edge of lesions and spread by rain splash and wind-blown rain. They germinate on leaves and petioles, or are washed into the soil where they can infect taro corms.
If the A1 and A2 mating types are present, antheridia and oogonia can form, producing oospores. Oospores may act as survival structures in plant tissue between crops.
The pathogen survives as mycelium in plant tissues or as encysted zoospores in soil. Infected planting material, along with sporangia and zoospores in wind-driven rain, are the main sources of inoculum in new plantings.
Sporangia and zoospores are spread by rain splash and wind-blown rain between plants (alloinfection) (Figure 16), or within the same plant (autoinfection) (Figure 17). Due to the slanted aspect of taro leaves and their thick, waxy cuticle, many sporangia and zoospores are washed into the soil or splashed onto petioles (Figure 18). Petiole lesions grow rapidly and may produce reddish-orange exudates (Figure 19). They should not be confused, however, with the small lesions caused by the taro planthopper (Tarophagus proserpina), which also produce exudates (Figure 20). In wetland (flooded) taro production, sporangia and zoospores are spread between plants and fields by paddy water.
The warm, wet climate of the tropics allows taro to be grown throughout the year, ensuring a continuous supply of host plants. Taro is vegetatively propagated, and P. colocasiae is spread from field to field and over long distances by infected planting material. Corms left in the field after harvest can also serve as inoculum sources in newly planted taro plots. Mycelium usually lasts less then five days in the soil, but encysted zoospores can survive for up to 3 months. Oospores and chlamydospores are formed in culture and may act as survival structures in infected plant tissue or soil, but have not been reported in nature.
Early disease management is aimed at reducing the inoculum level and relative humidity in the field. Taro leaf blight is an explosive disease, however, and cultural and physical control methods are usually ineffective during an epidemic. Roguing (removing all or parts of infected leaves) reduces inoculum levels. As disease severity and roguing intensify, however, physical leaf removal mimics the blight by further reducing total leaf surface area.
Field sanitation may decrease inoculum levels early in the season, but sporulating leaf lesions supply enough propagules (sporangia, zoospores) to increase disease. Spacing plants farther apart does little to decrease taro leaf blight disease. It has been demonstrated experimentally, both in the presence and absence of leaf blight, that planting taro closer together improves yield. Close spacing (e.g. 0.5 m or 2 ft) may increase leaf blight severity, but it also increases the total weight and number of corms, though individual corms are smaller.
Protectant chemical sprays containing copper, manganese, or zinc, have been effective against taro leaf blight, but heavy rains make repeated applications necessary. Good results have also been reported with metalaxyl, a systemic agent used against the oomycetes. In many countries and island nations, however, taro is a subsistence crop and routine chemical use is neither economically practical nor environmentally suitable.
Resistant cultivars offer the best long-term control of taro leaf blight. However, desirable cultural characteristics and eating qualities are often lost during breeding. Current breeding efforts therefore are focused on improving yield, suckering (desirable for vegetative propagation), time to maturity, taste, and texture.
Isozyme analysis and DNA markers (RAPD) have identified significant genetic differences in isolates of P. colocasiae within and between countries, which may affect the pathogenicity of the isolates. With this in mind, taro from breeding programs should be tested against P. colocasiae isolates already present in countries before new breeding lines are introduced.
Taro, Colocasia esculenta, is grown under flooded or dry land conditions on over 1.8 million ha (4.5 million acres) and is the fourteenth most consumed vegetable worldwide. It is an important staple crop throughout the tropics and part of the traditional culture in places like Hawaii and the Samoan Archipelago. The leaves are eaten cooked, and the corm is baked, boiled, fried, pounded into a paste (poi), or made into flour.
Marian Raciborski first reported Phytophthora colocasiae, the cause of taro leaf blight disease, in Java in 1900. From its origin, possibly in eastern India or Indo-Malaysia, it spread throughout the tropical climates of Southeast Asia and the Pacific, Africa, the Caribbean, and the Americas. Reductions in corm yield of 25-50% have been reported in the Pacific and 25-35% in the Philippines. Losses may be greater among highly susceptible cultivars. Leaf yield losses of 95% were reported for susceptible varieties in Hawaii.
The most recent epidemic of taro leaf blight occurred in the Samoan Archipelago in 1993-1994. In 1993, the taro export market for Independent Samoa was US$3.5 million, accounting for 58% of Samoa’s exports. By 1994 leaf blight had swept the islands and exported taro was valued at less than US$60,000. American Samoa, 90 km (75 miles) to the east, produced 357,000 kg (786,000 lb) of taro in 1993. This dropped to about 22,000 kg (50,000 lb) in 1994 and 5,000 kg (11,000 lb) in 1995. The severity of this epidemic was mainly due to extensive cropping with a single, susceptible cultivar throughout the archipelago.
The warm, humid days and cool, wet nights of the tropics are ideal for reproduction and spread of P. colocasiae. During rainy weather, leaves on taro cultivars that normally live for 30-40 days may be destroyed in less than 20 days. Therefore, a healthy plant that carries 5-7 functional leaves may have only 2-3 leaves when infected. This reduces net photosynthesis, resulting in a reduced corm yield. Highly susceptible cultivars appear to be melting in the field, producing smaller and smaller leaves on shorter and shorter petioles.
CAB International. 2003. Crop Protection Compendium. CAB International, Wallingford, UK.
Cox, P.G., and C. Kasimani. 1990. Effect of taro leaf blight on leaf number. Papua New Guinea Journal of Agriculture, Forestry and Fisheries 35: 43-48.
Gollifer, D.E. and J.F. Brown. 1974. Phytophthora leaf blight of Colocasia esculenta in the British Solomon Islands. Papua New Guinea Agricultural Journal 25: 6-11.
Erwin , D.C. and O.K. Ribeiro. 1996. Phytophthora Diseases Worldwide. American Phytopathological Society Press, St. Paul, MN.
Lebot, V., C. Herail, J. Pardales, M. Prana, M. Thongjiiem, and N.Viet. 2003. Isozyme and RAPD variation among Phytophthora colocasiae isolates from South-east Asia and the Pacific. Plant Pathology 52: 303-313.
Raciborski, M. 1900. Parasitic algae and fungi, Java. Batavia Bulletin of the New York State Museum 19: 189.
Trujillo , E.E. 1965. The effect of humidity and temperature on Phytophthora blight of taro. Phytopathology 55: 183-188.
Zhang, K.M., F.C. Zheng, Y.D. Li, P.J. Ann, and W.H. Ko. 1994. Isolates of Phytophthora colocasiae from Hainan Island in China: evidence suggesting an Asian origin of this species. Mycologia 86: 108-112.