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Taro Leaf Blight Plant Disease July 2011 PD-71 Taro Leaf Blight in Hawai‘i Scot Nelson,1 Fred Brooks,1 and Glenn Teves2 1Department of Plant and Environmental Protection Sciences, Honolulu, HI 2 Department of Tropical Plant and Soil Sciences, Moloka‘i Extension Office, Ho‘olehua, HI aro (Colocasia es- ha (2.8 US tons/acre) Tculenta (L.) Schott) (FAOSTAT 2010 esti- grows in Hawai‘i and mates; Ramanatha et throughout the tropical al. 2010). Pacific as an edible In 2009, approx- aroid of historical and imately 1814 tonnes contemporary signifi- (2,000 US tons) of C. cance (Figure 1). Farmers esculenta were har- cultivate kalo (Hawaiian vested in Hawai‘i from for taro) in wet lowland 100 farms on 180 ha (Figure 2) or dryland (445 acres). More than (Figure 3) taro patches 80% of Hawai‘i’s pres- for its starchy, nutritious ent-day taro production corms. The heart-shaped occurs on the island of leaves are edible and Kaua‘i. The farm value can also serve as food Figure 1. A taro (Colocasia esculenta) patch in Hawai‘i. of Hawai‘i’s taro crop wrappings. Historically, in 2009 exceeded $2.4 taro crops provided nutritious food that helped early million (United States Department of Agriculture Polynesians to successfully colonize the Hawaiian 2011). Processors use mature corms of Hawaiian Islands. cultivars to make poi by steaming and macerating “Taro” refers to plants in one of four genera the taro. Cultivars processed into poi commercially within the family Araceae: Colocasia, Xanthosoma, are predominantly ‘Lehua’ types, and to a lesser Alocasia, and Cyrtosperma. These aroids vary in extent ‘Moi’ and ‘Api‘i.’ However, many Hawaiian use and extent of cultivation among Pacific Island cultivars are processed into poi for subsistence use societies. In Hawai‘i, the genus most commonly or steamed and eaten as pieces. The Chinese taro grown and consumed is Colocasia, and “taro” cultivar (Bunlong taro) is prepared as deep-fried or “dasheen” commonly refers to C. esculenta. “chips” or as taro “baskets” in restaurants, as dim Globally, taro is the fourteenth most consumed sum, as hash patties or as steamed taro “cakes.” vegetable, with 12 million tonnes (~13 million US Other modern Hawaiian taro products include taro tons) produced from about 2 million hectares (~5 batters, taro breads or rolls, and taro pancakes, in million acres) and yielding on average 6.5 tonnes/ which poi is added to wheat flour. Cooks bake, boil, Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in co- operation with the U.S. Department of Agriculture, under the Director/Dean, Cooperative Extension Service/CTAHR, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822. Copyright 2011, University of Hawai‘i. For reproduction and use permission, contact the CTAHR Office of Communication Services, [email protected], 808-956-7036. The university is an equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i without regard to race, sex, gender identity and expression, age, religion, color, national origin, ancestry, disability, marital status, arrest and court record, sexual orientation, or status as a covered veteran. Find CTAHR publications at www.ctahr.hawaii.edu/freepubs. UH–CTAHR Taro Leaf Blight in Hawai‘i PD-71 — July 2011 Figure 2. Taro cultivation in a wet, lowland setting (lo‘i) Figure 3. A dryland taro field in Hawai‘i. Plants are in Hawai‘i. The lo‘i can be filled with water or drained by grown here in deep soil and irrigated by rainfall. the farmer. or steam taro leaves as food wrappings in laulau, susceptible cultivars appear to melt in the field, a Hawaiian delicacy prepared with steamed pork the leaves getting smaller and smaller, on shorter enclosed in layers of taro leaves and wrapped in ti and shorter leaf stalks. Unfortunately, there is little leaves. to be done to prevent a severe epidemic of taro Taro leaf blight, caused by Phytophthora leaf blight if pathogen inoculum is present and colocasiae Racib., is historically the most important environmental conditions are favorable for infection and damaging disease of taro worldwide and is and disease development. In this article we discuss responsible for major taro yield losses globally. the causal pathogen, symptoms of the disease, and This disease was responsible for the widespread management options for taro leaf blight in Hawai‘i. taro crop failure in the Samoan archipelago in the 1990s. Taro crops in American Samoa were Hosts of P. colocasiae devastated by an epidemic of taro leaf blight from Phytophthora colocasiae infects taro and dasheen 1993-1994 (Trujillo et al.1997). Taro production (C. esculenta, Colocasia esculenta var. globulifera), fell from 357,000 kg (786,000 lb) per year before and some cultivars of ape or tamu (Alocasia the epidemic to less than 5,000 kg (11,000 lb) by macrorrhiza). The most susceptible hosts are among the end of 1995 (Brooks 2008). Similarly, in 1993 cultivars of C. esculenta. the value of exported taro for Western Samoa (now Biology and morphology of P. colocasiae Samoa) was US$3.5 million, about 58% of Samoa’s Organisms in the genus Phytophthora are no longer agricultural exports. By 1994 the value of exported classified as fungi, but rather as pseudo-fungi taro was less than US$60,000. in the Stramenopiles. Phytophthora colocasiae The severity of the Samoan epidemic was is heterothallic, requiring two opposite mating partly due to widespread planting of a single types (designated A1 and A2) to form oospores, susceptible taro cultivar throughout the archipelago. the sexual stage. Only the A1 mating type occurs During such epidemics, leaves on taro cultivars in Hawai‘i and in most other taro-growing areas. that normally live for 30-40 days are destroyed Oospores have thick walls and are well suited for in 10 days or less. As leaves die, photosynthesis long-term survival in the absence of their taro is reduced and corm yields diminish. Highly hosts. This characteristic suggests that migration 2 UH–CTAHR Taro Leaf Blight in Hawai‘i PD-71 — July 2011 Figure 4. Sporangia and mycelia of Figure 5. A sporangium of Phytoph- Figure 6. Germinating zoospores of Phytophthora colocasiae (magnified). thora colocasiae releasing zoo- Phytophthora colocasiae spores, which swim in free water to (magnified). infect taro leaves (magnified). and sexual recombination may play important Taro leaf blight disease occurs in all taro-growing roles in the population dynamics of the pathogen. areas of Hawai‘i. Chlamydospores are thick-walled asexual spores that sometimes grow in laboratory cultures. Like Yield loss oospores, they may also be involved in the natural Taro leaf blight can reduce taro corm yield by 50% or survival of the pathogen in soil between crops. This more for highly susceptible taro cultivars. Leaf yield pathogen produces another type of asexual spore, losses of up to 95% have occurred for susceptible called a sporangium (Figure 4). Sporangia (plural of varieties in Hawai‘i. Epidemics can completely sporangium) are oval in shape, hyaline (colorless), destroy susceptible cultivars in the field. Poi quality and semi-papillate (the tip of the spore is not can be reduced, as reduced photosynthesis results in pointed). They have a short stalk or pedicel attached decreased production of gums and starches. to the base of the sporangium and are deciduous Disease symptoms (spores fall from the mycelium). Sporangia can Phytophthora colocasiae primarily infects leaves, germinate directly and infect host tissue via germ but petioles and corms are also susceptible (Brooks tubes. If there is a brief chilling period in the 2008). weather and moisture on the leaf, however, they can also germinate indirectly, releasing swimming, Leaves infective spores (zoospores) (Figure 5). After a short Early leaf infections often occur where rainfall or period, the zoospores lose their swimming tails and dew droplets tend to accumulate, at the margins encyst. The encysted spores then germinate (Figure and tips of leaves. The first symptoms appear on 6), and their germ tubes penetrate the taro tissue. the upper surface of leaves as small, brown to olive- Phytophthora colocasiae grows well between 20- green flecks, or as spots surrounded by faint, diffuse 25°C (68-77°F) and most rapidly from 27-30°C (80- halos (light green or yellow tissue around lesions) 86°F). Minimum and maximum temperatures for (Figure 7). The circular spots enlarge rapidly, growth are 10°C (50°F) and 35°C (95°F), respectively. forming zonate, brown to purplish-brown lesions (Figure 8). The zones in the leaf spots result from Disease distribution differential radial growth of the pathogen within The first report of taro leaf blight was from Java in leaf tissues during periods of fluctuating temperaure 1900 (Raciborksi 1900). Phytophthora colocasiae and relative humidity. This radial expansion of now infects taro throughout the Pacific, Asia, East leaf spots during moist weather occurs rapidly Asia, Africa, the Caribbean, and the Americas. on leaves of susceptible cultivars, rotting large 3 UH–CTAHR Taro Leaf Blight in Hawai‘i PD-71 — July 2011 sections of leaves in a few days. On the undersides As the pathogen destroys the tissue, petioles turn of leaves, spots appear water soaked, or dry and soft and may break if unable to support the weight tan or gray (Figure 9). As the spots expand in of the leaves (Figure 17). Petiole lesions expand diameter they combine, causing a blight that can rapidly and may produce a reddish-orange exudate. destroy the leaf in seven days or less (Figure 10, These lesions and exudates may sometimes be Figure 11). Conspicuous droplets of amber, bright confused with injury caused by the taro planthopper orange, or reddish-brown plant exudate often ooze (Tarophagus proserpina [Kirkaldy] Hemiptera: from lesions (Figure 12).
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