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Taro Leaf Blight (''Phytophthora colocasiae'') is a highly infectious
plant disease Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...
that is characterized by the formation of large brown lesions on the leaves of infected
taro Taro () (''Colocasia esculenta)'' is a root vegetable. It is the most widely cultivated species of several plants in the family Araceae that are used as vegetables for their corms, leaves, and petioles. Taro corms are a food staple in Africa ...
plants.Nelson, S., Brooks, F., and Teves, G. July 2011. Taro leaf blight in Hawaii. College of tropical agricultural and human resources. Lesions are the result of
oomycete Oomycota forms a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, called oomycetes (). They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the resul ...
s leaching nutrients out of the leaves via
haustoria In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates t ...
to create white powdery rings of
sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cy ...
.Singh, D., Jackson, G., Hunter, D., Fullerton, R., Lebot, V., Taylor, M., Iosefa, T., Okpul, T., and Tyson, J. July 2012. Taro leaf blight - a threat to food security. Agriculture. 2, 182 - 203. This pathogen grows best in high humidity and high rainfall environments offering the pathogen means of dispersal via rain splash as well as a warm humid environment that favors
hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...
l growth across the infected plant.''Phytophthora colocasiae''. 2013. http://www.cabi.org/isc/datasheet/40955 .CABI. (11/1/14). The Taro Leaf Blight epidemic of
Samoa Samoa, officially the Independent State of Samoa; sm, Sāmoa, and until 1997 known as Western Samoa, is a Polynesian island country consisting of two main islands (Savai'i and Upolu); two smaller, inhabited islands (Manono Island, Manono an ...
in 1993 is an example of extreme devastation this plant pathogen can cause if preventative measures are not used to control spread and symptoms.Brooks, F. 2005. “Taro Leaf Blight.” http://www.apsnet.org/edcenter/intropp/lessons/fungi/Oomycetes/Pages/TaroLeafBlight.aspx American Phytopathological Society Link. (10/27/14).


Hosts and symptoms

First described in
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's List ...
by
Marian Raciborski Marian may refer to: People * Mari people, a Finno-Ugric ethnic group in Russia * Marian (given name), a list of people with the given name * Marian (surname), a list of people so named Places * Marian, Iran (disambiguation) * Marian, Queensla ...
in 1900, taro leaf blight is caused by the oomycete ''Phytophthora colocasia'' which infect primarily ''
Colocasia ''Colocasia'' is a genus of flowering plants in the family Araceae, native to southeastern Asia and the Indian subcontinent. Some species are widely cultivated and naturalized in other tropical and subtropical regions. The names elephant-ear ...
spp.'' and ''
Alocasia macrorrhizos ''Alocasia macrorrhizos'' is a species of flowering plant in the arum family (Araceae) that it is native to rainforests of Maritime Southeast Asia, New Guinea, and Queensland and has long been cultivated in South Asia, the Philippines, many Paci ...
''. ''P. colocasiae'' primarily infects leaves, but can also infect petioles and
corm A corm, bulbo-tuber, or bulbotuber is a short, vertical, swollen underground plant stem that serves as a storage organ that some plants use to survive winter or other adverse conditions such as summer drought and heat (perennation). The word ' ...
s. Symptoms on leaves initially occur where water droplets accumulate and eventually form small, brown spots surrounded by halos on the upper surface of leaves. These spots expand very quickly and form large brown lesions. The entire leaf can be destroyed within a few days of the initial appearance of symptoms under wet conditions.Misra, R.S, Sharma, K., and Mishra, A.J. September 2008. Phytophthora leaf blight of taro (Colocasia esculenta) - a review. The Asian and Australian Journal of Plant Science and Biotechnology. 2 (2), 55-63. The undersides of leaves have spots that look water-soaked or gray, and as they expand, blight forms and the leaf is destroyed within a few days. Symptoms occur in a day/night pattern where water soaked areas expand during the night and then dry out during the day. As a result, additional water marks form leading to increasingly larger lesions. As the lesions expand, sporangia develop most actively at the margin of the lesion and progress to attack healthy tissue. One characteristic feature found on leaves is the formation of bright orange droplets oozing out from above and below water soaked leaf surfaces. As a result, the droplets dry out during the day and become crusty. Another sign of ''P. colocasiae'' infection are masses of sporangia that form a white, powdery ring around the lesion. Symptoms on petioles includes gray to brownish black lesions that can occur anywhere on the petioles. Petioles become soft and may break as the pathogen destroys the host. Symptoms on corms are often rubber-like and soft as well as having a light tan color. These symptoms occur rapidly and can arise anywhere on the corm and are often subtle in early stages. Decayed corm tissue appears brown and turns purplish in advanced stages of infection. Lesions can also be formed by sporangia that are splashed by rain. The dead central area breaks and falls out as the lesion gets larger. The rate of spread for this disease is very high which results in a high percentage of yield loss.


Disease cycle

''P. colocasiae'' is an oomycete and is thus characterized by
oospore An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia, ...
s and
coenocytic A coenocyte () is a multinucleate cell which can result from multiple nuclear divisions without their accompanying cytokinesis, in contrast to a syncytium, which results from cellular aggregation followed by dissolution of the cell membranes insid ...
hyphae. Oospores have very thick-walls which provide durable survival structures. As a result, oospores overwinter in soil, underground storage organs, or on leaf debris left in the field after harvest. However,
inoculum In biology, inoculum refers to the source material used for inoculation. ''Inoculum'' may refer to: * In medicine, material that is the source of the inoculation in a vaccine * In microbiology, propagules: cells, tissue, or viruses that are used t ...
does not survive for very long on leaf tissue. Other ''Colocasia'' plants such as Elephant-ear and
Dasheen Taro () (''Colocasia esculenta)'' is a root vegetable. It is the most widely cultivated species of several plants in the family Araceae that are used as vegetables for their corms, leaves, and petioles. Taro corms are a food staple in Africa ...
are an additional means of survival for this pathogen. Finally,
chlamydospore A chlamydospore is the thick-walled large resting spore of several kinds of fungi, including Ascomycota such as '' Candida'', Basidiomycota such as ''Panus'', and various Mortierellales species. It is the life-stage which survives in unfavourable c ...
s have been produced under ideal laboratory conditions in culture, and may also serve as a survival structure in addition to oospores. However, chlamydospores have not yet been observed in the field. Therefore, it is not known if chlamydospores are really part of the ''Phytophthora colocasiae'' disease cycle. Upon infection, oospores that overwinter on leaf tissue and petioles give rise to sporangiophores which have lemon-shaped sporangia at their tips. Sporangia can infect taro leaves either directly via
germ tube A germ tube is an outgrowth produced by spores of spore-releasing fungi during germination. The germ tube differentiates, grows, and develops by mitosis to create somatic hyphae.C.J. Alexopolous, Charles W. Mims, M. Blackwell, ''Introductory Myc ...
s or indirectly by producing
zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Diversity Flagella types Zoospores may possess one or mo ...
s. Whether sporangia infect directly or indirectly depends on weather conditions. If weather conditions are favorable, such as warm temperatures, sporangia infect directly via germ tubes. Germ tubes give rise to
appressorium An appressorium is a specialized cell typical of many fungal plant pathogens that is used to infect host plants. It is a flattened, hyphal "pressing" organ, from which a minute infection peg grows and enters the host, using turgor pressure capable ...
which form haustorium and allow the pathogen to extract nutrients without penetrating the host’s cell membrane. As a result, more sporangia are formed and if weather conditions remain favorable, additional sporangia are produced and infection continues either directly or indirectly on other hosts. Indirect infections occur under unfavorable or very wet conditions by releasing zoospores from sporangia. Zoospores lose their
flagella A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates. A microorganism may have f ...
, become
cyst A cyst is a closed sac, having a distinct envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac (like the manner in which ...
s,
germinate Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fer ...
and feed on the host via a germ tube, and produce more sporangia to continue the disease cycle.Jackson, G.V.H. (1980) Diseases and pests of taro. South Pacific Commission, Noumea, New Caledonia. 51 pp. The slanted shape of the taro leaf encourages sporangia and zoospores to spread to other hosts via splash from rain. The pathogen can also be transmitted across fields by infected plant material or contaminated tools. The pathogen can survive as
mycelium Mycelium (plural mycelia) is a root-like structure of a fungus consisting of a mass of branching, thread-like hyphae. Fungal colonies composed of mycelium are found in and on soil and many other substrate (biology), substrates. A typical single ...
for a few days in dead and dying plant tissues as well as in infected corms. On the other hand, encysted zoospores of ''P. colocasiae'' can survive for several months without a host. Once infection season comes to an end,
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
occurs to form an
oospore An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia, ...
. In order to have successful sexual reproduction, weather conditions must be favorable and
mating type Mating types are the microorganism equivalent to sexes in multicellular lifeforms and are thought to be the ancestor to distinct Sex, sexes. They also occur in macro-organisms such as fungi. Definition Mating types are the microorganism equivalent ...
s must match. Two mating types, A1 and A2, exist for ''Phytophthora colocasiae''. Hormonal signaling allows for sporangia of the two mating types to come together and initiate the development of
oogonia An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes. In the mammalian fetus Oogonia are formed in larg ...
and
antheridia An antheridium is a haploid structure or organ producing and containing male gametes (called ''antherozoids'' or sperm). The plural form is antheridia, and a structure containing one or more antheridia is called an androecium. Androecium is also t ...
. An
oogonium An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes. In the mammalian fetus Oogonia are formed in larg ...
can be likened to a female reproductive organ while an antheridium carries out the role of a male reproductive organ. The penetration of an oogonium via an antheridium leads to the formation of a sexual spore or an oospore. The oospore will overwinter and germinate to produce infectious sporangia once conditions improve.


Environment

The pathogen grows voraciously in areas with high humidity and heavy rainfall in addition to an optimal pH of 6.5 and temperature of . The aforementioned cool, wet, and humid conditions favor both the asexual and sexual reproduction of ''Phytophthora colocasiae''. The ideal temperature range for this pathogen is . Sporangia, which develop most rapidly at margins of leaf lesions, can germinate directly on leaves at temperatures ranging from , or spread to neighboring leaves via rain splash. During less ideal conditions, such as low temperatures nearing and high humidity, sporangia release zoospores for indirect germination. Germination occurs over a period of approximately two hours, followed by a 2-4 day incubation period between germ tube penetration and the onset of disease symptoms. The ideal temperatures for reproduction of the ''Phytophthora colocasiae'' pathogen has led to its distribution throughout cool tropical areas of
Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...
, from where the pathogen is thought to have originated. ''P. colocasiae'' has been observed in
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
,
China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's most populous country, with a population exceeding 1.4 billion, slightly ahead of India. China spans the equivalent of five time zones and ...
,
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
, the
Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...
,
Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...
,
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
,
Papua New Guinea Papua New Guinea (abbreviated PNG; , ; tpi, Papua Niugini; ho, Papua Niu Gini), officially the Independent State of Papua New Guinea ( tpi, Independen Stet bilong Papua Niugini; ho, Independen Stet bilong Papua Niu Gini), is a country i ...
, and the
British Solomon Islands The British Solomon Islands Protectorate was first declared over the southern Solomons in 1893, when Captain Gibson, R.N., of , declared the southern islands a British protectorate. Other islands were subsequently declared to form part o ...
.


Management

In the past, control of ''P. colocasiae'' has been aimed largely at limiting the amount of inoculum via cultural practices. An example of such a practice is
roguing In agriculture, roguing refers to the act of identifying and removing plants with undesirable characteristics from agricultural fields. Rogues are removed from the fields to preserve the quality of the crop A crop is a plant that can be grown a ...
, which involves the removal of all or part of infected leaves. This practice has ultimately proved ineffective as the removal of leaves largely mimics the defoliating effects of the Taro Leaf Blight disease itself, and exacerbates the losses in yield already devastating the crop. Increased spacing between taro plants has been explored as a method of limiting transmission. However, this method has also proven ineffective as taro plants grow best when planted close together. Therefore, increasing space between plants also decreases yield. Chemical control of ''P. colocasiae'' has offered some relief in the form of preventative sprays containing
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
,
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
, and
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
. The slanted shape of taro leaves, and prevalence of the crop in wet climates, necessitates numerous applications which may not be economically practical. The systemic
fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...
metalaxyl Metalaxyl is an acylalanine fungicide with systemic function. Its chemical name is methyl N-(methoxyacetyl)-N-(2,6-xylyl)-DL-alaninate. It can be used to control ''Pythium'' in a number of vegetable crops, and ''Phytophthora'' in peas. Metalaxyl ...
has also been found to be effective in controlling ''P. colocasiae'', especially when applied in conjunction with the pesticide
mancozeb Mancozeb is a dithiocarbamate non-systemic agricultural fungicide with multi-site, protective action on contact. It is a combination of two other dithiocarbamates: maneb and zineb. The mixture controls many fungal diseases in a wide range of field ...
at the first sight of disease symptoms. While chemical control is one of the more effective methods of managing this pathogen, the fact that taro is usually a subsistence crop makes chemical control economically impractical and environmentally unsustainable. A recent study obtained promising results using '' Eucalyptus globus'' essential oil to combat ''P. colocasiae''. Upon chemical analysis of the
essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the o ...
, researchers determined the most abundant compounds to be 1,8-cineole,
α-pinene α-Pinene is an organic compound of the terpene class, one of two isomers of pinene. It is an alkene and it contains a reactive four-membered ring. It is found in the oils of many species of many coniferous trees, notably the pine. It is also ...
, and
p-cymene ''p''-Cymene is a naturally occurring aromatic organic compound. It is classified as an alkylbenzene related to a monoterpene. Its structure consists of a benzene ring ''para''-substituted with a methyl group and an isopropyl group. ''p''-Cym ...
. These compounds make up 26.4%, 14.1% and 10.2% of the essential oil, respectively.
In vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
studies demonstrated complete inhibition of sporangia germination and mycelial growth following applications of ''Eucalyptus globus'' essential oil at a concentration of 0.625 mg/mL and inhibition of zoospore germination at a concentration of 0.156 mg/mL.
In situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
, these compounds completely inhibited sporulation,
necrosis Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...
, and overall disease symptom expression at a concentration of 3.5 mg/mL. Despite the promise of chemical control, genetic resistance currently offers the best long-term control of ''P. colocasiae''. In 2013, researchers were able to confer resistance to the taro plant via the
oxalate oxidase In enzymology, an oxalate oxidase () is an oxalate degrading enzyme that catalyzes the chemical reaction: :oxalate + O2 + 2 H+ \rightleftharpoons 2 CO2 + H2O2 The 3 substrates of this enzyme are oxalate, O2, and H+, whereas its two products a ...
(OxO) gene gf2.8 of wheat (''
Triticum aestivum Common wheat (''Triticum aestivum''), also known as bread wheat, is a cultivated wheat species. About 95% of wheat produced worldwide is common wheat; it is the most widely grown of all crops and the cereal with the highest monetary yield. Ta ...
''). The researchers were able to isolate a transgenic line which did not develop disease symptoms following inoculation with ''P. colocasiae'' zoospores.He, X., Miyasaka, S., Fitch, M., Khuri, S. & Zhu, Y. 2013. Taro (''Colocasia esculenta'') Transformed with a Wheat Oxalate Oxidase Gene for Improved Resistance to Taro Pathogen ''Phytophthora colocasiae''. HortScience 48, 22–27. Several Taro Leaf Blight-resistant varieties, such as the K333, K345, and Ainaben, have also been identified. While genetic resistance appears to be the most powerful tool for managing ''P. colocasiae'' thus far, certain genetic modifications result in changes in the taste and appearance of the taro plant. Due to its grave cultural and economic significance, the challenge of developing resistant varieties without decreasing the cultural and economic value of the crop remains.


Importance

Taro is the fourteenth most consumed vegetable worldwide and is a
staple crop A staple food, food staple, or simply a staple, is a food that is eaten often and in such quantities that it constitutes a dominant portion of a standard diet for a given person or group of people, supplying a large fraction of energy needs and ...
both in the diet and economy of the
tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
. Many tropical nations rely on taro as a main export. Taro Leaf Blight causes varying losses in corm yield depending on how susceptible the cultivars are to Taro Leaf Blight infection and damage. Reductions in corm yield of 25-50% have been reported in various locations across the
Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
. Losses of 25-35% of corm yield have been recorded in the Philippines while in some extreme cases, losses of 95% have been recorded in various cultivars across Hawaii. The most recent epidemic of Taro Leaf Blight spanned the
Samoan Archipelago The Samoan Islands ( sm, Motu o Sāmoa) are an archipelago covering in the central South Pacific, forming part of Polynesia and of the wider region of Oceania. Administratively, the archipelago comprises all of the Independent State of Samoa an ...
from 1993-1994. Taro export made up 58% of Samoa’s economy and brought in 3.5 million US dollars annually immediately prior to the epidemic in 1993. In 1994, taro exports brought in only $60,000 US dollars - a 99.98% drop in profit in just one year’s time. The reason Taro Leaf Blight was able to ravage Samoa’s taro crop was because taro is vegetatively propagated via cuttings and not
seed A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiospe ...
s. Taro
monoculture In agriculture, monoculture is the practice of growing one crop species in a field at a time. Monoculture is widely used in intensive farming and in organic farming: both a 1,000-hectare/acre cornfield and a 10-ha/acre field of organic kale are ...
s with no leaf blight resistance had been created with very little genetic variance.Moorhead, A. May 2011. Lesson in Diversity from Samoa’s taro blight. Plant Protection.18-19. This narrow gene variance left the monocultures devoid of resistant plants to buffer the spread of the pathogen allowing it to quickly spread in the warm humid climate of Samoa, devastating taro production throughout the archipelago. Currently, the taro exportation of Samoa has rebounded due to crossing the Samoan variety with resistant varieties from Southeast Asia. The implementation of virus free tissue testing has ensured that no infected vegetative tissue can be sold and grown on Samoan soil. Samoa’s epidemic served as an example for other taro exporting countries to ensure Taro Leaf blight resistance among plantations and to test tissue to ensure that the disease does not spread across other countries.


References

{{Taxonbar, from=Q25220361 Fungal plant pathogens and diseases Phytophthora