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Taro Leaf Blight (''Phytophthora colocasiae'') is a highly infectious
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.
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 inJava
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:
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* Mari people, a Finno-Ugric ethnic group in Russia
* Marian (given name), a list of people with the given name
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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 byoospore
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 sporangiophore
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