Cassava mosaic virus
Encyclopedia

Introduction

African cassava mosaic virus (ACMV), East African cassava mosaic virus (EACMV), and South African cassava mosaic virus (SACMV) are distinct species of circular single-stranded DNA viruses that are whitefly
Whitefly
The whiteflies, comprising only the family Aleyrodidae, are small hemipterans. More than 1550 species have been described. Whiteflies typically feed on the underside of plant leaves.-Agricultural threat:...

-transmitted and primarily infect cassava
Cassava
Cassava , also called yuca or manioc, a woody shrub of the Euphorbiaceae native to South America, is extensively cultivated as an annual crop in tropical and subtropical regions for its edible starchy tuberous root, a major source of carbohydrates...

 plants. These have thus far only been reported from Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

; related species of viruses (Indian cassava mosaic virus
Indian cassava mosaic virus
Indian cassava mosaic virus is a plant pathogenic virus of the family Geminiviridae.-External links:**...

, ICMV) are found in India and neighbouring islands (Sri Lankan cassava mosaic virus, SLCMV), though cassava is cultivated in Latin America as well as South East Asia. Nine species of cassava-infecting geminiviruses
Geminiviridae
Geminiviruses are plant viruses which have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication . According to the Baltimore classification they are considered class II viruses...

 have been identified between Africa and India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

 based on genomic sequencing and phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

 analysis. This number will probably grow due to a high rate of natural transformation associated with CMV

The viruses are members of the Family Geminiviridae and the Genus Begomovirus
Begomovirus
The genus Begomovirus contains more than 200 species and belongs to the taxonomic family Geminiviridae. They are plant viruses that as a group have a very wide host range, infecting dicotyledonous plants...

. The first report of cassava mosaic disease (CMD) was from East Africa in 1894 . Since then, epidemics have occurred throughout the African continent resulting in great economic loss and devastating famine
Famine
A famine is a widespread scarcity of food, caused by several factors including crop failure, overpopulation, or government policies. This phenomenon is usually accompanied or followed by regional malnutrition, starvation, epidemic, and increased mortality. Every continent in the world has...

. In 1971 a resistant line of cassava, the predominant host of this plant pathogenic virus, was established and used by the International Institute of Tropical Agriculture
International Institute of Tropical Agriculture
International Institute of Tropical Agriculture is one of the world's leading research partners in finding solutions to hunger, malnutrition, and poverty. IITA's award-winning research-for-development addresses the development needs of tropical countries...

 in Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

. This resistance worked as an effective control for many years. However, in the late 20th century, a more virulent virus broke out in Uganda
Uganda
Uganda , officially the Republic of Uganda, is a landlocked country in East Africa. Uganda is also known as the "Pearl of Africa". It is bordered on the east by Kenya, on the north by South Sudan, on the west by the Democratic Republic of the Congo, on the southwest by Rwanda, and on the south by...

 and quickly spread to East and Central Africa. This highly virulent virus was later discovered to be a chimaera
Chimaera
Chimaeras are cartilaginous fish in the order Chimaeriformes, known informally as ghost sharks, ratfish , spookfish , or rabbitfishes...

 of two distinct begomovirus species .

Currently, CMD is managed through phytosantitation practices as well as the use of conventional resistance breeding. Additionally, vector management and cross-protection help to minimize transmission and symptom development. Though management practices are useful, the viruses’ high rate of recombination
Recombination
Recombination may refer to:* Recombination , the process by which genetic material is broken and joined to other genetic material* Recombination , in semiconductors, the elimination of mobile charge carriers...

 and co-infection capabilities have caused CMD to be one of the most detrimental diseases affecting food supply in Africa .

Hosts and Symptoms

Cassava originated in South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

 and was introduced to Africa in relatively recent times . It is known to be a very drought-tolerant crop with the ability to yield even when planted in poor soils. When cassava was first grown in Africa, it was used for subsidiary purposes though it is now considered to be one of the most important food staple crops on the continent. Its production is moving toward an industrialized system in which plant material is used for a variety of products including tarch, flour, and animal feed .

As cassava is vegetatively propagated, it is particularly vulnerable to viruses and thus Cassava geminiviruses lead to great economic loss each year.. When these infect a host plant, the host plant’s defense system is triggered. Plants use gene silencing
Gene silencing
Gene silencing is a general term describing epigenetic processes of gene regulation. The term gene silencing is generally used to describe the "switching off" of a gene by a mechanism other than genetic modification...

 to suppress viral replication, though begomoviruses have evolved a counter-acting suppressor protein against this natural host defense . Because different species of begomovirus produce different variants of this suppressor protein, co-infection by multiple species typically leads to more severe disease symptoms .

Initially following infection of a cassava geminivirus in cassava, systemic symptoms develop . These symptoms include chlorotic mosaic of the leaves, leaf distortion, and stunted growth . Infection can be overcome by the plant especially when a rapid onset of symptoms occurs. A slow onset of disease development usually correlates with death of the plant.

Though the cassava-infecting geminiviruses causes most of their economic damage in cassava, they are able to infect other plants. The host range depends on the species of virus and most are able to be transmitted and to cause disease on plants of the genera Nicotiana
Nicotiana
Nicotiana is a genus of herbs and shrubs of the nightshade family indigenous to North and South America, Australia, south west Africa and the South Pacific. Various Nicotiana species, commonly referred to as tobacco plants, are cultivated and grown to produce tobacco. Of all Nicotiana species,...

and Datura
Datura
Datura is a genus of nine species of vespertine flowering plants belonging to the family Solanaceae. Its precise and natural distribution is uncertain, owing to its extensive cultivation and naturalization throughout the temperate and tropical regions of the globe...

.

Causal Agent and Disease Cycle

Cassava geminiviruses are transmitted in a persistent manner by the whitefly Bemisia tabaci, by vegetative propagation using cuttings from infected plants and occasionally by mechanical means . Cassava produces its first leaves within 2-3 weeks of planting; these young leaves are then colonized by the viruliferious whiteflies . This is the key infection period for CMD geminiviruses, as they cannot infect older plants . As the genome of the viruses has two components, DNA A and B, that are encapsidated in separate geminate particles, it requires a double inoculation to cause infection .

Generally, Whitefly requires 3 hours feeding time to acquire the virus, a latent period of 8 hours, after which it requires 10 minutes to infect the young leaves . There is variation in the literature on this score, however, with other sources citing a 4 hour acquisition time and 4 hour latent period . Symptoms appear after a 3-5 week latent period. . Adult whiteflies can continue to infect healthy plants 48 hours after initial acquisition of the virus . A single whitefly is sufficient to infect the host, however, successful transmission increases when multiple infected whiteflies feed on the plant .

After entering the plant via the leaves, virus remains in the leaf cells for 8 days . As it is a single stranded DNA virus, it needs to enter the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 of the leaf cells in order to replicate itself . After this initial period, the virus then enters the phloem
Phloem
In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...

 and travels to the base of the stem and out into the branches . Travel to the branches of the plant is much slower than travel through the stem, so cuttings of branches from infected stems may be free of disease . Some literature has indicated that infection is limited to above ground tissue, but it is not clear why this would be the case .

Environment

The severity of cassava mosaic disease is impacted by environmental factors such as light intensity, wind, rainfall, plant density and temperature. Given that the viruses are transmitted by whitefly, the spread of the virus is going to depend largely on the vector. Temperature is the most important environmental factor controlling the size of the vector population . In the literature vector preferred temperature estimates vary from 20 to 30°C to 27 to 32°C but generally high temperatures associated with high fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

, rapid development, and greater longevity in whitefly . Increased light intensity has also been shown to increase activity of the whitefly vector .

Whitefly can fly at speeds up to 0.2 mph, but in high wind conditions they can move much greater distances in a shorter period of time thus increasing rate of virus spread . This wind dependent spread is reflected in the location of the whitefly in cassava fields, with populations greatest in upwind boarders and lowest within the field .

Virus incidence increases when cassava is growing vigorously . Thus, plant density impacts the spread of the virus, with low density fields encouraging faster disease propagation than high density fields . In dry areas, rainfall can be a limiting factor for cassava growth so higher rainfall will be associated with higher incidence of disease. Populations of whitefly will increase with rainfall, but heavy rains may impede whitefly spread and thus decrease incidence of virus. Timing of planting can also play an important role in the severity of disease, with cassava planted in march showing a 74% incidence rate of CMV, compared with 4% in August . Seasonal distribution of the virus will vary with the climate. In tropical rain forest type climates, where it is wet and humid most of the year, rapid virus distribution of the virus occurred from November to June, and slow progress occurred from July to September. This timing correlated with higher and lower temperatures. In a study of the disease in the Ivory Coast of Africa, maximum rate of disease spread was reached by two months after planting. Little to no infection occurs after three months, and variation in spread was due to varying in temperature, radiation and population levels of whitefly.

Control Strategies



Control strategies for cassava mosaic disease include sanitation and plant resistance. In this case sanitation means using cuttings from healthy plants to start with a healthy plot, and maintaining that healthy plot by identifying unhealthy plants and immediately removing them. This strategy does not protect them from being inoculated by white flies, but research shows that the virus is more aggressive in plants infected from contaminated cuttings than by insect vectors. There are also specific varieties that fare better against some viruses than others, so plant resistance is possible. For example, hybrids that are a result of crossing cassava and other species, such as Manihot melanobasis and M. glaziovii have been shown to have considerable resistance to CMV .

Importance

Mostly grown as a food source in Africa, cassava is the third largest source of carbohydrates in the world.
In recent times, cassava production has turned from subsistence to commercial production .
CMD was first described in 1894, and is now considered one of the most damaging crop viruses in the world . Annual economic losses in East and Central Africa are estimated to be between 1.9 and 2.7 billion USD . Although cassava is also cultivated in Latin America and South East Asia, the geminiviruses infecting it are only found in Africa and the Indian sub-continent. This has been mainly attributed to the inability of B. tabaci to colonize cassava effectively in this part of the world.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK