Geminiviruses transmitted by Bemisia tabaci
In a recent paper, Dr J.K. Brown has tried to evaluate the global impact of whitefly-transmitted geminiviruses and to establish the current distribution of Bemisia tabaci (EPPO A2 quarantine pest) with emphasis on the B biotype (B. argentifolii). This biotype has shown an extraordinary ability to adapt to an extremely broad host range. It has developed resistance to several compounds (e.g. DDT, methyl-parathion, endosulfan) and can be distinguished by using an esterase marker, and essentially by its ability to cause phytotoxic disorders (e.g. squash silver leaf). The B biotype occurs in subtropical and temperate regions in the Old World (India, Africa, Arabian Peninsula, Europe and Mediterranean Basin, Japan) and in the New World. In the southern States of US in the 1988/89: A and B biotypes were present in Arizona and California; in Florida and Texas only the B biotype occurred. Now in south US only the B biotype is present (Arizona, California, Florida, Georgia, Tennessee, Texas), and it is also present in New York and Hawaii. It is present in Mexico (Yucatán Peninsula (Quintana Roo), Sonora, Tamaulipas). In the Caribbean only the B biotype is now present (Antigua ; Barbuda, Dominican Republic, Grenada, Guadeloupe, Puerto Rico, Trinidad ; Tobago, St Kitts ; Nevis). In Central and South America, it is present in: Belize, Brazil, Guatemala, Honduras, Panama. In Jamaica, the occurrence of squash silver leaf symptoms suggests its presence. It appears that there is a southward spread of the B biotype into the West coast regions of Mexico and throughout Venezuela. The present situation of the most important diseases caused by geminiviruses transmitted by B. tabaci is summarized below.
- Diseases of cucurbits
So far, two diseases have been identified and documented.
- Squash leaf curl geminivirus (EU Annex I/A1) is present in Costa Rica*, Dominican Republic*, Guatemala*, Honduras*, Mexico* (Northern States - Sonora, Sinaloa), Nicaragua*, USA (Arizona, California). It appears to be a complex of strains, including watermelon curly mottle virus and melon leaf curl virus.
- Watermelon chlorotic stunt geminivirus has been found in watermelons in Yemen. It causes yellow foliar mottle, stunting and reduction of fruit set.
- Diseases of Euphorbiaceous crops
African cassava mosaic and Indian cassava mosaic geminiviruses both occur in the Old World on cassava and can cause serious losses.
- Diseases of Malvaceous crops (cotton and okra)
- African cotton mosaic geminivirus has been reported for the first time in Sudan on Gossypium barbadense and was called at that time cotton leaf mottle. The virus has been reported throughout Africa, but essentially in the regions south of the Sahara: Benin, Cameroon, Central African Republic, Chad, Côte d'Ivoire, Ghana, Mali, Nigeria, Sudan, Togo, Tanzania.
Mosaic symptoms on cotton have been observed elsewhere (Brazil, Colombia, Dominican Republic, El Salvador, Guatemala, India, Nicaragua, Puerto Rico) but there is not sufficient information to determine similarities or differences between these disorders and the African cotton mosaic geminivirus.
- Cotton leaf crumple geminivirus was described in US cotton-growing areas in Arizona and California (near the Colorado river Valley) in the 1950s and later in Mexico*. Recently its incidence has increased in these regions.
- African cotton leaf curl geminivirus was first reported in 1912 in Nigeria. The disease is now present in Africa north of the Equator (except Egypt and Maghreb), in Benin, Burkina Faso, Chad, Côte d'Ivoire, Togo. In Pakistan, yield losses due to this disease have recently been very significant.
Diseases caused by uncharacterized whitefly-transmitted geminiviruses are now very important in cotton-growing regions of Africa (Mali, Sudan) and in the Indian sub-continent (India, Pakistan).
Preliminary molecular studies on cotton leaf crumple, African cotton mosaic, leaf curl from Sudan and 4 whitefly-transmitted geminivirus of cotton from the Americas (Dominican Republic, Guatemala, Texas) have shown that the New World viruses are similar though distinct from one another and present differences with the Old World viruses. However, further studies are still necessary.
- Okra leaf curl geminivirus has been reported from Côte d'Ivoire and Nigeria and is likely to occur elsewhere in Africa and Asia where okra is grown in whitefly-infested areas. Similar symptoms (vein thickening, upward leaf curl, stunting) have been observed in the Caribbean Basin and in Guatemala, but the diseases have not been characterized.
- Leguminous crops
Very serious epidemics have been reported throughout the world. In the Tropical Americas and the Caribbean Basin, whitefly-transmitted geminiviruses have been the most destructive diseases since the 1960s. Several of them have been characterized.
- Bean golden mosaic geminivirus (EU Annex I/A1) is present in the Caribbean Basin and Latin America. A golden mosaic-like geminivirus from Florida had been reported for the first time in 1993 and has recently been identified as bean golden mosaic geminivirus (see EPPO Reporting Service 95/142)
- Bean dwarf mosaic geminivirus from Colombia differs significantly from bean golden mosaic virus and is therefore thought to be a distinct virus.
- Bean calico mosaic geminivirus (the northernmost isolate in the Americas) has been shown to be a distinct virus.
- Mung bean yellow mosaic geminivirus from Thailand was shown to have sequence similarities with both the Old World virus (African cassava mosaic virus) and two New World viruses (tomato golden mosaic and bean golden mosaic geminiviruses).
Other diseases of legumes have been reported but have not yet been characterized: soybean crinkle leaf in Thailand, soybean yellow mosaic in India and yellow mosaic disease of soybean in Venezuela.
- Diseases of Solanaceae
Diseases characterized by leaf curl, yellow or golden mosaic, poor yield and reduction of fruit quality have been reported from many parts of the world. Epidemics on tomato and pepper have been seen throughout the Mediterranean Basin and Asia (India, Sri Lanka, Taiwan). The earliest reports involve diseases of tomato, capsicum, tobacco: in India in 1946 and 1963/64, in Israel and the Near East in 1966. Similar symptoms were then described in tomato or tobacco in Brazil, Cape Verde, Turkey, Côte d'Ivoire, Japan, Jordan, Nigeria, Philippines, Sri Lanka, Venezuela. Since 1984, whitefly-transmitted geminiviruses in pepper, tobacco and tomato began to be widespread in Southern parts of US (Florida, Texas), Mexico, Caribbean (Antigua ; Barbuda, Dominican Republic, Puerto Rico, Trinidad ; Tobago), Central America (Belize, Costa Rica, Guatemala, Nicaragua, Panama). And recently, outbreaks on tomato have been reported from Brazil.
Tomato yellow leaf curl geminivirus (EPPO A2 quarantine pest) is now a well characterized virus. 'Florida tomato virus' appears in EU Annex I/A1 and is now correctly named tomato mottle geminivirus. Further studies are needed on the relations between these and other more or less characterized whitefly-transmitted geminiviruses of tomato and capsicum, and on the relationships between Old World and New World viruses.
Dr Brown suggests that the changes observed are linked to the worldwide dispersion of the B biotype of B. tabaci (rather than a genetic change occurring simultaneously in local populations throughout the world), which can be transported on ornamental and vegetable plants (especially Euphorbia pulcherrima and chrysanthemum). She stresses that additional data is needed concerning uncharacterized whitefly-transmitted geminiviruses and associated diseases. Better knowledge on the disease incidence, virus identity, distribution, composition of mixed infections and investigations of the molecular mechanism of virus transmission in the hope to design resistant modified plants are obviously needed. Finally, only an IPM approach could provide control of these diseases (reduction of B. tabaci population pressure, host plant resistance, etc.),
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* new records according to the EPPO Secretariat.
Sources
Brown, J.K. (1994) Current status of Bemisia tabaci as a plant pest and virus vector in agro-ecosystems worldwide.
FAO Plant Protection Bulletin, 42 (1-2), 3-32.