Whitefly-transmitted closteroviruses
A recent paper describes in detail the current knowledge on whitefly-transmitted closteroviruses. These viruses typically cause symptoms (stunting, interveinal yellowing or reddening of affected plants) which may easily be attributed to other causes, such as physiological or nutritional disorders, or even pesticide toxicity. They usually exist in low titres in infected plants and are generally restricted to phloem tissue. This has rendered diagnosis and isolation difficult. In recent years, this group has been rapidly expanding, and so far it includes the following viruses.
- Beet pseudo-yellows closterovirus (BPYV) was the first described whitefly-transmitted closterovirus. It was isolated from Californian glasshouses in 1965, and later found throughout the world. This virus is transmitted by Trialeurodes vaporariorum only. It has a broad host range: vegetables (carrot, cucumber, endive, lettuce, melon, spinach, courgette, sugar beet), ornamentals (Aguilegia, Callistephus, Godetia, Gomphrena, Tagetes, Zinnia), weeds (dandelion). Symptoms in cucurbits are first characterized by chlorotic angular spots on lower leaves. Leaf interveinal areas may become completely chlorotic except for the veins which remain green. Significant economic losses in cucurbit crops have been reported in North America, Europe and Asia. Other viruses transmitted by T. vaporariorum, cucumber yellows virus (found in Japan) and muskmelon yellows virus (found in France) are probably the same as beet pseudo-yellows closterovirus. Other probable synonyms are cucumber chlorotic spot virus (found in France) and melon yellows virus (found in Spain).
- Cucurbit yellow stunting disorder closterovirus (CYSDV) was first found in United Arab Emirates in 1982 on cucurbit crops. It was also found in cucurbits from Egypt, Israel, Jordan, Saudi Arabia, Spain and Turkey. This virus is apparently only present in the Old World. Its host range is limited to Cucurbitaceae. Symptoms are similar to those of beet-pseudo yellows closterovirus. It is transmitted efficiently by Bemisia tabaci biotype B, relatively inefficiently by B. tabaci biotype A and not transmitted by T. vaporariorum.
- Lettuce infectious yellows closterovirus (LIYV) (EPPO A1 quarantine pest) was found in the desert regions of California and Arizona in 1981, on lettuce, cucurbit and sugar beet. It is transmitted efficiently by B. tabaci biotype A and less efficiently by biotype B (100-fold less efficient). This virus is primarily restricted to the desert south-western USA. In 1981, severe losses (over 20 million USD) were observed on lettuce, cucurbit and sugar beet crops. Compared with previous years, yield reductions of 50 to 75 % and 20 to 30 %, were respectively seen on lettuce and sugar beet. In the 1980s and early 1990s, the displacement of biotype A by biotype B occurred. Due to this change, the cropping pattern had to be modified and in particular the melon crops planted in late-summer (which constituted very favourable hosts for the virus at that time of the year) had to be abandoned due to heavy damage caused by B. tabaci biotype B. As a result, since 1992, lettuce infectious yellows closterovirus has been present with a very low incidence (less than 0.1 %) and is no longer considered as a problem (provided changes in cropping patterns or vector populations do not occur).
- Lettuce chlorosis closterovirus (LCV) was observed in south-western USA when the incidence of LIYV became very low. Symptoms of lettuce chlorosis closterovirus are similar to those of LIYV in lettuce anõd sugar beet. An important difference is that lettuce chlorosis closterovirus does not infect cucurbits. It is efficiently transmitted by both A and B biotypes of B. tabaci. At present, this virus has not become a significant problem, and its incidence remains low.
- Tomato infectious chlorosis closterovirus (TICV) (see also EPPO RS 98/096) was first found in tomato fields in Orange county, California in 1993. In one season, the growers of this county suffered 2 million USD losses. This virus is specifically transmitted by T. vaporariorum. It affects tomatoes, several ornamentals (Ranunculus, china aster, petunia) and has also been detected in weeds (Picris echoides, Nicotiana glauca, Cynara cardunculus). It can also infect lettuce. Since its discovery, tomato infectious chlorosis closterovirus has been found mostly in tomatoes grown in commercial glasshouses and in tomatoes used in breeding programmes. The virus has now been identified in Italy, North Carolina and in several areas of California.
- Tomato chlorosis closterovirus (ToCV) was recently characterized, but it was known in Florida since 1989 as the yellow leaf disorder of tomato. It is transmitted by T. vaporariorum, B. tabaci biotypes A and B and by T. abutilonea. It appears that tomato chlorosis closterovirus is widely distributed in USA, as in addition to several counties in north central Florida, preliminary surveys showed that it is also present in glasshouse tomatoes in Colorado and Louisiana. During these surveys on tomatoes, another unnamed whitefly-transmitted closterovirus distinct from TICV and ToCV has been found, but further studies are needed.
- Sweet potato closteroviruses. Several virus names have been proposed in association with diseases of sweet potato in various countries (North and South America, Kenya, Israel): sweet potato chlorotic stunt virus, sweet potato virus-disease associated closterovirus, sweet potato virus disease associated closterovirus, sweet potato sunken vein virus, but it currently felt these are strains of the same virus called sweet potato chlorotic stunt closterovirus (SPCSV).
In addition to these viruses, more closteroviruses or clostero-like viruses have been reported, although not fully characterized: abutilon yellows virus, diodia vein chlorosis virus, nandina stem pitting virus. With the availability of biological and molecular diagnostic tools for whitefly-transmitted viruses, it is likely that new closteroviruses will be found. Finally it can be noted that taxonomy of closteroviruses is currently evolving, and it is now proposed to distinguish two genera within the family Closteroviridae:1) the genus Closterovirus to include monopartite aphid-transmitted viruses and 2) the genus Crinivirus to include bipartite and whitefly-transmitted viruses.
Sources
Wisler, G.C.; Duffus, J.E.; Liu, H.-Y.; Li, R.H. (1998) Ecology and epidemiology of whitefly-transmitted closteroviruses.
Plant Disease, 82(3), 270-279.