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The symptoms of groundnut rosette disease are of three main types (Reddy, 1991; Naidu et al., 1998b), known as chlorotic rosette, mosaic rosette and green rosette.
- Chlorotic rosette is prevalent throughout sub-Saharan Africa. Younger leaflets first show faint mottling and all subsequent leaflets are pale yellow with green veins. Plants infected when young, produce progressively smaller, chlorotic, twisted and distorted leaflets, but when infected at a later stage, they show symptoms in only a few branches or the apical portion of the plant. Internodes are shortened and stems thickened, especially in plants infected when young. Affected plants, especially those infected young, are severely stunted.
- Mosaic rosette has only been recorded only in East and Central Africa (Storey and Bottomley, 1928; Storey and Ryland, 1957). Younger leaflets show conspicuous yellow and green mosaic symptoms. Later symptoms resemble those of chlorotic rosette. Stunting is less pronounced than for chlorotic rosette.
- Green rosette occurs in West Africa, Uganda, northern Malawi, and possibly in Angola. Younger leaflets show mild mottling and isolated flecks. Older leaflets are very dark green, reduced in size, and show downward lateral rolling. The plants are severely stunted and resemble plants infected with peanut clump furovirus.
Previously it was thought that the different symptoms were caused by different strains of GRV. It is now known that groundnuts infected with GRV alone show few or no obvious symptoms, and that rosette symptoms are caused by the GRV satellite RNA (Murant et al., 1988). The green and chlorotic forms of rosette are caused by 'green' and 'chlorotic' variants of the satellite RNA. Mosaic rosette, thought by Storey and Ryland (1957) to be caused by mixed infection by 'chlorotic' and 'mottle' strains of GRV, is actually caused by mixed infection with 'chlorotic' and 'mottle' variants of the satellite RNA (Murant and Kumar, 1990).
The following measures have been proposed (Reddy, 1991): removal of groundkeepers and volunteer plants, as these are primary sources of infection; early sowing at high plant densities (Booker, 1963; A'Brook, 1964; Farrell, 1976a); timely application of aphicides (Davies, 1972). The use of resistant varieties is the best control procedure.
Resistance to groundnut rosette disease was first found in groundnut from the region between Côte d'Ivoire and Burkina Faso (Sauger and Catharinet, 1954; De Berchoux, 1958). This material was the source of resistance in all rosette-resistant cultivars developed since. The resistance is controlled by two independent recessive genes and is effective against both chlorotic and green forms of rosette (Nigam and Bock, 1990). Additional sources of resistance to the disease have been identified in groundnut germplasm collections available at ICRISAT (Subrahmanyam et al., 1998). However, the resistance is directed only against GRV and there is no resistance to GRAV (Bock et al., 1990).
Resistance to Aphis craccivora has also been found in groundnut (Padgham et al., 1990) and could prove a useful additional approach to the control of rosette.
Groundnut rosette disease is important only in sub-Saharan Africa, where it is by far the most destructive of all groundnut diseases. The disease is not prevalent every year, and its unpredictability is one of its most harmful aspects. When epidemics occur they can be devastating. Thus in 1975 rosette destroyed 0.7 million hectares of groundnut in Nigeria (Yaycock et al., 1976), causing yield losses estimated at over 0.5 million tonnes. Groundnut rosette disease decreased plant height and spread 12 weeks after sowing by 40 and 48%, respectively. Leaf, stem and root dry matter at 8 weeks after sowing were decreased by 73, 81 and 88%, respectively. The number of leaflets/plant was reduced, the LAI was reduced from 1.0 in 1974 to 0.2 in 1975, the number of mature pods/plant was reduced by 80%. The shelling percentage decreased from 65.8-76.4% to 39.3-41.3% and pod yield was reduced from 2.29-2.8 6 t/ha to negligible amounts (Yayock, 1977).
In Zambia in 1995, losses were estimated at 5 million (Naidu et al., 1998b; P. Subrahmanyam, personal communication).
Losses due to groundnut rosette have also been reported from India. In Tamil Nadu, a survey revealed that rosette was found in 10 districts. The disease incidence was 4.92% in bunch and 3.26% in spreading varieties of groundnut and was nearly twice as high in irrigated as in rainfed crops. The loss in yield varied from 27 to 100%. Maximum infection was found when the crop was 75 days old (Kousalya et al., 1973).