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The curly top disease is named because of the bunchy, twisted foliage characteristic of many infected crop plants. In sugarbeet, the most reliable, early diagnostic feature is an inward rolling of the leaf margins and a clearing of the minute veins on the youngest, innermost leaves. Initially, these symptoms are confined to a portion of youngest leaf but in a few days, the entire leaf is affected. Subsequently, wart-like protuberances develop on the veins of the lower leaf surfaces. Leaves are dark, dull-green in colour, thick, crisp and brittle. Occasionally, clear, viscid droplets are exuded from the petioles, midribs or veins on the lower leaf surfaces of infected plants. This liquid subsequently becomes black and sticky and then dries, forming a brown crust. A hairy or woolly condition of the roots develops in badly diseased sugarbeets as the number of rootlets increases. In cross section, affected sugarbeets show black concentric rings which alternate with light areas, while in longitudinal section the dark discoloration is seen extending lengthways. In tomatoes in the field, there is an inward rolling of leaflets along the midrib; the petiole and midrib frequently curve downwards, giving the leaf a drooping but not wilting appearance. Leaves become thickened and crisp and may later assume a yellow colour with purple veins. The pith dries out, leaving the stems hollow. As foliar symptoms appear, the plant ceases growth and takes on an erect, rigid habit. Fruits, if formed, ripen prematurely and seeds are aborted. Before aerial symptoms become apparent, roots may decay beginning at the tips. The stems and leaves turn brown and the plant finally dies.
In glasshouse-grown tomatoes, initially, the most diagnostic symptom is the appearance of transparent veins. Purple veination is usually absent. Inward curling of leaflets occurs, especially in older plants. White excrescences sometimes appear on the veins and a yellowing develops between them. There is a marked stunting of plants infected at an early stage. Eventually, the whole plant turns yellow and dies. In potatoes, the plants are stunted with yellowish, inward-rolled leaflets and sometimes a bending of the petioles. In advanced stages of infection, dwarfed shoots frequently develop in the axis of leaves near the tip of the plant. Ultimately, infected plants turn yellow and die (Bennett, 1971; Duffus, 1986).
Owing to the epidemiological complexity of the curly top disease, strategies for effective control are quite limited. However, successful control is possible through a combination of measures. Chemical control of the vector should be applied inside the production area as well as outside in order to eliminate the breeding areas of the leafhoppers (Duffus, 1986). Resistant cultivars should also be used, as described by Martin and Thomas (1986) and Lewellen (1989). Since most host crop plants are more readily infected by BCTV in the younger stages, seed treatment with systemic insecticides can provide some protection to susceptible plants. Therefore, the use of specific systemic insecticides when plants are most susceptible to BCTV infection is an effective alternative component in curly top disease control. It will also avoid undesirable effects on nontarget organisms by massive insecticidal spraying (Wang et al., 1999).
The disease was first recognized causing important damage in 1888, in Nebraska, USA, and has since caused frequent and often very destructive outbreaks throughout the area to the west of the Rocky Mountains and locally to the east of them. During the last 20 years in the USA, BCTV isolates have increased in severity to the point that isolates considered severe in the 1950s are now considered mild; infection of a resistant cultivar with severe BCTV isolates, even 10 weeks after seeding, can cause losses of over 13% when only 72% of the plants show symptoms. Furthermore, current virulent isolates have a serious impact on the sugar content of both susceptible and resistant sugarbeet cultivars (Duffus and Skoyen, 1977). In the EPPO (European and Mediterranean Plant Protection Organization) region, in contrast, BCTV is not considered a significant pest.