Cotton leaf curl disease, a multicomponent begomovirus complex

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Abstract
SUMMARY Cotton leaf curl is a serious disease of cotton and several other malvaceous plant species that is transmitted by the whitefly Bemisia tabaci. The disease is, at this time, endemic throughout Pakistan and epidemic in Western India. Affected cotton plants exhibit a range of symptoms such as leaf curling, stunted growth and a poor yield of cotton fibre. In addition, affected plants may develop leaf-like outgrowths from the veins on the undersides of leaves. A number of distinct begomoviruses have been shown to be associated with infected plants, all of which require a satellite component (DNA beta) to induce symptoms in cotton. DNA beta components are a group of recently identified, symptom modulating, single-stranded satellite molecules. An additional, satellite-like component, DNA 1, is invariably found in diseased plants, although it is not required for disease development. The viruses associated with the CLCuD complex on the Indian subcontinent, five of which have been identified thus far (Table 1), are all single component begomoviruses (genus Begomovirus family Geminiviridae). The satellite (DNA beta) and satellite-like (DNA 1) components have yet to be classified, although the DNA 1 components are closely related to, and thought to have originated from, components of a second group of single-stranded DNA viruses, the nanoviruses (family Nanoviridae). Physical properties: The begomoviruses associated with CLCuD, like all geminiviruses, have geminate (twinned) particles, approximately 18-20 nm in diameter and 30 nm long, consisting of two incomplete T = 1 icosahedra joined together in a structure with 22 pentameric capsomers and 110 identical protein subunits. It is probable, although not conclusively proven, that the DNA 1 and DNA beta components, being half the size of the viral component, are encapsidated in monomeric, rather than geminate particles. Disease symptoms: Symptoms in cotton usually appear within 2-3 weeks of inoculation by B. tabaci (determined experimentally (Singh et al., 1997)) and are initially characterized by a deep downward cupping of the youngest leaves. This is followed by either upward or downward curling of the leaf margins, swelling and darkening of the veins as well as the formation of enations on the veins, which frequently (dependant on variety) develop into cup-shaped, leaf-like structures (Fig. 1). Disease control: Control of CLCuD is mainly based on insecticide treatments against the insect vector (Bemisia tabaci). Roguing, the removal of affected plants, particularly of ratoon cotton from the previous seasons crop, is recommended but appears to have little affect in reducing the incidence of the disease. More recently, resistant cotton cultivars have been introduced that were developed by conventional breeding/selection. After initially showing promise in the control of CLCuD, recent reports have suggested that the virus complex has overcome the resistance. , .