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Abstract
Cotton is one of the most important crops of Pakistan, accounting for over 60% of
foreign exchange earnings. The present epidemic of cotton leaf curl disease (CLCuD)
originated in the Punjab region near the city of Multan and was first reported in
1985, although it was noted in this region as early as 1967. By the early 1990s, CLCuD
had become the major limitation to cotton production in Pakistan and it has now spread
into India and, more recently, south and west into other provinces of Pakistan. The
very characteristic symptoms include leaf curling, darkened veins, vein swelling and
enations that frequently develop into cup-shaped, leaf-like structures on the undersides
of leaves. Identification of the vector of CLCuD as the whitefly Bemisia tabaci (Genn.)
quickly led to the suggestion that the causative agent of the disease is a geminivirus.
Researchers soon confirmed the presence of such a virus that is currently ascribed
to the genus Begomovirus of the family Geminiviridae, However, in 1999, the aetiology
of the disease was shown to be more complex than was originally assumed. Despite the
identification of both a begomovirus and a so-called nanovirus-like component, the
precise causal agent of CLCuD remains uncertain.