The vector of leaf curl disease of cotton is the whitefly.
A vector is a type of insect that can spread a disease from one plant to another. In the case of leaf curl disease in cotton, the whitefly is the insect that spreads the virus that causes the disease. The whitefly feeds on the sap of the cotton plant, and in doing so, it can spread the virus from an infected plant to a healthy plant. This can lead to the spread of the disease and can cause significant damage to cotton crops.