Hi there.  Great questions. Black Rot is a common fungal disease of grapes and requires several steps for control. (Captan is one type of fungicide)  One of the steps is to use a fungicide application to break the disease cycle.  This fungus overwinters in spores that are found on the dead brown grapes (called mummies) that can still be stuck to the vines, as well as in dead leaves that maybe around the base of the vines.  So cleaning up any and all debris can be very helpful in slowing the spread.  Fungicides are not treatments, but act as prevantative measures, so applying them at correct times based on the disease cycle and according to label directions for the specfic product that you purchase to use is really important. The funcicide application will cover the buds, leaves and fruit so that fungal spores can not attach and infect the plsnt.  The fungicide needs to be labeled for use on grapes and to control black rot. This factsheet is excellent.  Many fungucides can be bought in Ready To Use forms, which do not require mixing. They generally come in a spray bottle, and can be a little more costly,because of the convenience of not having to mix.  Additional information can be found here. Please reply with questions.