All fungicides have strong and weak points. We all want one fungicide that works great on all fungi and can be sprayed on all our different species of fruit trees without any label restrictions. Unfortunately it doesn’t exist. You want to try to learn what some of the common diseases and pests look like and use that knowledge to pick fungicides and insect sprays that work well against those problems.
Captan is a reasonable choice if you are only going to buy one fungicide and inexpensive. But you need to adjust the pH of the spray water to the acid side since Captan breaks down quickly in basic water. The pH issue is explained in this post.
First Enterprise apple - #25 by mroot
Another option is to use a second fungicide to help the first in the weak areas. I use Captan and Immunox mixed in the same tank. I also use Daconil. For stone fruits you could use Captan mixed with Daconil up to shuck split. Daconil isn’t labeled for use on apples but the label allows use on many vegetables and ornamental plants in addition to stone fruits.
How many trees do you have? If your number of trees is small, shelf life of fungicides and insecticides can be a problem since you don’t spray much of it each year. Also is your orchard in a residential area? Some products are restricted to use in non-residential areas either by the label or state law.
At first all of this may seem overwhelming. I felt overwhelmed in the beginning when I started spraying. If you don’t know where to start you should look at Alan’s spray schedule. It’s a good place to start.
I also recommend that you get a copy of the label and read it before you buy any fungicide or insecticide.