Be certain that you are using the correct product for the type of plant, as well as for the disease or pest you are using it to control. If leaf curl is severe, it is important to maintain tree vigor by thinning more fruit than usual, irrigation, and applying extra nitrogen fertilizer. As far as the ants go, I doubt that they are causing to much of a problem to your tree, unless you also have a problem with aphids. Ants supposedly herd and protect aphids like cattle. Aphids tend to congregate on new plant growth.
Salivary secretions cause a stunting of leaf and stem growth.
If aphids appear to be a problem, there are many insecticides on the market, which will effectively control them, as well as the ants. A preferred and safer method would be to purchase a quantity of lady bugs next spring, and allow them to cure the aphid problem for you. The ants will just go about their own business, doing ant things.