It is not a good substitute if you are spraying apples and Cornell only recommends chlorothalonil against cherry leaf spot, peach leaf curl, brown rot, blossom blight and black knot ( BK is mostly about plums) and different timing for these is involved.
Myclobutanil (Funginex) is for rust, scab, and mildew on apples and protects against all the diseases on stonefruit that Chlor does besides peach scab and brown rot.
Don’t spray anything but stonefruit with Chlor and go on-line to get the exact timing- also following the label, or tell us what you are gunning for.
Spraying really is something that should be done with care and precision.