While the first part of that title may have been true when trying to control powdery and downy mildew with Bordeaux mixture in wine grapes, it is no longer the case when controlling small grain diseases with the current systemic fungicides.
When does adding a fungicide to your herbicide program in small grains make sense?
The simplest answer is “Only when you have found disease in your field or have insufferable early-season disease risk”. Adding fungicides in the absence of a disease does not buy you anything other than risking crop injury, as certain fungicide formulations may ‘heat up’ specific herbicides. It is therefore important to read the label restrictions on the herbicides and fungicides you plan to use.
Source : umn.edu