Hi Rachel,

Zephirine Drouhin is a lovely rose, and also one of the heirloom varieties that can be prone to black spot, and powdery mildew. Neither of those seem to be the specific issue based on the photograph.  Yellowing of rose leaves is called chlorosis. The yellowing can be due to nutritional deficiencies - typically too little iron, too much, or too little water, lack of sun, insect pests, plant disease, or a combination of these. While the very yellowed leaf in the photograph, with it's single dark green blotch is suggestive to me of a nutritional deficiency the only way to be most certain is to do both a soil test, and also look more closely for insects or disease.  

In Summit County you can get a soil test through your county's Soil and Water Conservation District.  Their website has the instructions for collecting soil samples, forms, and cost for testing:  http://sswcd.summitoh.net/   The only way to know that you are providing roses, or any plant, with the necessary major, and trace nutrients is to know the soil's pH, and current nutrient levels.  

Spider mites, or leaf miners are two insect pests that can cause leaf yellowing - I cannot tell from your photograph if these are present. Neither can I tell if there is a virus in play. Spraying for insects or disease without knowing exactly what you are trying to eradicate can create more trouble than help.  The Ohio State University C.Wayne Ellet Plant and Pest Diagnostic Clinic (CWE-PPDC) diagnoses plant pests and diseases and you can get instructions for submitting samples on their website,  https://ppdc.osu.edu 

The American Rose Society provides a guide to rose diseases that you will also find helpful  for identifying problems:   https://www.rose.org/_files/ugd/b152c0_d42726a1f5b444d5967e72192a8b3c62.pdf

The Maryland State Extension service also provides some online education on rose diseases, and identification: https://extension.umd.edu/resource/rose-identify-and-manage-problems/

Both of the above also offer some guidance on rose maintenance. 

Thank you, and happy gardening!