Answer: There can be many reasons, some are cause for concern and some are not.
To diagnose, look beyond the yellow for other identifying marks. 1. Are there blackspots? The fungus blackspot causes leaves to develop black spots, turn yellow and drop off. 2. What about the leaf veins? Signs of nutrutional defiency are when the veins are yellow or darker than rest of the leaf. 3. Next turn the leaf over and look at underside. The fungal disease rust appears as small orange pustules, and causes yellowing. 4. See any webbing? In very hot weather spider mites ( look like tiny salt and pepper specks) attack the underside of the leaf. Spider mites are especially a problem if you use insecticides. No beneficial insects, bigger chance of getting mites.
If you don't see any of the above, one of the most common - and natural - reasons for leaves turning yellow is they are simply worn out! Sometimes though leaves meet their demise before their time because of stress. Lack of water or heat stress can really take its toll. So when the weather spikes, make sure the roses have lots of H2O to avoid stressed out roses, droopy bushes and leaf drop.
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