Answer: Your roses are probably going through some temporary stress from lack of water. Light textured planting mixes, especially in clay pots (which "wick" away water), can dry out quickly. The roses respond to the temporary "drought" by yellowing and dropping leaves. Strangely enough, too much water (soggy soil) can cause similar symptoms. This usually happens when a container doesn't have drainage holes.
Other possible causes are lack of nitrogen (the first number on a fertilizer product) or black spot fungus. This fungus causes dark spots on the leaves. The area around the spots typically turns yellow and the leaves soon fall off. Since you didn't mention seeing the spots, I suspect water (too much or not enough) is the probable cause.
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