Answer: Based on your description I suspect it may be overwatered. Roses need an evenly moist yet well drained soil. This means not saturated or sopping wet. Sometimes when you sink a container into the ground the hole works like a sump and collects extra water in the bottom. This means the potting mix is sitting in water and absorbing it like a sponge and stays constantly wet. Another possibly soil problem is if the new soilyou used when you repotted it is not draining at the same rate as the original potting mix -- most soilless mixes draing quickly. If the roots are overly wet they may rot, one of the signs of this is foliage problems.
Another possibility is that a fungal infection such as black spot might be starting. I would suggest you work with your retailer to try to determine the specific cause of the problem and then knowing that, decide how to proceed. I hope it is nothing too serious. Good luck with your tree rose.
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