Rose leaves suddenly falling off bush
My formerly prolific cluster rose bush suddenly dropped all of its leaves. It is in a 24in x 24in cedar planter box on a south-facing 10th story terrace and is 5-6 years old. This happened well after the last frost, and shortly after I did mild pruning. A leave few shoots have formed but shortly thereafter dried up. The plant has gotten ample water. The other roses bushes (climbers and ramblers that have never bloomed as frequently or vigorously as my distressed rose) are doing fine, although the black spot has started. Any guidance is appreciated. I loved that bush and would hate to see it die.
First, carefully examine the rose for cankers, mites, thrips and sign of foliar disease. If these check out, I would remove the rose from the planter. Examine the roots for rot -anything mushy or obviously off kilter. There are several fungal diseases that can cause root rot as can overwatering. I would go so far as to remove and replace all potting mix in the pot. If there is a lot of root rot, use a hose or large bucket of water to remove the soil from the roots; it is probably contaminated. Trim off any bad roots, reset the rose, water it in and add long-acting fertilizer. Create shade or move the plant to a shady location for a few days while the plant settles in.