Potted roses may be transplanted in hot summer. Just water thoroughly. If you should disturb the root ball much, and the plants wilt, you may need to provide temporary shade for ten days. Ideally, daffodil time for bare roots and dogwood time for potted roses.

Right, don't fertilize until they have recovered and have put out plenty of leaves. Bloom Buster is the wrong kind of fertilizer--regular Miracle Gro or Miracle Gro for Roses is fine, or any reasonably balanced fertilizer such as 10-10-10 or 18-6-12. Watering correctly is the most important thing. Keep a 2" mulch of leaves or bark around the roses. When the soil under the mulch feels dry-ish, water thoroughly with 2-3 gallons for small plants or 5 gallons for larger, plus or minus for extremely hot or very mild, cloudy weather. Usually once a week is fine. For new plants, I divide the ration in half and water twice a week.

Prune well beneath the canker and dispose of all the prunings by bagging them and throwing them away. Disinfect the pruners between plants. Cut at about a 45 degree angle and make sure your pruners are nice and sharp before you start so they'll make clean cuts. I hope the canker is not too low on the canes or you'll be cutting back the Knockout roses lower than generally is recommended. I remember when Knockouts were first introduced and were touted as being extremely disease-resistant. I have been surprised how many people have had major black spot and canker issues with them....but that is a reminder to us all that resistance is merely resistance, not immunity.

midorit, ALL roses go through rest periods between what are known as "flushes" of bloom, and even though Knockouts are known for fairly continuous bloom, even they have down periods. And yours has been going through some trauma and stress. Don't overthink it, and don't kill it with love, lol. Keep it moist, not wet, and let it recuperate a while. Consider it normal for your rose to rest up to several weeks between flushes, depending on its health and the weather. When it finishes each flush, you can fertilize with a good rose fertilizer according to directions. Epsoma Rose-tone is good for long term health; a granular food is good for a quick boost. DON'T overuse the granular, it can burn the roots. But there are as many types of rose food as there are rose growers, lol. A time-release food works, too, and it's harder to overdo it. By the way, if that pot is sitting directly on the ground, it won't drain properly and you'll eventually cause the roots to rot. Raise it up on pot feet (or anything solid to get it off the ground) so the drain hole is unobstructed. Any Home Depot or Lowe's carries the nice terra cotta pot feet.