The thing with using hot caps is to make sure they have a decent opening at the top to allow good airflow. Without a good opening, you can easily cook the plants inside on a sunny day, even if it isn't very warm. Even with the opening, the plants are protected from wind and will be warmer and grow rapidly. I normally use wood shingles to shade the transplants from afternoon sun, but lately have been using more and more gallon milk jugs, sometimes 1/2 gallon milk or juice jugs with the bottoms cut out and the tops cut off to leave a 4 inch or larger hole in the top. Works great. If the plants didn't cook under the caps, it is possible that under the hot caps the plants got pretty moist from the rain, then it warmed up and the humidity rose quite high under the caps and made a perfect environment for various fungal diseases to develop and quickly take the plants. I've never grown melons from indoor transplants (always 100% transplant loss on any transplanted watermelon), so always grow them from seed and have great results. No covers needed because there is no transition from a protected environment to a garden or transplant shock. Have transplanted volunteer melons in the garden and it does seem to slow them down enough to let the seeded ones catch up.

Your roses might not be dead; there could be live cane below the soil line, or at the soil line. I would prune off the dead wood, and see what happens this spring. I'm not familiar with Don Juan, but it might be prone to dieback in a cold zone.

Paul3: Yes, consistently (little variation) dark (not really extremely dark), cool, dry are very important for dormancy storage of bananas. The interactions of light with non-freezing but low temperature causes chilling injury in most tropical plants which can progress to cell tissue death and subsequent attack by molds and fungi. What happens in this case is that when light strikes the leaves or stems or any photsynthetic tissues, the electrons gets fired up. But the low temperature lowers metabolic activities such that there are no active electron acceptors down the photosynthetic factory. The excited electrons when its energy cannot be diffused would literally damage tissues around it. Damaged tissues are susceptible to attack by fungi or bacteria. Only small wattage light are okay to use, and intermittently only when you need it. The dryness is important to discourage rotting away the corm. The cool is something natural during winter time and induces dormancy of tropical plants as long as it is non-freezing. These three things should be played out in balance. I have kept mine in garage where I have stable temperature that varies only by 10 deg F every day (45 deg F to 55 deg F) during the winter. If your attic temperature varies only by 10 deg F during the entire 24 hours, then that should be okay.

Jerijen says to get rid of any roses that had canker but my potted rose is doing great still. I know I can never put it in the soil or around other roses because the canker is in the wood. I have not looked at the roots to see if new canker is growing again because the plant is doing so well. Karl advised the bleach treatment to someone else and it worked for me. There is a treatment that will kill the critters that cause canker for use on the soil but it isn't 100% effective and its costly. I had a rose die off exactly as you described. It was healthy in December. I cut it down and moved it in January to a new spot. The old leaves fell off. The new ones grew a few inches then suddenly died. The canes turned dark at the base and continued until the whole cane was dead. I was wondering if I overwatered. One outlying cane is still growing leaves. I'm going to dig up the dead canes and look for cankers. Usually the plant goes into slow deline with canker-fewer, smaller flowers and small slow growing leaves. Eventually canes begin to die. This was a rapid die off of all the canes from the bottom up. Roses on either side are doing great. Wish I had not decided to move it but too late now. I had buried an old possum there the year before. Maybe he didn't appreciate me disturbing him when I planted the rose.