Sorry, the purple,red, yellow leaves have shriveled up and dropped but the leaves are still tiny at about palm wide and hand long. I forgot to take pictures, but there doesn't seem to be any holes in the stem(which is about a cm thick) although there are tiny holes scattered on the leaves.

The primary edible parts of the rabe plant are the leaves. The best varieties have more leaves that bulbs and stalks. I planted a similar plant called spigiriello that is more stalky, but the leaves are great; tastes a lot like collards.

What ever that is it is not black spot. With black spot you get very distinct black spots on the GREEN leaves first. Then the leaf dies and turns yellow and drops. I do not see any black spots on your leaves at all. If the leaves are toward the very bottom and on the inside of the plant it could just be shedding leaves it not longer needs. As the plant grows and leafs out the bottom and inner leaves become shaded and no longer produce food for the plant. The plant then drops them to conserve its resources. Why continue to feed and hydrate a leaf that is longer contributing to the health of the plant?

Several different fungal diseases can cause the leaf spotting shown in the photograph. There are preventative sprays, but none that I know of that provide a cure once the problem is visible. While unsightly, there is no real danger that the infected plants are going to die or lose all their leaves, so most rhododendron growers ignore it. Since it's the lower leaves mostly, they were likely to be ready to fall anyway. Stress from heat or other causes does seem to play a role in encouraging such infections. Picking up and disposing of infected leaves can help in preventing any spread to uninfected plants.