Tomato stem primordia are tiny bumps that may appear on the stem of a tomato plant. They are the beginning of a new root node which will develop when in the presence of water or if the plant should lose contact with its existing root system.
Small stipples appear along the stem of the plant. There may only be a few small patches or it might cover the majority of the stem.
None. This is usually nothing to worry about. The development of primordium is usually caused by high humidity, sustained wet weather or overwatering. Primordia are critical to the proper positioning and development of plant organs, in this case the root system. Advice on planting tomatoes often includes burying a proportion of the stem when potting on as this promotes the primordia to develop.
Bumps can, however, be a sign of a problem with the root system. Provided that the plants are not overwatered; continue to monitor any further growth.
Ensure the plants are not kept in excessively humid or wet conditions.