It is hard to tell much from the photos, but usually red stems is caused by nutrient deficiencies, of which nitrogen and phosphorus is the most common(K and Mg also possible). Feed with a mild dose of a good all around fertilizer and they should recover in a few days. If that does not work, then you probably have some lockout issues in your soil due to nutrient build up. If that is the case flush with plain water then foliar feed with a mild all around fertilizer to get the soil up take going again. Good luck, you are catching it early so it should be fine. My first guess is that they just need a little more juice. Has the growth slowed down? Are any of the leaves getting a little yellow? Also signs of possible deficiencies.
