Black Petiole is due to a collapse of the cytoplasm in the parenchyma cells and believed to be associated with a potassium and phosphorus imbalance when the potassium level in the soil is low and the phosphorus concentration high.
The dorsal side of the internal leaf petioles or midribs turns dark grey or black at or near the point where the midrib attaches to the core. The affected area may be quite limited or may extend for 5 to 8 cm along the midrib.
High rates of nitrogen may also contribute to the problem. Black petiole tends to become more evident when cabbage has been stored for a period of time. Soft rot can become a problem in broccoli when optimum nitrogen is exceeded.