Black spots on cabbage leaves concern both gardeners and consumers, raising questions about the plant’s health or edibility. These discolorations can affect the outer wrapping leaves or penetrate the internal head structure, sometimes only becoming visible after the cabbage is cut. The spots can result from a range of issues, from highly contagious infectious diseases to simple environmental stress or nutrient disorders. Identifying the visual characteristics of the spots is the first step in determining whether a plant requires treatment or if the issue is merely cosmetic.
Fungal Pathogens
The most frequent fungal culprit behind dark spots on cabbage is Alternaria Leaf Spot, caused by the fungus Alternaria brassicicola. The lesions start as small, dark specks, often on older, lower leaves, that enlarge into circular or slightly irregular spots with a characteristic “target pattern.” These spots feature concentric rings of dead tissue, resembling a bull’s-eye, and may be surrounded by a yellow halo.
The fungus thrives when leaves remain wet for extended periods, especially when temperatures are moderate (55 to 75 degrees Fahrenheit). Spores of Alternaria are dispersed by wind and splashing water, allowing the disease to spread quickly. Severe infections can lead to the merging of multiple spots, causing large sections of the leaf to die and potentially leading to head rot in storage.
Bacterial Pathogens
Black Rot is a disease caused by the bacterium Xanthomonas campestris pv. campestris. The bacteria typically gain entry through the leaf’s hydathodes, which are natural pores located along the leaf margins where water droplets collect. The infection first appears as yellowing at the leaf edge, which then expands into a characteristic “V-shaped” lesion with the point directed toward the center of the leaf.
As the disease progresses, the bacteria move systemically into the plant’s vascular system, causing the veins within the affected area to turn black. This internal blackening of the water-conducting tissue is a signature symptom of Black Rot. The disease flourishes in warm, wet conditions and, once systemic, can travel into the head, causing internal decay.
Environmental and Physiological Factors
Not all black spots indicate a contagious disease, as many result from non-infectious physiological issues or physical damage.
Peppery Spot
One such disorder is Peppery Spot, characterized by numerous, very small black specks, typically less than one millimeter in diameter. These tiny spots are randomly scattered across the inner and outer leaves.
Peppery Spot is considered an aesthetic flaw rather than a spreading pathogen. Since it does not spread from plant to plant, it is often only noticed when the head is sliced open.
Tip Burn
Another common non-infectious issue is Tip Burn, a disorder related to inadequate calcium transport to the rapidly growing inner leaves of the cabbage head. This lack of calcium, often exacerbated by fluctuating water availability or high levels of nitrogen fertilizer, causes the tissue at the leaf margins to break down. The resulting internal necrosis manifests as a brown or black discoloration along the edges of the inner leaves.
Physical and Cold Damage
Physical and cold damage can also lead to dark spots, especially on the outer wrapping leaves. Severe or prolonged freezing temperatures can cause cellular damage that appears as blackening. Upon thawing, the leaves may become water-soaked and translucent, sometimes with the outer layer of tissue separating, leading to superficial black or brown spots. This type of damage is entirely external and does not progress or spread like an infectious disease.