Black Rot

• V-shaped yellow lesions pointing inward from leaf margins — the wedge shape is the most distinctive diagnostic feature
• Dark, blackened veins running through the yellow lesion and into the leaf midrib
• Yellowing and browning that progresses inward along the vein network, not in soft, wet patches like bacterial soft rot
• Stunted heads on cabbage and broccoli when infection enters the plant early in the season
• In cross-section, the vascular tissue (ring of bundles) in the stem appears dark brown or black rather than white
• Symptoms may appear first on one side of a plant or row — spread is uneven and follows water splash and insect movement

Xanthomonas campestris pv. campestris is a bacterial pathogen that spreads primarily through infected seed and transplants — many infections begin from contaminated seed lots that show no visible symptoms. The bacteria enter through leaf margin pores (hydathodes) where guttation water exits the plant, or through wounds. Once inside the vascular system, the bacteria spread systemically. Warm, wet weather and overhead irrigation that splashes bacteria between plants accelerate spread. The pathogen survives on infected crop debris for several weeks and on infected seed for years. Crucifer weeds near the garden can harbor the pathogen between seasons.

1. 01Start with certified disease-free or hot-water-treated seed — black rot is frequently seed-borne and this step prevents most primary infections
2. 02Rotate brassica crops on a 2–3 year cycle and remove all brassica debris thoroughly at season end
3. 03Avoid working in the brassica bed when foliage is wet — the bacteria spread on hands, tools, and clothing
4. 04Water at the base of plants rather than overhead; overhead irrigation is a reliable way to spread the pathogen through a planting
5. 05Remove and bag symptomatic plants early — do not compost brassica waste from an infected bed
6. 06Copper-based bactericides can slow spread when applied preventively, but will not cure systemic infection — apply before symptoms appear if the disease is established in your garden

If V-shaped lesions are appearing despite using certified seed and you're rotating properly, a diagnostic lab can confirm whether the pathogen is black rot or another brassica disease with similar symptoms, such as downy mildew or clubroot.

• Black Rot of Crucifers— University of Minnesota Extension
• Brassica Black Rot— Penn State Extension