Bacterial Head Rot

Young sunflowers track the sun across the sky, reaching new heights with every passing day - except, sometimes they don’t. Sometimes, a small injury can become infected and a black rot spreads around the base of the flower, finally engulfing it in a black goo that dries and hardens into a smelly shadow of what might have been. What causes this, and can it be prevented?

​Head rot, also known as pin rot, is a disease of sunflowers, lettuce, and broccoli, caused by the Pectobacterium carotovorum, subsp. carotovorum and P. atrosepticum bacteria.

Symptoms of bacterial head rot

The first symptom of bacterial head rot is nothing more than a small, brown, greasy or water soaked looking area on the surface of a cluster of unopened flowers or leaves. These lesions are usually seen at the sight of mechanical injury caused by bird and insect feeding, hail, or falling twigs. Bacteria enter the damaged tissue and that’s where the infection begins.

Affected areas turn from brown to black as the infection spreads into surrounding plant tissue. There is a distinctly bad smell, similar to rotten potatoes, but it is rare for secondary fungal growths to occur when head rot is present. If the bad smell is absent and other bacterial and fungal infections are present, the infection is more likely to be caused by Alternaria fungi.

Bacterial head rot prevention and control

Cool winter and spring temperatures combined with prolonged periods of rain, fog, and dew provide the perfect medium for bacterial head rot pathogens. This means good air circulation between plants can go a long way toward preventing this disease. That’s a good thing, because chemical sprays and other treatments have not been consistently effective in preventing bacterial head rot.

The best way to prevent this problem in your garden is to start with resistant cultivars, such as broccoli with dome-shaped heads, space plants properly, and avoid overhead watering.