A poinsettia plague: Bacterial canker

As a grower, it is natural to focus on the health of poinsettias’ roots. They have so many enemies: Thielaviopsis, Pythium, Phytophthora and Rhizoctonia, let alone fungus gnats. Focusing on root health is indeed important for healthy crops. But there are some other, less common things to watch for, including the bacterial canker of poinsettia.

Bacterial canker is a disease caused by Curtobacterium flaccumfaciens pv. poinsettiae that was first described after it appeared in New Jersey in 1942. The disease had probably gone unidentified for a number of years before plant pathologists were able to identify the pathogen. It is a Gram-positive bacterium, very different from the Xanthomonas that causes angular leaf spots and the Dickeya and Pectobacterium species that cause cutting rots. In the laboratory, it may grow relatively slowly compared to the soft rotters and leaf spotters, a trait that makes it easy to overlook in a petri plate. The appearance of this disease on poinsettia in Europe and the United States in recent years suggests that it is time to review the problem and work together as an industry to resolve it.

The bacterial canker pathogen is primarily disseminated within plant material. It is presumed to spread from poinsettia to poinsettia during irrigation or handling of the plants. The bacteria gain entry to the leaves via the stomates.

This particular bacterium, known previously under the name Corynebacterium poinsettiae, infects only poinsettia. There are other pathovars (host-specialized strains) of the species that affect other crops, including C. flaccumfaciens pv. oortii on tulips. A high proportion of the published research on Curtobacterium flaccumfaciens focuses on the disease that affects beans, which is caused by C. flaccumfaciens pv. flaccumfaciens.

Bacterial canker is actually a systemic disease, a bacterial wilt, that blights leaves, bracts and stems. The disease is an insidious threat in greenhouse production systems because infections can remain latent for some time: symptomless infections have been detected in poinsettia cuttings early in the growing season. Bacterial canker disease symptoms develop relatively slowly compared to other bacterial diseases. Symptoms often appear only on crops that are close to finishing.

Monitoring

Watch closely for symptoms during production.

If you see wet-looking stem lesions, this should trigger sending cuttings to a lab for ID.

Check for elongated, water-soaked streaks on the stem, or brown cankers that girdle the stem.

Look closely if you notice any defoliation: leaves on poinsettias with bacterial canker may show irregular brown (necrotic) areas and drop from the plant.

If plants appear severely diseased, look for longitudinal cracks in the stems and amber-colored droplets of bacterial ooze.

Prevention

Preventing bacterial disease requires excellent sanitation and pathogen exclusion by the propagator as well as by the finisher.

Within a greenhouse business, keeping sources and cultivars separated is ideal. Have workers change gloves, use a hand sanitizer or wash hands when they move to a new cultivar.

Avoid excessive nitrogen levels or high-temperature growing conditions, both of which have been associated with more serious bacterial canker problems.

When disease is suspected, discard the affected plants, retaining some to send to a laboratory for diagnosis. Poinsettias with bacterial canker cannot be cured.

Treatment

In the event of an outbreak, your best bet is to discard all infected plants. Treatments used for protection against the bacterial leaf spot caused by Xanthomonas axonopodis pv. poinsettiicola might bring side benefits against bacterial canker, but this has not been tested. Copper fungicides and Bacillus biocontrols (e.g. Cease, Triathlon BA) join sanitation in being the current weapons against bacterial disease spread in the greenhouse.

The author is a plant pathologist specializing in ornamentals at Cornell University’s Long Island Horticultural Research & Extension Center. She aims to help growers outwit diseases. mld9@cornell.edu