Bacterial Stalk and Head Rot
Bacterial Stalk and Head Rot (Pectobacteria spp.)
Bacterial stalk and head rot diseases are both caused by the same bacterial pathogens, Pectobacterium carotovorum  subsp. carotovorum  and P. atrosepticum, belonging to a group of soft-rot plant pathogenic bacteria.  They are known to occur in Russia and the United States and throughout Europe, South Africa, and several central African countries. These Pectobacteria belong to the soft-rot-causing group of plant pathogenic bacteria.  The diseases are capable of reducing yield and seed quality but are seldom economically damaging.
 
The stem rot phase exhibits dark green to black discoloration on the outside of affected stems, and the blackening is often centered around a petiole axil.
bacterial head rot
Figure 1. Dark discoloration associated with bacteria stalk rot.

The rotting of tissues is odorless unless stems are in an advanced state of decomposition.  Infected stalks may soften and dry up, becoming dark brown to black.
bacterial stem rot
Figure 2. Severely bacterial stalk rot.
Flowers and seeds are also discolored. Infected plant stalks will often lodge due to the weight of maturing heads on the weakened stalks.
bacterial stalk rot
Figure 3. Sunflower stem lodged from bacterial stalk rot.

The head rot phase is characterized by symptoms that may be initially confused with other head rot diseases caused by Rhizopus, Botrytis, or Sclerotinia.  Coalescing lesions develop watery, soft rot symptoms that dry and become dark brown as disease progresses.
bacterial stalk rot
Figure 4. Browning and softening symptoms of bacterial head rot.
Bacterial head rot can be distinguished from the other head rot diseases by producing an odor of rotting potatoes, the absence of fungal mycelium, and the presence of slimy masses of bacterial growth within infected tissues.
bacterial head rot
Figure 5. Bacterial exudate and ooze inside infected sunflower head and stem.
Due to bacterial activity, exudates and gases are formed as the sugars in the infected plants ferment. This may then result in drops of exudate that drips onto leaves
bacterial head rot
Figure 6. Bacterial ooze dripping from infected sunflower.
 
and/or a foam appearing on infected stem or head tissues.
 
bacterial head rot
Figure 7. Bacteria stalk rot infected sunflower ‘foaming’.
 
These two bacterial diseases often occur after storms or extended wet periods late in the growing season, which suggests that only stressed or senescing plants are highly susceptible.  Infection occurs at the petiole axis, a site that collects water and serves as a favorable environment for bacterial residence and insect activity (see Figure 1).

Under conditions of high humidity, the pathogen may be seen exuding out of wounds in slimy masses
bacterial stem rot
Figure 8. Slimy bacterial exudates on bacterial stalk rot infected sunflower.

and severely affected plant tissues will eventually disintegrate. If the weather becomes warm and dry, the affected tissues will also dry out and turn black as infection ceases.
bacterial stalk rot
Figure 9. Bacteria head rot infected sunflower drying and turning black.
 
Similar to Rhizopus head rot, infection is encouraged by wounds caused by any form of mechanical damage, insects, hail, other diseases such as Alternaria leaf spot.
bacterial stalk rot
Figure 10. Physically wounded and bacteria stalk rot infected sunflower.

Disease may begin on either the stalk and progress to the head or vice versa, depending upon the initial site of wounding and subsequent infection.  The pathogen survives in infested crop debris and as a pathogen on many other plants, including potato, onion, and carrot.
Figure 11. Progression of bacteria stalk rot into head tissue.
 
Other NSA Resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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