Bacterial Leaf Spot
Bacterial Leaf Spot (Pseudomonas syringae pv. helianthi)
Bacterial leaf spot (BLS), caused by Pseudomonas syringae pv. helianthi, is now commonly recognized throughout temperate sunflower production regions worldwide.  Although it is seldomly considered to be an economically significant problem, increased severity of the disease with subsequent foliar death and defoliation could pose problems in locations where high levels of humidity or rainfall are present.
 
Symptoms of bacterial leaf spot can vary greatly depending on cultivar and environmental conditionsGenerally, lesions begin on foliage as small necrotic spots of varying size and shape.
bacterial leaf spot
Figure 1. Necrotic lesions of various sizes caused by bacterial leaf spot.
 
These spots are water-soaked and angular, and later become necrotic.
bacterial leaf spot
Figure 2. Small angular lesions caused by bacterial lead spot.

The lesions may also be surrounded by chlorotic haloes,
bacterial leaf spot
Figure 3. Very small necrotic BLS lesions surrounded by chlorotic halos.

which also can coalesce to form large chlorotic areas on affected leaves.  As disease progresses, the leaf spots coalesce, causing tissues to dry, die and fall out, often in linear strips between leaf veins.
bacterial leaf spot
Figure 4. Coalescing lesions caused by bacterial leaf spot.

In some instances, infection becomes systemic and spreads along veins,
 
bacterial leaf spot
Figure 5. Systemic lesions caused by bacterial leaf spot.

but without the chlorotic bleaching of plant tissues associated with the toxins produced by the related apical chlorosis pathogen (Pseudomonas syringae pv. tagetis). 

Some cultivars tend to be more susceptible than others, yet the disease is still very commonly observed in temperate climates.  As the disease is often found occurring primarily on leaves in the lower part of the canopy, it seldom causes yield reductions. 
 
bacterial leaf spot
Figure 6. Severe bacterial leaf spot infections. Note: infection is most severe on lower leaves.
The pathogen can be both seedborne and soilborne on plant residues.  Like many other bacterial pathogens, it can be spread by splashing rains and high winds.  It also infects plants naturally through open stomates on leaves and through wounds created by hail, sandblasting, or other forms of mechanical damage.  Although the disease is readily observed in sunflower production throughout the high plains of North America, no control measures are warranted due to the normally low degree of damage to affected plants.
Images
Figure 1. Necrotic lesions of various sizes caused by bacterial leaf spot (Bob Harveson, University of Nebraska).
Figure 2. Small angular lesions caused by bacterial lead spot (Bob Harveson, University of Nebraska).
Figure 3. Very small necrotic BLS lesions surrounded by chlorotic halos  (Bob Harveson, University of Nebraska).
Figure 4. Coalescing lesions caused by bacterial leaf spot (Bob Harveson, University of Nebraska).
Figure 5. Systemic lesions caused by bacterial leaf spot (Bob Harveson, University of Nebraska).
Figure 6. Severe bacterial leaf spot infections. Note: infection is most severe on lower leaves (Bob Harveson, University of Nebraska).
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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