Apical Chlorosis
Apical Chlorosis (Pseudomonas syringae pathovar tagetis)
Apical chlorosis is a bacterial disease that is found globally, but of minimal economic impact. However, the symptoms are striking and noticeable from a great distance, making the disease a frequent source of questions among sunflower growers and others. The disease was originally thought to be a nutrient deficiency or virus problem (and can still be confused with them), but is now known to be caused by a soilborne bacterium.

The bacterium infects and causes symptoms only on young (not fully developed) leaves. Symptoms are caused by a toxin which disrupts chloroplast development in foliar tissue, resulting in yellow to white leaves (Fig. 1, 2, 3, 4).
apical chlorosis
Figure 1. Sunflower plant, artificially inoculated, showing extreme Apical chlorosis due to inhibition of chloroplasts during leaf development
apical chlorosis
Figure 2. Sunflower seedlings with Apical Chlorosis
apical chlorosis
Figure 3. Single sunflower plant with Apical Chlorosis
apical chlorosis
Figure 4. Older (pre-bloom) sunflower with Apical Chlorosis
Infected plants can be seen while the plant is actively growing, including the seedling stage in spring to early summer and prior to bloom. The disease appears to be more common during periods of cool weather, aided by rain, which splashes the soilborne bacteria onto leaves. Infected plants may occur singly and randomly in a field (Fig. 5), or there may be a series of chlorotic plants in a row (Fig. 6).
 
apical chlorosis
Figure 5. Individual sunflower plant visible from field edge
apical chlorosis
Figure 6. Row of sunflower plants with Apical Chlorosis
The bacterium causes similar chlorotic leaves on various thistles (Fig. 7), cultivated marigolds (Fig. 8) and zinnias (Fig. 9), and other “composite flowered plants” in the Asteraceae. 
apical chlorosis
Figure 7. Canada thistle plant with Apical Chlorosis
apical chlorosis
Figure 8. Marigold plants with Apical Chlorosis
apical chlorosis
Figure 9. Zinnia plant, artificially inoculated, with Apical Chlorosis
Once the weather warms up and rapid plant growth resumes, subsequent upper leaves develop a healthy green color, while the affected chlorotic leaves will remain so for the life of the plant.

Since the affected plants recover, the few chlorotic leaves will not affect yield. While USDA research has identified inbred lines with resistance, there is little need to incorporate this into commercial hybrids due to the lack of yield-limited potential of the disease.  This bacterium is closely related to that causing bacterial leaf spot, which likewise is of minimal economic impact.
Images
Figure 1. Sunflower plant, artificially inoculated, showing extreme Apical chlorosis due to inhibition of chloroplasts during leaf development (Tom Gulya, USDA-ARS).
Figure 2. Sunflower seedlings with Apical Chlorosis (Tom Gulya, (USDA-ARS).
Figure 3. Single sunflower plant with Apical Chlorosis (Tom Gulya, USDA-ARS)
Figure 4. Older (pre-bloom) sunflower with Apical Chlorosis (Tom Gulya, USDA-ARS).
Figure 5. Individual sunflower plant visible from field edge (Tom Gulya, USDA-ARS).
Figure 6. Row of sunflower plants with Apical Chlorosis (Tom Gulya, UDSA-ARS).
Figure 7. Canada thistle plant with Apical Chlorosis (Tom Gulya, USDA-ARS).
Figure 8. Marigold plants with Apical Chlorosis (Tom Gulya, USDA-ARS)
Figure 9. Zinnia plant, artificially inoculated, with Apical Chlorosis (Tom Gulya, USDA-ARS).
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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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