Phoma Black Stem
Phoma Black Stem (Phoma macdonaldii)
Phoma black stem is the most commonly occurring stem disease in the US Northern Great Plains. While the disease occurs in the High Plains and Southern Plains states, it is less prevalent. Yield loss has been documented, but is believed to be rare.
 
The fungus survives on infested sunflower residue. Infection occurs when fungal spores are splash dispersed from residue and land on leaves during a period of free moisture. Infection often begins on leaf margins, and a chlorotic and necrotic angular lesion will form and progress across the leaf, through the petiole and into the stem.  Foliar lesions are difficult to distinguish from other sunflower diseases or ailments (such as Phomopsis stem canker), and generally not considered diagnostic. A small (approximately 2 inches long by 1 inch wide) coal-black stem lesion forms on the stem at the base of the petiole.
phoma black stem
Figure 1. Phoma black stem lesions on a single stem
 
Many lesions can occur on the same plant, and numbers lesions can occur in a canopy.  As spores are dispersed by rain splash and high humidity are needed to begin the disease cycle, infected leaves (and subsequent stem lesions) are often limited to the lower half of the plant.
phoma black stem
Figure 2. Numerous Phoma black stem lesions in a sunflower field
 
Lesion color can be diagnostic for sunflower stem lesions.  Phoma lesions are distinctly black, while Phomopsis stem canker lesions are distinctly brown. Sclerotinia mid-stalk lesions are distinctly cream to tan colored. Additionally, Phoma black stem lesions will remain superficial and healthy white tissue is apparent if the lesion is shaved off.  Conversely, lesions caused by Phomopsis stem canker and Sclerotinia mid-stalk rot will degrade and weaken the pith.
phoma black stem
Figure 3. Phoma black stem lesion (Right) compared to a Phomopsis stem canker lesion (Left)
 
While yield loss is rare, it has been documented in at least two situations. Yield loss may occur if the pathogen is vectored inside and within the stem by burrowing sunflower stem weevils (Apion). Additionally, yield loss has been observed in portions of fields with very high yield. Active management of Phoma black stem is generally not considered necessary. Crop rotation, avoiding planting adjacent previously cropped sunflower residue, and selection of a less susceptible hybrid (if known) may help manage the disease. Foliar application of an efficacious fungicide can also effectively manage the disease, but is thought to be rarely economically viable.
Images
Figure 1. Phoma black stem lesions on a single stem (Sam Markell, NDSU)
Figure 2. Numerous Phoma black stem lesions in a sunflower field (Sam Markell, NDSU)
Figure 3. Phoma black stem lesion (Right) compared to a Phomopsis stem canker lesion (Left) (Sam Markell, NDSU)
Important Phoma black stem Resources:
Fungicide efficacy trials for management of Phoma black stem conducted from 2017-2019.
https://www.sunflowernsa.com/uploads/90/15.Hansen.etal_phoma_2020.pdf

https://www.sunflowernsa.com/uploads/81/2.Hansen_etal_import.mgmt.phoma.black.stem_2019.pdf

https://www.sunflowernsa.com/uploads/71/gilley_management.phoma_2018.pdf

https://www.sunflowernsa.com/uploads/research/1356/2020PHOMANSAResearchForumPaper.pdf
 
https://www.sunflowernsa.com/uploads/research/1341/2019NSAResearchForumPaper.pdf
 
https://www.sunflowernsa.com/uploads/research/1330/ManagementofPhomablackstemwithfungicide.pdf
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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