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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.

Many lesions can occur on the same plant, and numerous lesions can occur in a canopy. Infected leaves (and subsequent stem lesions) are often limited to the lower half of the plant due to spores dispersed by rain splash. High humidity also contributes to the disease cycle.

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.

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 to 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)

Additional Resources

Fungicide efficacy trials for management of Phoma black stem conducted from 2017-2019 (Powerpoint) The Importance and Management of Phoma Black Stem in Sunflowers (Powerpoint) Management of Phoma black stem with fungicide (Powerpoint) Fungicide Management for Phoma Black Stem in Sunflowers (Research paper) The Importance and Management of Phoma Black Stem in Sunflowers (Research paper)   Management of Phoma black stem with fungicide (Research paper)

Other NSA Resources

Photo Gallery The Sunflower Magazine's Archive section Approved Chemicals

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).