Phoma Black Stem
Phoma black stem
Black stem from Phoma

Phoma is the most common sunflower disease.  In the 2007 NSA Field Survey, field incidence ranged from 10% in Manitoba to 40% in Minnesota.  This disease is the result of a soil borne fungus.  The infection is characterized by large dark lesions on the stem.  The infection usually starts on the leaf and follows the petiole to the stalk.  The large patches on the stalk become most noticeable after petal drop.
 
phoma stalks
Stalks with Phoma

Life Cycle: The fungus overwinters in plant debris and is spread by splashing rain.  The disease is often spread by the stem weevils.  The larvae can spread the fungus while tunneling in the stalk.

Damage: Losses are generally minimal.  Infected plants may produce smaller heads and lighter seed. The stalk lesions are on the surface only and the inner pith is not destroyed.  Thus, Phoma by itself may not be a lodging factor.

Economic Thresholds: This is not well defined since there is not a fungicide treatment available.

Scouting Method: Not available    

Management: There is no fungicide treatment available for this fungus.  A four-year crop rotation is a good tool.  Controlling the stem weevil will reduce this disease as well.  Tolerant hybrids are available and most seed companies have rated their hybrids for resistance/tolerance to Phoma.

Research: Good progress has been made in overall plant health in the public and private sectors.  Expect more enhancements in the near future.

Photos: Visit the Photo Gallery

Another resource about Diseases can be found in the Archive section of The Sunflower magazine.

Source:  NDSU Extension Bulletin 25 Sunflower Production Handbook, NDSU Extension Service, September 2007 and High Plains Production Handbook June 2005.
 
Additional Resources
High Plains Sunflower Production Handbook  file size: 1554 kb

NDSU Extension Bulletin 25 - Revised 9/2007 file size: 5591 kb

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