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.