Alfalfa

Spring Black Stem and Leaf Spot

 Ned Tisserat and Gary Franc

Cause: Phoma medicaginis var. medicaginis

Occurrence:  The disease is widespread in the High Plains but often does not result in significant defoliation.  It is most important on spring growth before flowering.

Key Symptoms:

• Small dark leaf spots with irregular borders
• Spots appear first on lower leaves
• Spots expand and coalesce with time
• Spotting may occur on petioles and lower stem
• Spots may enlarge to blacken lower stem
• Diseased leaves turn yellow and drop prematurely

Pathogen:  The fungus overwinters on dead leaves and infected stems.  Spores are splashed to emerging shoots and leaves.  The disease is favored by extended periods of cool, wet weather.

Management Approaches

Biological Control

No biological control practices have been developed for spring black stem.

Cultural Control

• Use resistant varieties.  See the following sources for varietal information:

o University of Wisconsin forage pages  http://www.uwex.edu/ces/forage/pubs/varinfo.htm

• University of Wyoming Alfalfa Disease Management Publication B1136 http://ces.uwyo.edu/PUBS/B-1136.pdf
• Plant high-quality seed free of the bacterial leaf spot pathogen
• Avoid reuse of irrigation water
• Harvest early if disease is severe and defoliation is occurring

Chemical Control

Several fungicides are labeled for spring black stem but are generally not recommended.

Categories: Alfalfa, Disease, Spring black stem

Date: 03/01/07