HPIPM:Spring Black Stem

Author: Ned Tisserat and Gary Franc

Taxonomy

Domain	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Pseudomonadales
Family	Pseudomonadaceae
Genus	Pseudomonas
Species	Pseudomonas syringae

Scientific Name

Pseudomonas syringae pv. aptata

Scientific Name Synonyms

Pseudomonas aptata

Common Name

bacterial blight

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.

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:

• 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.

The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.