Alfalfa

Bacterial Leaf Spot

Howard F. Schwartz, Ned Tisserat and Gary Franc

Cause: Xanthomonas axonopodis pv. alfalfae (synonym =X. alfalfae or X. campestris pv. alfalfae).

Occurrence:  Damage from bacterial leaf spot is not common or severe in the High Plains regions, but it maybe severe locally under warm temperatures (80 to 90ºF).  Symptoms are most common in mid-summer.

Key Symptoms:

• Chlorotic areas with tiny circular, water-soaked spots
• More pronounced on  bottom sides of leaves
• Under favorable conditions, irregularly shaped, water-soaked lesions develop
• Fungi may invade old lesions
• Water-soaked greasy spots may also develop on stems
• Severe infestations cause extensive defoliation
• May be confused with other fungal pathogens that cause defoliation
• Infected seedlings in fall plantings may damp-off or be severely stunted

Pathogen:  Bacteria can overwinter in alfalfa crop debris and soil, and may survive in hay and seed for several years. Bacteria are spread by wind, rain, and irrigation water and infect the plant through wounds and stomata. 

Management Approaches

Biological Control

No biological control practices have been developed for bacterial leaf spot.

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
• Spring seeding in temperate regions can avoid severe seedling losses.

Chemical Control

No pesticides are registered specifically for bacterial leaf spot, but some copper-based fungicides may provide some disease suppression.  Copper fungicides can be phytotoxic to some sensitive alfalfa varieties such as ‘Lahontan’.

Product List for Bacterial Leaf Spot: 

Pesticide	Rate per Acre	Application Frequency	Remarks (days)
Copper Fungicides
Kocide 101	2 lbs	—	Apply 10-14 days before harvest
Kocide DF	2 lbs	—	Apply 10-14 days before harvest
Kocide 4.5LF	1.33 pts	—	Apply 10-14 days before harvest
Champ Dry Prill	1.33 lbs	—	Apply 10-14 days before harvest
Champ Formula 2 Flowable	1.33 pts	—	Apply 10-14 days before harvest
Champion Wettable Powder	2 lbs	—	Apply 10-14 days before harvest
Cuprofix Ultra 40 Disperss	1.25 lbs	7-14 days	7 day PHI
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.

Categories: Alfalfa, Disease, Bacterial Leaf Spot

Date: 03/01/07