HPIPM:Bacterial Leaf Spot

Author: 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.

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:
• 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

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.