Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Bacterial leaf spot has been reported in Australia (Queensland), Egypt, El Salvador, India, Japan, Nicaragua, Sudan, and the United States (Florida, Iowa, Kansas, Maryland, and Wisconsin). It occasionally causes locally severe defoliation and post-emergence damping-off and stunting. The disease is likely widespread but causes only minor damage. Symptoms: Leaf symptoms begin as small, round water-soaked spots 0.5 to 1 mm in diameter. As spots enlarge and coalesce, they become irregular in shape and have a dry pale yellow to tan center with brown border and can have a chlorotic halo. Severe spotting causes the leaflet to become chlorotic, drying from the tip toward the base, and becoming papery in texture. Severe defoliation can occur. Under unfavorable conditions, spots may not enlarge but remain as small brown specks resembling symptoms of common leaf spot or Leptosphaerulina leaf spot. Symptoms on stems start as small water-soaked greasy circular spots. These areas elongate and often coalesce to form lesions several internodes in length that may extend half to three-quarters around the stem. Lesions have a margin that is first tan then becomes dark brown, purple, or black with a central greenish-yellow region. A thin film of dried bacterial exudate is sometimes seen over lesions. Old lesions become brown and may resemble symptoms of spring black stem (Phoma medicaginis) or summer black stem (Cercospora medicaginis). Broken stems can occur as a result of stem weakening. Bacterial leaf spot can be distinguished from fungal diseases by streaming of bacteria from lesions. Causal Organism: Bacterial leaf spot is caused by Xanthomonas alfalfae subsp. alfalfae. The bacterium is a motile gram-negative rod and forms yellow, convex, round, mucoid colonies on YDP. It can be identified from the 16S-23S rDNA intergenic spacer sequence (GenBank accession DQ660896). The host range under artificial inoculation conditions includes a number of legumes such as annual medic species (M. truncatula, M. scutellata, M. lupulina), sweet clover species (Melilotus officinalis, M. indicus), Phaseolus vulgaris, and Pisum sativum. Some strains are weak pathogens of citrus. Disease Cycle and Epidemiology: The bacterium overwinters in soil and infected plant debris. It is spread by rain and wind-driven soil. The bacterium enters through stomata or wounds. Hot, rainy weather favors disease development. However, the disease can also be severe under hot, dry conditions, especially in thin stands and space-planted nurseries where windblown infested soil particles wound and inoculate plants. Management: Highly resistant germplasm has been developed, but resistant cultivars are not available. Crop management methods that reduce soil erosion by wind and use of windbreaks can reduce infection. Spring seeding prevents loss of seedlings to the disease.