Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Crown gall is uncommon in alfalfa. The disease has been reported from alfalfa stands in the Imperial Valley of California, but rare infected plants can be found occasionally in other parts of the U.S. Symptoms: The galls or tumor-like overgrowths form on the crown branches at or slightly below the soil line and on lateral roots. Initially, the galls are small and roundish with a smooth surface and are white, tan, or light green in color. As the galls enlarge, the surface becomes rough, convoluted, and dark. Galls can persist for several years and become greater than 2 cm in diameter. Affected plants may show some loss of vigor and slowly die; however, the disease has not been noted often enough on alfalfa to assess its effect on yield and stand loss. The galls appear similar to the overgrowths of crown wart, caused by Physoderma alfalfae. However, P. alfalfae can be identified in gall tissue by the presence of globose resting spores (40-50 um in diameter), which are absent in crown gall tissue. Causal Organism: Agrobacterium tumefaciens is a gram-negative rod. It is a common soil bacterium that infects dicotyledonous plants from more than 90 plant families. Disarmed (non-tumorigenic) strains are widely used for genetic transformation of plants, including alfalfa. Isolation of bacteria is easiest from young, actively growing galls. Disease Cycle and Epidemiology: Little is known about the progress of disease in alfalfa, but the interaction is influenced by plant cultivar and bacterial strain. The bacterium is known to overwinter in soil or decaying organic matter. It enters plants through wounds caused by mechanical injury or from insect feeding. The bacterium transfers a portion of DNA from the tumor inducing plasmid (pTi) into a plant cell, where it integrates randomly into a plant chromosome. Expression of transferred genes causes cell proliferation and enlargement resulting in the formation of a gall. Plant carbohydrates are diverted to gall tissue, and growth of the gall crushes and distorts vascular tissues, reducing plant vigor. Bacteria are released to the soil upon plant death or when portions of the gall are detached from the plant. Non-dormant alfalfa cultivars (fall dormancy rating of 9) are more susceptible than dormant cultivars. Differences among A. tumefaciens strains in host range and virulence has been observed. Management: No information on management of the disease on alfalfa is available. Use of dormant cultivars, which appear to have greater resistance than non-dormant cultivars, should reduce incidence of the disease.