Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Alfalfa dwarf occurs rarely in alfalfa fields. Dwarf has been identified only in California, where it is found at a low frequency. Plants with symptoms of dwarf were reported in the 1950s in Mississippi, Georgia, and Rhode Island, but experimental confirmation of the disease in those States was not recorded. The bacterium causing dwarf, Xylella fastidiosa subsp. fastidiosa, is found in the southeastern and western U.S., Mexico, Costa Rica, Peru, and Argentina, where it causes economic losses in grapevine (Pierce's disease) and almond (almond leaf scorch). Although dwarf was locally severe in California alfalfa fields in the 1930s to the 1950s, current management of alfalfa likely limits development and spread of the disease. Symptoms: As the disease name implies, plants affected with dwarf have a short stature; however, stunting occurs long after infection. Initial symptoms are shortened internodes and reduced leaflet size. As the disease progresses, stems become shorter and more slender with small leaves until the plant has only a few short slender stems with small, dark-green leaves. In the root the first symptoms are yellow streaks in the vascular tissue. Ultimately, the stele and the entire cortex have a yellow to brown discoloration. Growth of the pathogen occludes xylem vessels leading to water stress, but other factors are responsible for stunting. The symptoms of dwarf can be confused with bacterial wilt. However, leaves of plants with bacterial wilt are usually distorted and pale green in color, and the inner surface of the bark has brownish stained areas, symptoms that are not observed in dwarf-infected plants. Causal Organism: Xylella fastidiosa subsp. fastidiosa is a gram-negative xylem-limited bacterium. It causes disease in a large number of agricultural, native, and weedy plant species, although infection in some hosts, notably weeds, causes no symptoms. Strains infecting grapevine and almond also infect alfalfa. The pathogen is transmitted by several species of xylem-sap feeding insects, the most important of which are the sharpshooter leafhoppers (Hemiptera, Cicadellidae). Although early research on the association of alfalfa dwarf with Pierce's disease led to identification of vectors, alfalfa is currently not considered an inoculum source for chronic infection of grapevine. The bacterium is nutritionally fastidious, is difficult to grow in culture, and requires special culture media. Sensitive and specific PCR assays have been developed for detection of Xylella fastidiosa in plant tissue. Disease Cycle and Epidemiology: The pathogen is limited to the insect vectors and xylem of host plants. There is a very short latent period after acquisition, and the bacterium persists until the insect molts, indicating that the bacterium multiplies in the foregut or mouthparts. Mechanical transmission with forage harvest is possible but requires verification. The bacterium spreads slowly through the alfalfa plant suggesting that frequent forage cutting schedules reduce systemic infections. Cool winter temperatures lower pathogen populations but do not eliminate infections. The incidence of dwarf is greater in older alfalfa fields. This suggests that shorter rotations of 3-4 years reduce the inoculum potential and results in a low incidence of the disease. Cold winter temperatures may also limit the geographic range of the disease. Management: California Common 49 was selected for resistance to dwarf but is not grown due to its susceptibility to spotted alfalfa aphid. The disease is likely limited due to removal of infected stems by frequent harvesting and short rotations.