Symptoms and Signs
The primary symptom of plants infected with the bacterium Xylella fastidiosa is stunted regrowth after cutting. The stunting may not be apparent for many months after initial infection. Leaflets on affected plants are smaller, often a slightly darker (bluish) color but not distorted, mottled, or yellow. The taproot is normal sized, but slicing it diagonally or horizontally down the root reveals abnormally yellowish wood with fine dark streaks of dead tissue. In recently infected plants, the yellowing is mostly in a ring beginning under the bark, with a normal white cylinder of tissue in the center. The inner bark is not discolored, there are no gummy pockets underneath the bark, and there are no large brown or yellow patches as is the case with bacterial wilt caused by Clavibacter insodiosum. Dwarf disease progressively worsens over 1 to 2 years after first symptoms and eventually kills the plant.
Comments on the Disease
Sharpshooters are vectors of the bacterium Xylella fastidiosa, which causes alfalfa dwarf disease, but only the green and red-headed sharpshooters are known to spread this disease in alfalfa. Other diseases in California caused by this bacterium include Pierce's disease of grapes and almond leaf scorch. Sharpshooters acquire the bacterium while feeding on an infected host plant and spread it to noninfested host plants through subsequent feeding. Sharpshooters continue to be able to transmit the bacterium to plants until they molt.
Alfalfa dwarf has rarely been reported since the 1950s and is primarily distributed only in Southern California and from Madera County south in the San Joaquin Valley. It is not recognized as an economic disease of alfalfa; however, the bacterium that causes alfalfa dwarf, Xylella fastidiosa, is the same pathogen that causes Pierce's disease of grapes, a very important grape disease in California. The role that alfalfa plays in the epidemiology of Pierce's disease is important. Leafhoppers, including the blue-green sharpshooter, spread the disease from alfalfa to grapes. Increased levels of Pierce's disease in grapes located adjacent to alfalfa has been documented in the San Joaquin Valley.
To protect grapes, minimize the attractiveness of an alfalfa stand to sharpshooters by preventing the growth of grassy weeds. Green and red-headed sharpshooters require grasses such as bermudagrass, watergrass, cultivated fescues and perennial ryegrass to breed. Annual grass weeds or cover in orchards and vineyards do not seem to develop significant numbers of sharpshooters if weeds are removed at least annually. This practice will reduce the transfer of the bacterial pathogen between alfalfa and grape vineyards.
When possible, avoid planting alfalfa adjacent to grape vineyards in areas where Pierce's disease is prevalent