Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Alfalfa witches'-broom was first reported in 1969 in Australia and later in South Africa, Canada, and Saudi Arabia. More recently, specific phytoplasmas associated with alfalfa witches'-broom have been identified from symptomatic plants in the United States (Wisconsin), Italy, Lithuania, Oman, Iran, China, Argentina, and Bolivia. The disease can result in significant losses in forage and seed production. Annual losses of approximately 25% in hay production were reported in Oman due to alfalfa witches'-broom. Symptoms: The disease is characterized by a proliferation of shoots leading to a bush- or broom-like appearance. Leaves are small and yellow. Flower abortion is observed in severely infected plants. Causal Organism: Alfalfa witches'-broom is caused by phytoplasmas, formerly termed mycoplasma-like organisms (MLOs). Phytoplasmas are small, wall-less prokaryotes that have not been cultured in vitro. The 16S rRNA sequence is the primary tool for identification and classification of phytoplasmas. The majority of phytoplasmas associated with alfalfa witches'-broom were identified in the 16SrI (aster yellows group). Phytoplasmas in the 16SrII (peanut witches'-broom group) and 16SrVII (ash yellows group) were also shown to be associated with alfalfa witches'-broom. Disease Cycle and Epidemiology: Symptoms can occur in plants of any age. Incidence of the disease has been reported from 20 to 95% of plants in a field. The pathogen is limited to phloem sieve tubes and is spread by phloem sap-feeding insects such as leafhoppers in a persistent manner. Seed transmission has not been reported. Management: No specific management strategies have been reported. As for other diseases caused by phytoplasmas, controlling insect vectors and removing plants serving as reservoirs for the pathogen can reduce disease incidence. Alfalfa cultivars with resistance to the potato leafhopper (Empoasca fabae) have been developed, but their utility for reducing alfalfa witches'-broom has not been tested.