Submitted to: Compendium on Alfalfa Diseases
Publication Type: Book / Chapter
Publication Acceptance Date: December 5, 2012
Publication Date: N/A
Technical Abstract: Crown wart has been found widely distributed in Australia, New Zealand, and European countries. It has been recorded sporadically in India (Punjab), South Africa, South America (Ecuador, Chile, Peru), Panama, Mexico, and Canada (British Columbia). In the United States, it has been found more frequently in the Pacific Coast States and occasionally in the Southeast (Alabama and Mississippi), Pennsylvania, and Indiana. The disease is usually confined to fields with excessive soil moisture during early spring months. Although apparently not a common disease, it can cause damage in the fields where it occurs. Symptoms: Galls (2-20 mm in diameter) form on stems or crown buds at or slightly below the soil surface. Originally, galls are white when young and turn gray to brown as they dry and decay in midsummer. Mature galls contain cavities containing yellow-brown resting sporangia. The presence of resting spores distinguishes the disease from crown gall, caused by the bacterium Agrobacterium tumefaciens. Infrequently, young leaves develop small, blister-like galls from local infections. Crown wart decreases yield and shortens plant lifespan. Because symptoms are transient and concealed below ground, the disease may be overlooked. Causal Organism: Physoderma alfalfae is classified in the order Blastocladiales and has not been cultured. The fungus exists intercellularly in galled host tissue as rhizomycelium, 0.5-1.0 um in diameter when young but thickening with age to 3-5 um. Intercalary mycelial swellings become enlarged to 17-19 X 10-15 um in diameter; haustoria or rhizoids emanate from these. Thick-walled, light brown to golden brown hemispherical resting sporangia (about 40-50 X 25-35 um) form within galls. When conditions are favorable, the resting spores crack open and release uniflagellate zoospores. Disease Cycle and Epidemiology: Galls mature in early summer. The fungus passes through summer droughts as resting spores within the galls. If moisture is abundant, most galls decay. When dry conditions prevail in late summer, galls do not decay but remain alive through the fall and winter. When galls disintegrate, the spores are liberated and serve as inoculum the following spring. Penetration of the host has not been observed, but resting spores apparently release zoospores that germinate and penetrate young crown buds. The pathogen causes marked enlargement of infected cells. Management: Provide good drainage; avoid excessive irrigation. Any measure that reduces moisture near the soil surface during the infection period should reduce the incidence of disease. Alfalfa should not be followed with alfalfa on infested land. Cultivars tolerant of wet soils have exhibited resistance to the disease.