Verticillium Wilt

Verticillium wilt symptoms include yellowing of leaf tips, sometimes in a V-shaped pattern. The edges of some apical leaflets will roll upward. As symptoms progress, leaves become desiccated and sometimes reddish in color. They may defoliate, leaving behind a stiff petiole. The infected stem does not wilt and remains green until all the leaves are dead. Xylem tissue in roots becomes brown in color.

Verticillium wilt of alfalfa is a warm-season disease and can be serious in susceptible varieties. Yields have been reduced by 50% in the second year of production. This disease has been found in alfalfa growing areas in the Mojave Desert and Riverside and San Bernardino counties. It has not yet been identified in the Central Valley or Imperial, Palo Verde, or Coachella valleys but has been found in a few coastal areas.

Verticillium albo-atrum can be carried internally and externally on alfalfa seed. The fungus also survives in alfalfa hay and in animal manure. The fungus penetrates alfalfa roots directly or through wounds. Spread within an alfalfa field can also occur through infection of cut stems when swathing. The fungus has been detected on sheep, which are trucked from one region to another to graze fields in winter months.

The most practical control measure is to plant resistant varieties. Cultivars resistant to Verticillium are listed on the National Alfalfa & Forage Alliance website.

In areas where the disease does not occur, take care to prevent importation of infected seed or plant materials:

• Clean or power wash harvesting equipment when moving from infested fields into new plantings.
• Cut infested fields last in the harvest cycle to avoid spreading the pathogen from field to field.
• Be careful when using return water from infested fields because pathogen spores can be carried in recirculated irrigation water.