How to Manage Pests
UC Pest Management Guidelines
Pathogen: Clavibacter michiganense ssp insidiosus (formerly Corynebacterium insidiosum)
(Reviewed 3/17, updated 3/17)
In this Guideline:
Symptoms and signs
Aboveground symptoms of bacterial wilt include a yellow-green foliage and stunted growth. Leaflets may be mottled and slightly cupped or curled upward. Stems on affected plants may be thin and weak. Disease symptoms are most evident after clipping during regrowth. A cross section of an infected taproot reveals a yellowish-tan color in the center. Brown pockets on the inside of bark tissue are sometimes evident. Once infected, plants do not usually recover. Within 5 to 8 months after showing symptoms, plants usually die. Infected plants are prone to winter kill in areas where soil freezes in winter.
Comments on the Disease
Bacterial wilt is a warm-season disease that occurs in most areas of California but is rarely seen today because of the development and use of wilt-resistant cultivars in the Sacramento Valley.
The disease is currently not a serious problem in the southern San Joaquin and desert valleys. Symptoms rarely appear before the second or third year of a stand and flood irrigated fields in these areas are typically removed after 3 to 4 years or reseeded for an additional 2 to 3 years. However, subsurface drip irrigated fields may stay in production for up to 5 to 10 years so bacterial wilt may become a problem in the later years of production.
The bacterium survives on plant residues in the soil and enters plants through wounds in the roots and crown or through the cut ends of freshly mowed stems. Disease severity and incidence increase when root-knot nematodes are present. The bacterium can survive in dry plant tissue or seed for at least 10 years and can be disseminated over long distances in seed and dry hay. However, bacteria numbers in the soil decline quickly when infected plant residue decomposes. The bacterium also can be spread by surface water, tillage, mowing, and harvesting equipment. The greatest incidence of the disease occurs in poorly-drained areas of fields, and large areas can be infected during periods of continuous wet weather.
Use resistant cultivars, which are listed in the National Alfalfa & Forage Alliance website. If bacterial wilt is discovered in a field, cut it last during harvest to prevent spread of inoculum by the mower to younger stands. Within a field, harvest infested areas last and never mow when the foliage is wet.
Crop rotation can reduce inoculum in the field. For more information, see CROP ROTATION.
UC IPM Pest Management Guidelines:
R. M. Davis, Plant Pathology, UC Davis (Emeritus)