Cause A bacterium, Clavibacter michiganensis subsp. insidiosus (syn. = Corynebacterium insidiosum), which survives in living or dead alfalfa plant tissue in soil and in alfalfa seed. The disease occurs in all major alfalfa production areas of Oregon, Washington, and Idaho. Infection is through wounds in the roots or crowns or through cut stems after mowing.
Disease severity increases when the northern root-knot nematode is present, and the stem nematode can vector the bacterium.
Symptoms Infected plants are stunted, yellow, and exhibit leaf mottling. Leaflets may appear slightly cupped upward to severely distorted. When taproots are cut in cross-section, a yellowish or brownish ring is evident under the bark. Yellowish or brownish streaks may also appear in outer woody tissue. Disease symptoms usually appear during the second or third crop year and are most evident in regrowth after mowing.
Cultural control
- Resistant varieties are best means of control. The varieties, Ranger, Agate, Iroquis, Oneida, 120, 532, Lahontan, Orestan, Ladak, Vernal, Caliverdi, and numerous others carry resistance.
- Maintain good stand vigor by managing soil fertility (especially potassium) and carbohydrate reserve storage and by using winter-hardy, resistant varieties.
- Do not mow when plants are wet.
- Harvest young stands before old stands, and remove plant debris from equipment.
- Rotating to other crops for 2 or more years is beneficial.
Reference Stuteville, D.L., and Erwin, D.C. 1990. Compendium of Alfalfa Diseases, 2nd ed. St. Paul, MN:APS Press.