Symptoms and Signs
Symptoms of Stagonospora crown and root rot include rough and cracked bark tissue on infected roots and crowns. The presence of red flecks in diseased root tissue is a distinctive diagnostic symptom. Fine red streaks also occur in the xylem (water-conducting tissue) in the center of the root, below rotted portions of the crown. Affected crown tissue is generally firm and dry, unless secondary organisms invade the tissue. The pathogen also may infect leaves and stems, causing irregular, bleached lesions with diffuse margins. Infected leaves soon drop after lesions form.
Comments on the Disease
Stagonospora crown and root rot is a cool-season crown and root rot disease. Spores of the pathogen form in black pycnidia and are spread by water that splashes them from infected leaves, stems, or plant debris onto susceptible tissue. The fungus enters the crown through stems and grows slowly downward into the taproot. Although the infection can take 6 months to 2 years to kill a plant and aboveground symptoms may be indistinct, the disease reduces plant vigor and yield. Leaves and stems are generally infected during spring rains, but crown infections can occur anytime. The disease is most damaging when alfalfa is not actively growing.
Management
To minimize the effects of Stagonospora crown and root rot, provide optimum growing conditions for the alfalfa crop. Consider rotating out of alfalfa for 2 years to eliminate the sources of inoculum within a field. For more information, see CROP ROTATION. No resistant cultivars are available, but germplasm with moderate resistance has been released.