This disease will normally cause plants to die after several seasons. Symptoms of alfalfa mosaic are most noticeable on new growth in the spring. However, many infected plants never show recognizable symptoms. Symptoms vary from a few irregularly shaped blackened spots to large, sunken, oval to diamond-shaped lesions on stems. Examination with a magnifying glass reveals tiny black hairs within these spots.
Stems eventually turn straw colored and die.
These dead shoots are often easily seen scattered through the field in summer and fall. This disease can move into the crown of the plant causing a bluish-black discoloration which results in damage to the whole plant. Infected plants are usually scattered throughout a stand and are detected by their yellow-green color and stunted growth. Mild symptoms consist of leaf mottling with a slight cupping or upward curling of the leaflets and some reduction in plant height. Severely infected plants are stunted and yellow-green, with many spindly stems and small, distorted leaflets.