In the past few years, alfalfa mosaic disease has been found and confirmed in a number of vegetable crops throughout California. Caused by the Alfalfa mosaic virus (AMV), this disease is generally of low importance in these crops but is notable and noticeable due to its striking symptoms. In most vegetable crops, AMV causes a bright yellow mosaic, mottle, or calico (multi-colored) appearance to the leaves. In some cases, the loss of green chlorophyll is so extreme that the leaf can have large white patches and blotches. If infected at an early age, plants can be stunted. For pepper and tomato, infected plants can produce fruit that are deformed, discolored, and unmarketable. Pictured below are symptoms of AMV as confirmed from recent California outbreaks on lettuce, spinach, cilantro, basil, tomato, and pepper.
In nature, AMV can infect over 150 different crops and weeds. The virus is spread by a number of aphids, including important virus vectors such as the green peach aphid, pea aphid, and blue alfalfa aphid. For alfalfa and pepper, researchers have confirmed that AMV can be carried in seed. Outbreaks of alfalfa mosaic disease often develop where susceptible vegetable crops are planted near or next to alfalfa (which was the first crop known to be infected with AMV) or pasture and weedy areas.
While AMV infections result in quite striking symptoms, positive identification should not be based on symptoms alone. Because the symptoms caused by plant-infecting viruses vary greatly, symptoms caused by non-AMV pathogens could resemble those caused by AMV. In addition, some genetic mutations that occur in the leaves of vegetable crops can result in large patches of leaf that are bright yellow or white, thereby looking like a symptom caused by AMV. Our lab uses the serological method ELISA (see photo below) to confirm the presence of this virus in plant samples.
1. Alfalfa mosaic virus on lettuce. 2. Alfalfa mosaic virus on spinach.
3. Alfalfa mosaic virus on cilantro. 4. Alfalfa mosaic virus on basil.
5. Alfalfa mosaic virus on tomato. 6. Alfalfa mosaic virus on pepper.
7. Serological methods are used to confirm AMV.