By Emmanuel Byamukama
In South Dakota, alfalfa fields that were recently scouted were found to be infected with Alfalfa mosaic virus (AMV) at a very low incidence. AMV is a common virus in alfalfa that can also infect soybeans. AMV overwinters in alfalfa crowns and new growth in the spring from these crowns have leaves with yellow and green patterns (mosaic) virus symptoms (Figure 1). Plants infected with AMV have reduced vigor and overtime this will lead to stand decline.
Alfalfa mosaic symptoms differ depending on the age of the plant and also on the weather conditions. Symptoms developing this early in the season, especially during a cool wet spring, originate from infections from last fall growth. AMV is transmitted by aphids but since aphids are not present in the fields at this point, the virus would have come from the crown.
There are several species of aphids that are capable of vectoring AMV. The most commonly observed species are the pea aphid, blue alfalfa aphid and spotted alfalfa aphid. Of these, pea aphids are generally the most abundant in South Dakota. Although pea aphids enjoy cooler weather, current conditions are even a little too cool for them.
AMV management
There are no rescue treatments available once the plant has AMV symptoms. For new alfalfa plantings, select alfalfa cultivars that have resistance to AMV. Some broadleaf weeds such as black nightshade can be a source of inoculum for AMV. Therefore, proactive broadleaf weeds management can help reduce the spread of AMV. For fields with AMV present, scout closely for aphid populations and manage them early to prevent additional spread of AMV within the field.