Alfalfa mosaic virus (AMV)
Written by: Taylor Janecek
Posted: July 11, 2023
Disease symptoms
Symptoms of AMV include leaf mosaic, leaf discoloration, leaf curling or deformation, and abnormal growth of new leaves. Infected plants may be stunted in growth. Highly symptomatic plants will have multiple types of leaf symptoms. AMV symptoms usually appear at the top of the plant within a few weeks after pepper transplants are planted in the field, but symptoms may appear on any part of the plant and at any time during the season. Pepper plants can recover from AMV symptoms, but the virus will still be present in the plant. AMV symptoms can vary dramatically among pepper varieties.
Leaf mosaic and discoloration are the most common symptoms of AMV and these symptoms occur on the upper part of the plant in the younger leaves. Over time, the leaf discoloration will appear whitish or bleached. Leaf deformation may also occur with AMV, including leaf curling and stunted growth of the leaves. The leaf veins may also become deformed and develop in a squiggle or zipper shape.
Rarer symptoms of AMV include leaf necrosis, which appears as black splotches between the veins of the leaves. The fruits of plants may also have symptoms, which appear as mottling, circular discoloration, or necrotic pockmarks. Other viruses, such as cucumber mosaic virus or tobacco mosaic virus, can produce similar symptoms in plants. If it is necessary to confirm virus identity, leaf or fruit samples should be sent to the Colorado State University Plant Diagnostic Clinic.
Quick Facts
- Alfalfa mosaic virus (AMV) infects over 400 plant species across the world.
- The virus is spread by numerous aphid species, and its symptoms include bright yellow mosaic patterns on leaves, leaf crinkling, and stunted growth of leaves, stems, and fruit.
- AMV causes severe economic losses in pepper production in Colorado and planting resistant pepper varieties is the best method for AMV management
Alfalfa mosaic virus on pepper. This virus is spread by aphids and has become common on pepper in Colorado. Image credit: Taylor Janecek, Colorado State University
Peppers infected with Alfalfa mosaic virus. Image credit: Taylor Janecek, Colorado State University
Example of leaf discoloration, mosaic symptoms of AMV. Image credit: Taylor Janecek, Colorado State University
Example of leaf discoloration, mosaic symptoms of AMV. Image credit: Taylor Janecek, Colorado State University
Example of leaf deformation from AMV. Image credit: Taylor Janecek, Colorado State University
Example of leaf discoloration and leaf necrosis from AMV. Image credit: Taylor Janecek, Colorado State University
Example of fruit deformation and discoloration with pockmarks from AMV. Image credit: Carlo Vaughn, Colorado State University
Example of fruit discoloration from AMV. Image credit: Taylor Janecek, Colorado State University
Pathogen life cycle
AMV is an RNA virus that only infects plants and AMV can only replicate in living plant cells. AMV transmission occurs primarily by their insect vector, aphids, although the virus is capable of also being transmitted mechanically. Aphids transmit AMV in a non-persistent manner, meaning that the aphids can acquire the virus within seconds to minutes when feeding on an infected plant and transmit the virus within seconds when probing a healthy plant. The aphids do not need to colonize the pepper plants to transmit it.
The main reservoir for AMV in Colorado is alfalfa (Medicago sativa L.), and infection risk is highest for pepper plants near alfalfa fields. AMV overwinters in alfalfa or is propagated the next season through seed transmission, although seed transmission rate is relatively low. Alfalfa harvest triggers aphids foraging and colonizing alfalfa to move to neighboring and adjacent crops, potentially transmitting the virus.
Disease management
Alfalfa mosaic virus is a particularly difficult virus to manage. Aphids only need to probe the plant to spread the virus, and for this reason chemical control methods are ineffective for managing AMV spread. Since AMV is common in alfalfa, not growing vegetable crops near alfalfa fields will help prevent AMV infection.
Planting AMV resistant varieties, such as the pepper variety Mira Sol, is the best option for AMV management. Aphid management methods, such as mineral oil sprays, straw mulch or reflective plastic mulch, or AMV-resistant border crops may also reduce AMV incidence in peppers.
If pots or stakes used to grow pepper transplants are reused, they should be sanitized before a new crop is grown. Seedling transplants with virus symptoms should be removed from the greenhouse and should not be planted in the field. Alfalfa mosaic virus can also cause losses in legumes used as forage crops and high incidence of AMV or other legume viruses in pastures can degrade pasture quality.
References
Ahoonmanesh, A., Hajimorad, M.R., Ingham, B.J., Francki. R. I. B. 1990. Indirect double antibody sandwich ELISA for detecting alfalfa mosaic virus in aphids after short probes on infected plants. Journal of Virological Methods. 30: 271–281
Albrect, T., Bartolo, M., Cranshaw, W.S., Nachappa, P. 2020. Old virus new host: defining the disease cycle of Alfalfa mosaic virus in peppers. Phytopathology (11) 12:7
Jones, R. A. C. 2013. Virus diseases of perennial pasture legumes in Australia: incidences, losses, epidemiology, and management. Crop Pasture Sci. 64: 199.