Alfalfa Mosaic Virus and Lavender

Alfalfa Mosaic Virus (AMV) and Lavender
Curtis Swift, Ph.D.

 



Image 1: Symptoms of AMV on Lavender



Image 2: Distorted yellow spotted leaves are smptomatic of AMV. Yellow spots on flower stalks are less common.

 

Introduction:

 

The Alfalfa Mosaic Virus is found through out the world and no lavender field is free of possible infection. While this disease is easily spread by cuttings they are seldom tested to ensure they are free of virus.  This does not, however, mean infection will result in a loss of yield or plant vigor. Identifying infected plants is often difficult as symptoms of infection may persist or disappear soon after infection. (Hartman, et al.)

 

 AMV was first described in California in 1931 as a mosaic virus of alfalfa. Since viruses are named for the first plant in which they are identified, the virus was named Alfalfa Mosic Virus (Hall).  AMV is a pathogen of several vegetable crops causing up to 65% yield loss of peppers (Pernezny, et al.) while the strain that infects beans is reported to be of no economic importance whatsoever. Other crops reported to be susceptible to AMV include peas (Hagedorn, 1984), soybeans (Harman, et al. 1999), celery, celeriac, and other umbelliferous crops (Davis & Raid, 2002), head lettuce (Davis, et al., 1997), eggplant, and other dicots, many of which are weeds.   This virus has one of the largest host ranges of any virus known to infect 600 plant species in 50 genera.  Daughtrey et al. indicate AMV has been reported in greenhouse flower crops to include primrose, geraniums and hydrangea.  They suggest this virus may be more widespread in flowers than reports would indicated.

 

Stunted lavender plants (Lavandula angustifolia and L. x intermedia)  with yellow spots and distorted plant growth were reported in western Colorado lavender fields in 2011.  In 2012, samples of tissue was collected from plants exhibiting yellow spotting and stunting from three widely distributed lavender fields in western Colorado and tested for virus.  Suspect tissue was collected from 'Fat Spike', 'Royal Velvet' and the lavendin 'Grosso'. A total of five samples collected from different plants were sent by priority mail to AgDia, Elkhart, Indiana for testing.

 

The presence of virus is typically determined by the ELISA procedure. For many crops, tissue from the suspect plant is macerated in a buffer solution and paper strips containing antibodies to the suspect virus inserted into the mix of buffer solution and plant tissue. This technique gives us an answer as to which virus or viruses is/are present in the sample in about five minutes. 

 

Due to the essential oil of lavender tissue, a technique called PCR had to be used to characterize (identify) the virus to family.  Polymerase chain reaction (PCR) is a scientific technique that increases a single or a few copies of a piece of DNA, generating thousands to millions of copies of a particular DNA sequence.  This technique revealed all five samples were in the Bromoviridae family of virus.   

 

This family includes:

•   Genus Alfamovirus; type species: Alfalfa mosaic virus

•   Genus Anulavirus; type species: Pelargonium zonate spot virus

•   Genus Bromovirus; type species: Brome mosaic virus

•   Genus Cucumovirus; type species: Cucumber mosaic virus

•   and others, hence the reason to have this virus characterized to Genus.

The DNA samples were found to be 97%-99% related to Alfalfa Mosaic Virus. The strain of the virus was not identified in this process.  Many strains of AMV cause necrotic local lesions, while other strains can result in yield loss, reduce winter survival, and an increase in infection by other pathogens.  No data is available on the overall effect of AMV on lavender thus further research on this problem as it relates to lavender winter survival and yield is needed.

 

Disease Management

Myzus persicae, the green peach aphid, is the most efficient aphid vector of at least 15 different aphid species that spread this virus. Aphids can acquire the virus after only a few minutes of feeding on a virus-infected plant and can immediately transfer the virus to another plant.  The virus is picked up on the outside of the stylet (needle-like mouth parts). When the aphid feeds on the next plant some of the virus particles remain in the probed plant tissue causing the disease.  This type of virus transfer is referred to as non-persistent or stylet-borne transmission, as compared to viruses that are sucked into the insect’s gut prior to being trasmitted to the next plant.  The latter is referred to as `circulator/ persistent`, or `propagative` transfer depending on whether the virus passes directly through the insect gut system, or multiplies inside the insect prior to being passed on to the next plant.  

Like the virus, M. persicae is found world wide. This aphid is a common pest of peach, cherry, and other Prunus species and insecticide treatment of those trees to prevent spread of this aphid to nearby lavender field is strongly recommended. In cold climates this insect often overwinters in warm greenhouses. 

Alfalfa Mosaic Virus can be spread through transmission of sap thus disinfecting equipment used for pruning, shaping, and other operations is recommended to prevent the spread of this virus, especially when AMV-infected lavender plants or weeds or other crokps are in or near the field.  A solution of 10% bleach solution (sodium hypochlorate) created by mixing one part bleach with nine parts water, or a spray of alcohol, or other disinfectant is recommended to treat sickles, shears, and other cutting implements between plants to help prevent spread. Bleach is corrosive so frequent replacement of cutting implements will be need if used as the disinfectant.

 

Image 3: Pigweed with symptoms of AMV.

 

All AMV-infected plants are dicots and many are weeds. Controlling weeds around the production field is key to preventing the introduction of this disease into the lavender field. It is critical that weeds be treated with an insecticide prior to being eliminated.  Hoeing, mowing, or spraying weeds with a herbicide prior to treating for aphids results in the aphids moving to other plants, some of which could  be lavender plants. Applying an insecticide to weeds prior to control can be done with synthetic products or organic approved materials such as SucraShield™, neem oil, insecticidal soap, etc.

When neighboring fields of alfalfa and other aphid-infested crops are harvested, aphids move to find other plants to feed on.  Leaving a buffer of plants between the harvested crop and the lavender field will help keep aphids from moving to the lavender. This buffer could be treated with an insecticide.  If the neighboring buffer is not a viable option the use of a trap crop around the lavender field should be considered. This trap crop could consists of several rows of alfalfa treated with a fast-acting contact organic or synthetic insecticide.  The concept is simple.  Aphids moving into the area would hopefully alight and feed on the trap crop and would be killed before they have an opportunity to move into the lavender field. Organic products typically have no residual or have a shorter residual effect than synthetic products and thus would need to be applied more frequently to maintain control of the aphids feeding in the trap crop.

Insecticides applied to lavender plants are not effective in preventing AMV infection since the non-persistent transmission of the virus occurs too rapidly.

The use of a spun-bonded polyester insect barrier over the plants helps prevent virus infections. Drought and drying winds cause aphids to move from droughty crops and dry desert areas to other crops including lavender. During such times additional steps may be necessary to prevent AMV infection of lavender.

Dodder, a parasitic seed plant, can spread this virus from infected to non-infected plants thus keeping the field field of dodder is critical.  

Image 4: Dodder wrapped around bindweed

The tolerance level of lavender to this virus is not known.  AMV infection appear to have no effect on plant growth or yield in some fields. Some growers remove infected plants while others leave them be.  More research on this aspect of the disease needs to be conducted to determine its effect on lavender in general and cultivars specifically. 

 Creating virus-free transplants and cuttings

Greenhouses used to propagate lavender should be kept free of aphids to help ensure transplants are not infected when moved to the production field or retailer outlets. The use of insect screening installed over doors, intake vents, fan housings, and other openings should be considered.  Mother plants used for cuttings should be examined for visible symptoms of the virus and not used for that purpose if symptoms are noted.  Mother plants should be kept covered with an insect barrier spun-bonded fabric if not kept in an aphid-free glass house. Having mother plants tested for AMV would be ideal but costly. 

Cuttings taken from plants infected by viruses, phytoplasma, fungi, or fungal-like organisms are often cured of their pathogens by being placed in hot water for a certain number of a minutes. Azalea cuttings infected by Rhizoctonia can be cleared of this pathogen by placing the cuttings in 122^o F water for 20 minutes without damaging the plant tissue. Virus-infected daffodil bulbs placed in 129^o F. water for an hour are made virus-free. While hot water treatment will most likely inactivate virus in lavender cuttings information on the proper temperature or length of time necessary to achieve this virus-free status needs to be determined. 

Hot water treatments are also used to inactivate virus in seed.  When seed is colllected from lavender in the process of selecting new cultivars one should consider treating the seed with hot water therapy as AMV is seed transmitted. A hot water bath of 144^o F for 10 minutes is a common treatment regimen for seed.

 

References: 

·        Chaube, H.S. and Singh, U.S. 1991. Plant disease management: Principles and practices. CRC Press, Boca Raton, FL.

·        Davis, R.M., and Raid, R.N. 2002. Compendium of umbelliferous crop diseases. American Phytopathological Society, St. Paul, MN.

·        Davis, R.M., Subbarao, K.V., Raid, R.N., and Kurtz, E.A. 1997. Compendium of lettuce diseases. American Phytopathological Society, St. Paul, MN.

·        Daughtrey, M.L., Wick, R.L., and Peterson, J.L. 2006. Compendium of flowering potted plant diseases. American Phytopathological Society, St. Paul, MN.

·        Hadidi, A, Khetarpal, R.K., and Koganezawa, H., eds. 1998. Plant virus disease control. American Phytopathological Society, St. Paul, MN.

·        Hagedorn, D.J., ed. 1984. Compendum of pea diseases. American Phytopathological Society, St. Paul, MN.

·        Hartman, G.L., Sinclair, J.B., and Rupe, J.C. 1999. Compendium of soybean diseases: 4th edition. American Phytopathological Society, St. Paul, MN.

·        Hall, R. 1991. Compendium of bean diseases. American Phytopathological Society, St. Paul, MN.

·        Pernezny, K. Roberts, P.D., Murphy, J.F., and Goldbert, N.P. Compendium of pepper diseases. 2003. American Phytopathological Society, St. Paul, MN.

·        Sherf, A.F., and Macnab, A.A. 1986. Vegetable diseases and their control. John Wiley & Sons, Inc, New York, NY

·        Sutic, D.D., R.E. Ford, and M.T. Tosic. 1999. Handbook of plant virus diseases. CRC Press, Boca Raton, FL.