Bulletin #5070, Common Questions about Dahlia Mosaic Virus and Other Dahlia Diseases

By Dr. Alicyn Smart, Associate Extension Professor and Extension Plant Pathologist, University of Maine; Dr. Stephanie Shea, Assistant Diagnostician and Accreditation Manager, University of Maine Cooperative Extension; and Emma Herrero, Lab Aide IV, University of Maine Cooperative Extension.

Reviewed by Jason Lilley, Assistant Extension Professor, University of Maine; Dr. Manjot Sidhu, Assistant Extension Professor, University of Maine; and Dr. Ankit Singh, Assistant Extension Professor, University of Maine.

Dahlia Mosaic Virus (DMV)

What is Dahlia Mosaic Virus?

Dahlia Mosaic Virus (DMV) is one of the most prevalent viruses in dahlias (Dahlia variabilis) and causes leaf symptoms, including leaf vein discoloration, stunting, reduced plant vigor and mosaic patterns, which are intermingled patches of normal and light green or yellowish coloring. The severity of symptoms can vary and some infected plants may show no symptoms, yet still carry the virus.

To confirm this disease, plant material needs to be sent to a lab for molecular testing. The University of Maine Cooperative Extension Plant Disease Diagnostic Lab provides this testing. For more information about this test, you can view the list of diagnostic tests and fees.

If I have DMV on my dahlias, is it in the tubers I bring inside to overwinter?

Yes, the virus is throughout the plant and will persist in the tubers, and therefore, all newly formed shoots and tubers will be infected as well.

Does DMV survive in the soil or dead tubers?

No. DMV requires a living, viable host to survive. Once the host tissue is dead, the virus can’t survive. However, it can survive in soil as long as a viable host is present, such as undecomposed plant material. This means that if all plant material is removed, clean tubers can be replanted in the same soil; the soil is not contaminated.

Where did DMV originate? How is it spread? Does it transfer to other plants?

DMV has been detected in many regions worldwide where dahlias are grown, though its specific origin is unknown. The virus spreads to new locations by planting infected tubers or cuttings. It primarily transfers directly to other plants via aphids. Aphids are piercing and sucking insects that feed on a plant with DMV, then move to a healthy plant and transmit the virus while feeding. It can also be spread by manual transmission, where a person touches an infected plant and then touches a healthy plant. Pruners can also transfer the virus during harvest.

Can DMV be transmitted through irrigation water?

DMV is not known to be water-borne (unlike Pythium or Phytophthora). It is primarily transmitted via stylet-borne insect vectors (aphids) and mechanical inoculation (sap-to-sap contact).

Does the virus increase in concentration and disease severity in the subsequent tubers?

Yes. In the early stages of the disease, the plant may show no or only mild symptoms. As the virus persists in the tuber and spreads, the plant may exhibit more severe symptoms and, in turn, plants grown from these tubers may appear more severely infected.

Is composting dahlias with DMV an issue?

Composting dahlias with DMV is not recommended because the virus remains viable for transmission until the tissue is broken down. Burying the tubers is an alternative option.

What is the testing protocol if I suspect my plants are infected with DMV?

A minimum of three whole leaves (containing symptoms, if present) per sample is required to carry out this test and should be placed in a ziploc bag and mailed to the lab along with a completed submission form that can be found on the lab website. Cut-flower growers are encouraged to test each variety to detect the virus.

What is the best approach to minimize DMV risk on my farm?

Always start with clean planting stock. If buying tubers, ask the seller whether they have virus-testing results for the lot(s) of interest.
If you are saving your own tubers, thoroughly inspect plants in the fall prior to cutting foliage.
Scout young plants for symptoms early in the season before summer heat arrives, as well as during cool waves in August and September.
Sanitize tools between every plant during harvest or division.
Consider regular virus testing.
Thoroughly shred (mow) and incorporate shoots and plant materials in the fall. This accelerates tissue breakdown and reduces the risk of infected tissue remaining in the field the following season.
Burn, bury, or remove plant material each fall.
Rotate planting plots. While the virus cannot overwinter in the soil, depending on weather events and location, there is a risk that volunteer plants or residual infected plant tissue from the prior season may be present. Rotating planting locations reduces this risk.
Manage planting stock. Build replacement of tuber supplies into your long-term budget.
Manage for aphid control. Consult this UMass Extension Fact Sheet for additional information: Cut Flowers: Insects and Mites in Commercial Production of Field-Grown Cut Flowers

Other Dahlia Diseases

What is Leafy Gall?

Leafy Gall is a disease caused by the bacterial pathogen Rhodococcus fascians. The disease causes infected plants to produce buds that partially develop, leading to shoot proliferation known as leafy galls. These usually form near the base of the plant. The disease is introduced through infected material and the bacteria can spread through water splashes in irrigation systems. It can also spread via contaminated tools and through the handling of infected plants.

What is Crown Gall?

Crown Gall is a bacterial disease caused by Agrobacterium tumefaciens. It causes irregular tumor-like growths on the stem and roots of the plant. Crown gall enters the plant through wounds, and it spreads through infected plant material, soil, and tools.

What other pathogens should I be concerned about?

Dahlia is susceptible to many pathogens. The most common ones are fungal pathogens, which include powdery mildew, botrytis, and verticillium wilt.

Figure 1. Leaves expressing mosaic patterns as a result of dahlia mosaic virus.
Figure 2. Stunted leaves with mosaic patterns as a result of a dahlia mosaic virus infection.
Figure 3. Small flower bud as a result of a dahlia mosaic virus infection.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

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