Dahlia Virus: Symptoms, Spread and Solutions

Dahlia viruses seem to be an area that many gardeners and growers know little about. 

A number of viruses can effect dahlias including:

• dahlia mosaic virus,
• dahlia common mosaic virus
• cucumber mosaic virus,
• tomato spotted wilt virus,
• impatiens necrotic spot virus and
• tobacco streak virus.

Virus is a big deal because it is INCURABLE!

When I (Lorelie) first started to grow dahlias I was completely unaware of virus and I am grateful that none of the initial plants I bought were affected (as far as I know), however as I have added new varieties to our farm, I have inadvertently bought virus contaminated stock. Over time I have done more research and reading into this important area and had discussions with other growers about virus and what we can do to combat it. This is extremely important for the health of your own plants and even more so if you sell, swap or give tubers to others.

Why is virus a problem?

If a plant has a virus it is like being sick. Virus weakens the plants, it can cause stunted growth, distorted foliage and flowers and can ultimately kill the plant. It can mean that dahlia clumps divide into less tubers, that tubers are more prone to storage issues like rot and that they produce less vigorous plants (e.g. Shorter in growth, less productive, weaker plants). It becomes more of a problem over time as plants can become infected with multiple viruses and can never be 'cured' from virus, even if the symptoms appear to improve.

So first, how do we identify virus in our plants? What does it look like?

Virus usually presents as yellowing on the leaf in some form. It can look like “yellow spots or blotches, mosaic, vein yellowing, yellow line patterns, leaf margin yellowing and necrosis”. Twisted leaves can also be a symptom of virus, although in my experience twisted leaves can also be caused by mite damage. Flat leaves are usually a sign of healthy plants. Some virus symptoms can be difficult to see in full sun but are more visible in the shade according to Professor Hanu Pappu at Washington State University - so if you're not sure give that a go!

Many plants show no symptoms even if they are carrying virus.

Here are some photos of virus from plants I've seen or that have been shared with me from other growers and gardeners. I'd highly recommend that you listen to the Sustainable Flowers podcast episode on dahlia virus if you wish to learn more about this. In the podcast Clara and Heather explain that symptoms can appear and disappear which may make the plant look like it is better, however unfortunately this is not the case and once infected, a plant remains infected. You can see more photos of virus infected plants on the New Zealand Dahlia Society page.

SPREAD

Virus spreads through the sap of the plant, so if a vector such as an insect or bug (aphids and thrips are a big culprit here) bites a contaminated plant and then a healthy plant it can spread the virus that way. It can also be spread by contaminated tools (e.g. Snips, secateurs) such as when we are cutting flowers or dividing tubers and even using our fingers. Who else here pinches out the dahlia tips with their fingers? I find that easiest and quickest but need to be REALLY careful about doing this. I will endeavour to swap over to using snips for this task in the future and be vigilant about sterilising my tools between clumps when I am dividing too. Virus is also passed down to plants grown from cuttings (it is in the plant cells) and can also be passed down in its seed.

SOLUTION

What can we do to ensure healthy plants and to reduce the risk of virus in our plants?

Throw out infected plants as soon as symptoms are observed. I'm following the saying of 'if in doubt throw it out' from the brochure provided by the American Dahlia Society. I choose to put these in my rubbish bin - not my compost - although I have heard on the Sustainable Flowers Podcast that it is actually safe to put them in your compost if the temperature gets high enough to kill the virus. I am definitely on the conservative side and would not want to risk it, so I put them in my rubbish instead.

STERILISING

Sterilising your snips in between plants is also really important to minimise the risk of cross contamination. According to Dr Pappu from Washington State University who tested a number of methods, there are 3 methods which are effective; 

1) 10% bleach solution

2) detergent mixed with water

3) a solution you can purchase called Virkon S (a disinfectant often used by Vets according to some brief googling).

I personally choose to use the 10% bleach, 90% water solution. I loved the recommendation on the Sustainable Flowers podcast to just have 2 sets of snips, and alternate which ones you use for each plant so that one is sterilising while you are using the other pair. This is currently the most practical & efficient method I am aware of for a cut flower grower.

P.s Since writing this blog post we have now swapped to using methylated spirits to sterilise our snips/tools due to bleach irritating my skin and rusting our tools.

I hope you've found this helpful and that you can use the images throughout this page to compare with your own plants when you are nervous about whether they might have virus. If you'd like to learn more then I'd recommend listening to the Sustainable Flowers podcast on dahlia virus and checking out some of the resources on https://www.dahlia.org/docsinfo/virus-in-dahlia/understanding_virus_in_dahlia/

Frequently asked questions:

• Can I take cuttings from a virus infected plant or collect seeds from a virus infected plant?

No. It's not recommended as the plant can pass on the virus in its genetic material once it is infected. Virus also tends to accumulate and become worse over time, causing more damage the longer it is around.

• My plant had symptoms but it looks better now. It's cured right?

No. Dahlias CANNOT recover from virus even if the symptoms are not apparent. Once a plant has become infected it will remain so, you cannot cure it.

• Is it ok to plant a new dahlia in the same soil as a virused dahlia used to be?

Yes. Virus can only survive in a living host so once it is removed you can plant a new dahlia there.

• Can I compost my virus infected plants?

Maybe. It is recommended to put them in your bin and not in your compost as the virus is only killed if the compost reaches a high enough temperature to kill it.

If you've found this helpful then please share this so that others can learn from it too. If you have a question that I didn't answer please send me an email at hello@florelie.com.au and let me know, chances are there are others who are wondering the exact same thing and I can include it in the FAQ section below for future readers.

Happy growing!

Lorelie