At the 2018 National Dahlia Show at Longwood Gardens PA, I attended the presentation “Viruses in my Garden” given by Ron Minor. Much of what he covered is in the attached pdf document from the ADS but here are a few notes I took myself, followed by a recent post on dahlia.com about the same subject.
As you are digging your dahlias this fall, please inspect them and destroy those which appear to be infected.
First thing I noted was that often it seems that all dahlias are virused because they all seem to have more than one shade of green in the leaves. So I was glad that Ron showed this photo comparing a healthy plant on the left with one which displays DMV on the right. Here is a better photo of the plant with DMV on the right.
Healthy-vs-Dahlia Mosaic Virus
Dahlia mosaic virus
Tomato Spotted Wilt Virus can be detected by lightning bolt like pattern around the leaf.
Leaves that curl under, likely have a mite infestation rather than a virus.
Ron singled out Thrips as the main spreader of some viruses but aphids can also be a source. Natural predators include lady bugs, parasitic wasps, and lacewings. You can also use Safer BioNEEM or Soap to spray them in the cool part of the day. The most direct form of protection is insect netting.
Tomato Spotted Wilt Virus
We can be a big spreader of virus if we don’t take precautions. If using tools to cut plant material then Ron recommended dipping them in either 10% bleach water or Dawn soap water between each plant. He actually does both with two buckets. If you are pinching with your finger nail then you should use a washcloth with Dawn soap on it and some rinse water to clean your hands. But if you are just bending them till they break (or tear) then you should not need to.
Viruses in my garden
Virus in Dahlias – Become Part of the Solution! One of the opportunities that has evolved from the virus work at Washington State University is for you to become an important part of the solution to the problem of virus in our dahlia patches. There are two key items that need to become a part of your routine practice in your garden. First is to watch for the appearance of virus on the leaves of your plants. Virus symptoms were illustrated in the brochure distributed in the June, 2016, ADS Bulletin and those pictures can also be found on the ADS Website, dahlia.org. If the foliage on your plants match the appearance of the leaves in the brochure, the plant(s) should be removed from the garden and destroyed in an area away from the garden. Aggressive implementation of a “remove and destroy” strategy on a dahlia garden has been shown to lead to a virtually virus-free garden! However, you could also be removing some plants that don’t need to be removed. An intermediate approach that has been available at the WSU Clean Dahlia Center is to test your dahlias for virus. That way you will know which of those suspicious-looking plants really has virus. In 2017 and 2018, testing was available at a subsidized rate of 30 samples for $300. The rate was made possible by the cooperation of Professor Hanu Pappu and the financial support of the Scheetz-Chuey Foundation. We hope to be able to establish a similar arrangement for 2019.
It is virtually certain that your garden contains a mix of clean and virused dahlias. Some of the plants with virus could lack any evidence of virus on the foliage. When you work in your garden, there is a very real risk that you can and will move virus from the virused plants over to the clean plants unless you disinfect your tools between each plant. A 10% bleach solution is the “gold standard” for disinfecting your tools but there are a couple more options listed in the table below.
The dishwashing detergent has the advantage of avoiding the damaging effect of the bleach on the metal in your tools. It is also readily available and inexpensive. The Virkon S is an anti-viral product that is routinely used in veterinarian cleaning applications. It is readily available from internet suppliers. (My grandchildren tell me to “just Google it!”) Please be aware that anti-bacterial products like hand sanitizer can be completely ineffective in killing virus.
Disinfecting your tools is an essential part of dividing your clumps. There is no more aggressive “attack” on your plants than When you divide those clumps into tubers.
10% Solution of Bleach
Dawn Dish Soap 4 tsp in 5 cups Water
Be sure to have one of the sterilization options sitting on your cutting bench to sterilize those cutting tools between each clump. I will have two or three cutters and both the bleach and soap solutions on my bench. The tools will be used in turn. Each will get a bleach dip and a soap rest while waiting for the next clump!
To become part of the solution of the dahlia virus problem; 1) Remove obviously virused plants from your garden and 2) Disinfect your tools between working on different plants.
Professor Hanu Pappu, Ron Miner and the Virus Team: Brad Freeman, Nick Weber, Jerry Moreno, and Linda Taylor