The Dreaded Leafy Gall
Diseases impact plants as well as humans, although luckily they are not passed between us. You can not get sick from a plant disease, but you can act as a vector and move diseases between your plants. There are two bacterial diseases that seem to be more common these days: crown gall and leafy gall. This page will focus on leafy gall and will provide information from sources and the researchers who have studied this disease.
What is Leafy Gall?
Leafy gall is a bacterial disease caused by a microbe called Rhodococcus fascians. You’ll never have to remember that again, but I point it out so that you know that the microbe responsible for this disease is different from the microbe that causes crown gall. As a distinct species, this microbe is able to live on the plant as an epiphyte. I have always thought the word epiphyte was cool and what this means is that it can live on the top of the cells, not causing disease until an opportunity arises, like a cut on a tuber or a nick on a stem. As such, this microbe may be in your garden, but not causing disease. It’s causes opportunistic infections. Look at all the biology you are picking up!
What does leafy gall do to the plant?
When the bacteria gets into the plant through a nick, it appears to start disrupting the normal plant hormones and causes an abundance of shoots. The tissue grows in fascicles ( a bundle of structures) which is where the microbe gets it’s name.
While this is unsightly, the plant may still produce to a degree. BUT if it happens on a thin stem, the anatomy of the stalk becomes compromised and is more likely to break. BUT again, it will spread to other dahlias and to other plants in your garden. This where trouble can happen because if this bacteria gets into other plants, like orchards, it can significantly disease those trees. These are two compelling reasons to eliminate diseased plants from your garden space.
How to recognize leafy gall?
Normally a sprout would come up and it would be white for several inches until it comes up through the soil and gets the sun’s UV light. Chlorophyl should not be made until the sprout hits UV light, but as you can see in this mass that was underground, the hormone disruption has caused green bits on this growth. There also should be eyes/ sprouts that are not directly connected to each other. The microbe in disrupting the hormones that would keep those sprouts separate. This leafy gall lesion is large, but they may be small, so look at all areas of the stem and tuber necks for evidence of this bacterial infection. In all honesty, if in doubt-throw it out. Don’t risk your garden.
Spreading leafy gall
This is a disease and one that can be spread by touch. General practices include disinfecting sheers or snips between plants to avoid transmitting the bacteria. This extends to shovels and rakes used around the base of the plants, especially if those tools are likely to nick the stem or tubers and introduce the bacteria under the surface of the plant.
New plants should be grown in isolation to protect the rest of your garden (but there is no solid evidence that insects like pollinators can spread leafy gall). Ideally, plants that are new to you that may be infected should be planted so their leaves do not touch and water sprayed on one plant can not splash onto another plant. This provides an argument for soaker hoses for watering as opposed to overhead watering. If you have an infected plant, water splash and leaves touching means the plants immediately surrounding the diseased plant may have picked up the bacteria, even if they haven’t started showing symptoms.
There is a bit more debate on whether water in soil can move the bacteria. Studies at Oregon State University have found this bacteria when introduced via water to a pot can be collected in runoff water from the saucer for up to 98 days before the bacteria is gone. They also found that if they watered a healthy plant with water collected in a saucer under an infected pot, the new plant can develop leafy gall. This study was not on dahlias, but does show the possibility of transmission from irrigation and this is a well accepted fact in the OSU plant clinic. This is important information for those who are starting tubers inside in growth trays where water can act to move the bacteria. Note: if you are growing cuttings in a tray and they share water through the base of a tray, then you should assume every plant in that tray is affected and they should all be thrown out. If you reuse the tray, you have to thoroughly disinfect it before using again.
Many growers dig out an 18-20 inch hole about 10 inches-12 inches deep and carefully remove the contaminated soil (trash, woods, back 40) and then replace the soil in that location to help prevent future plants from being infected. You may hear the advice to grow other non-susceptible plants may be grown at that site for a few years to be sure the bacteria is gone from the soil. Another study on the persistence of this bacteria in the soil suggests this is not the case. Researchers applied heavy doses of R. fascians into several soil types and then over time tried to collect the bacteria from the soil. They found bacterial numbers were declining in 3 days. Granted, this was just soil and maybe with a plant in that soil, the bacteria may have better survival rates, but if you are lifting tubers and leaving the soil fallow until the following spring, the study suggests that the bacteria will be gone and significant declines in bacterial numbers can happen as quickly as three days after the plant material has been removed.
To date, studies have not shown the bacteria to blow in the wind and become aerial to spread, nor are there scientific studies that show pollinators/insects spread this bacteria, despite the discussions that go on in several dahlia social media groups.
Transmission via different plant parts – Can I save it?
Sadly, an infected plant needs to be considered infected from root to tip of the leaf. There will be a lot of the bacteria around the wound and growth, within several inches. Based on the fact most of these lesions occur near the tuber neck and base of the stalk, its safe to assume that all of these tissues are infected, even the tuber on the off side that isn’t near the lesion. All tuber and stalk need to be burned or thrown in the trash inside a bag. Don’t compost it because the bacteria will be in the plant matter remnants and you want to keep infected plant tissue away from healthy plants.
As for the leafy part of the plant, it’s possible that the plant may not have the bacteria up on the top of the stalk or leaves. Dr. Putnam, a researcher at Oregon State University I conversed with, said the bacteria does not get into the veins of the plant (phloem) and get transmitted to all parts of the plant internally. But that doesn’t stop the bacteria from being on the surface of the stem and leaves. Taking cuttings to try and salvage your dahlia variety is exceedingly risky because that stem grew and originated from the area that the lesions develop. You should weigh the risks to your garden against the value of that plant and the possibility of getting a replacement at a later date.
Scientific papers and posters researched and summarized here can be located at this link