Gardeners often wonder if unusual growths on dahlia plants are “lipomas.” However, plants do not develop true lipomas, as this term describes a fatty tissue tumor found in animals and humans. The growths on your dahlia are likely plant galls or other swellings.
What “Lipoma” Means for Plants
A lipoma is a medical term for a benign tumor composed of fatty tissue, typically found in animals and humans just beneath the skin. Plants lack fatty tissues, so they cannot develop true lipomas. Abnormal growths on plants are called galls, which are localized overgrowths of plant tissue. These galls form as a reaction to irritation or stimulation from various external agents, including bacteria, fungi, insects, mites, or nematodes.
Common Dahlia Growths and Swellings
Several types of growths can appear on dahlia plants. Crown gall, caused by the bacterium Rhizobium radiobacter, is common. This infection results in irregular, rough, woody swellings on tubers, roots, or lower stems, usually at or below the soil line. These galls inhibit the plant’s ability to transport water and nutrients, sometimes appearing cream-colored when young and darkening to brown as they age.
Leafy gall, caused by Rhodococcus fascians, affects the plant’s hormone activity. This leads to dense clusters of distorted, stunted, leafy shoots or buds, typically at the stem base. Leafy gall can be challenging to spot, especially early in the growing season, as the distorted growth may be obscured by normal foliage.
Swellings on dahlia roots can also be caused by root-knot nematodes, microscopic soil worms. These pests induce knobby, pimple-like galls on the roots, ranging in size up to an inch across. While visible only by digging up the plant, nematode galls can lead to above-ground symptoms like stunted growth, yellowing leaves, and wilting, particularly during hot weather. Other less common galls may occur on upper plant parts due to insect larvae or other pathogens.
How to Identify Dahlia Growths
Identifying the specific type of growth on your dahlia involves careful observation of its location and appearance. Note where the growth is situated: galls at the soil line, lower stem, or tubers often indicate crown gall, while dense, distorted leafy clusters at the base suggest leafy gall. Inspecting roots for knobby swellings requires digging up the plant to check for root-knot nematodes.
Examine the growth’s texture, color, and size. Crown galls are typically rough and woody, while nematode galls on roots are knobby. Also, look for other plant symptoms like stunted growth, unusual wilting, or yellowing leaves, which may suggest a root issue. The plant’s history, such as recent transplanting or exposure to infected plants, can also provide diagnostic clues.
Managing and Preventing Dahlia Growths
Once a dahlia plant has bacterial galls like crown gall or leafy gall, removing the entire infected plant is often the most effective measure to prevent further spread. Dispose of infected plant material properly; do not add it to compost piles. For soil-borne pathogens, implement crop rotation by planting dahlias in a different location for at least two years. Soil solarization, covering moist soil with clear plastic during hot weather, can also reduce soil-borne pathogens.
To prevent future issues, purchase healthy, certified disease-free dahlia tubers or plants from reputable sources. Avoid planting dahlias where infected plants previously grew. Maintain good garden sanitation by regularly disinfecting pruning shears and other tools with a bleach solution or alcohol, especially between plants. Minimizing wounds to dahlia plants during planting and cultivation can also reduce entry points for bacteria. While no chemical treatments exist for bacterial galls, planting marigolds in affected areas can help suppress nematode populations.