Got galls? Galls are overgrowths of the stems in plants that resemble tumors. In chrysanthemums, they appear on the main stem and peripheral twigs. The fat, ugly tumors are the most obvious of the chrysanthemum crown gall symptoms. What causes this and how do you prevent it? The disease affects plants in over 90 families and is as contagious to plants as the common cold is to humans.
Chrysanthemum Crown Gall Symptoms
Crown gall of mum plants disrupts the flow of nutrients and water to other parts of the specimen. The first observed symptoms are usually on the crown of the plant but can also be seen on the stem. The disease also affects the roots, but this is less easy to detect without digging up the plant.
The galls are warty tumors seen upon the basal or crown parts of the chrysanthemum. They are light green to whitish and soft when young, but become brown and woody as they age. Galls can also appear on leaves, generally at the mid-veins. They are smooth, tan, and about ¼ inch (0.5 cm.) across.
Over time, crown galls will cause stunted growth and limited vitality in the plant. Crown gall of mum plants can lead to lower production of flowers; yellowed, limp leaves; and overall diminished plant health. These symptoms can mimic many other issues such as lack of water, low nutrients, and plant injury.
What Causes Chrysanthemums with Crown Gall?
Agrobacterium tumefaciens is the culprit when crown galls appear. It is a naturally occurring bacterium in the Bacillus group that persists in the soil where aeration is adequate. It can also survive on the roots of plants. The most common soils in which the bacterium survives are sandy loams.
The disease spreads easily through poor sanitation practices and plant injury. Any slight nick on the plant surface can invite the bacterium to enter. Even tissue that has experienced frost damage may allow the disease into the plant’s vascular system. Using unsanitized pruning tools can also transfer the disease to the chrysanthemum.
Chrysanthemum Crown Gall Treatment
There are a couple of methods of treating mums with crown gall, but inspecting plants before planting can help prevent the spread of the disease in the garden. Often, nursery stock is already contaminated with the disease, which can be seen early in the roots of new plants.
Look for nodes and irregular growth on plants before planting. Additionally, disinfect your cutting shears to prevent the transfer of the disease.
In greenhouse situations, a creosote or copper-based product is used to some effect. In the home garden, the use of such products is not recommended and it is best to dig out and destroy any affected plants.
Before planting any susceptible stock in the soil again, solarize the soil to kill the bacteria and avoid re-infection in your garden. A useful pre-planting chrysanthemum crown gall treatment is to dip the roots of a new plant into Agrobacterium radiobacter, a biological control that essentially inoculates your plant. This can be hard to source, however, but good sanitation, crop rotation, and inspection of new plants are usually sufficient.