A new disease is appearing on loropetalum, or Chinese fringe flower plants, in the area.
It’s caused by a bacterium called Pseudomonas savastanoi, which also causes galls or knots on olives and oleanders, says LSU AgCenter plant pathologist Raj Singh. Other known host plants include ash, privet and forsythia.
“On loropetalum, the bacterium causes irregular, rough galls or knots with dark-colored callus,” Singh says. The galls can be found on both shoots and stems.
The bacteria rapidly girdle small lateral shoots, resulting in shoot dieback. As the disease develops, the galls enlarge and girdle the main stem, causing the plant to die.
The disease develops rapidly during extended periods of wet, warm weather. The bacterium is spread from infected to healthy parts of the same plant or to neighboring plants by splashing water from rain or sprinkler irrigation.
“The bacterium penetrates the host tissue through natural openings or wounds and causes infection,” Singh says.
After initial infection, galls may start to appear in two to three months.
The disease is introduced into nurseries or landscapes on infected plants and spreads to healthy plants during propagation in nurseries and during pruning and hedging in the landscape.
“Contaminated cutting tools play an important role in spreading the pathogen in the landscape,” he says.
The disease can be managed by removing entire plants that have galls on the main stem.
“If the disease is detected at an early stage when galls are present on small shoots, removing infected plant material several inches below the gall helps reduce disease spread,” Singh says.
Singh recommends buying disease-free, healthy plants and inspecting the stems and shoots for any galls or knots.
“Avoid unnecessary injuries and disinfect cutting or pruning tools between cuts,” he says.
In the landscape, avoid overcrowding plants and select sites with good air movement to promote rapid drying of plant tissue, Singh says. If overhead sprinklers can’t be avoided, plants should be watered early in the morning.
Preventive applications of copper-containing fungicides when the environment is favorable for disease development may help avoid infection and suppress bacterial growth in landscapes where the disease is prevalent.
“But remember,” Singh says, “fungicides will not cure the galls or knots.”
Got a gardening question? Write to GardenNews@agcenter.lsu.edu.