Bacterial gall on loropetalum stem is seen as irregular, rough galls or knots with dark-colored callus caused by Pseudomonas savastanoi on Loropetalum sp. (Photo by Yan Chen, LSU AgCenter) News Release Distributed 05/11/15
BATON ROUGE, La. – A new disease is making its appearance on loropetalum plants in Louisiana nurseries and landscapes. It’s caused by a bacterium called Pseudomonas savastanoi, which is also known to cause galls or knots on olives and oleanders, said LSU AgCenter plant pathologist Raj Singh. Other important known hosts include ash, privet and forsythia.
“On loropetalum, the bacterium causes irregular, rough galls or knots with dark-colored callus,” Singh said. 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, leading to plant death, Singh said.
The disease develops rapidly during extended periods of wet, warm weather. The bacterium is spread from infected to healthy parts of 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 said. 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, and cuttings from infected stock plants for propagation serve as a major source of pathogen spread in nurseries,” he said.
In landscapes 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 said.
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 said.
In the landscape, avoid overcrowding plants and select sites with good air movement to promote rapid drying of plant tissue, Singh said. If overhead sprinklers can’t be avoided, plants should be watered early in the morning.
“Nursery owners must take all precautions to avoid introducing the disease into their nurseries,” Singh said. “They should buy liners or stock plants from a reliable source.”
Nurseries should quarantine newly arrived loropetalums in a separate location for about three to four months, carefully inspecting these plants routinely for development of any galls, knots and shoot dieback symptoms, he said. “Remove symptomatic plants immediately and dispose them off properly.”
Nursery propagation areas should be kept as clean as possible and tools and other equipment should be disinfected. “Avoid reusing potting mix or pots to avoid infection carryover,” Singh said. “Maintain disease-free, healthy stock plants away from propagation area and keep them healthy.”
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, fungicides will not cure the galls or knots,” Singh said.
Rick Bogren