BATON ROUGE, La. – Beginning in the fall of 2011, Raj Singh, the LSU AgCenter’s “plant doctor,” began seeing an increase in the number of calls about damage to boxwoods.
“It actually started when I received four samples of plants showing damage,” Singh said. “Two were from homeowners, and two came from commercial landscapes.”
The symptoms on those plants mimicked Phytophthora root rot, he said.
“There was random dieback of twigs with light tan-colored foliage,” he said. “However, the roots and crowns of the affected plants looked normal.”
The infection also caused bright black discoloration of the stem immediately under the bark. Singh said this discoloration extends all along the infected twigs and differs from discoloration of the crown region caused by Phytophthora.
After several attempts, Singh said, he was not able to detect or isolate Phytophthora from the roots and crowns of the infected tissue.
“I then started working with the tissue taken from the transition zone of healthy and dead twigs,” Singh said. “After about five days, a fungus was consistently isolated from the infected twigs.”
Singh said he tentatively identified the fungus as Colletotrichum. Later on, DNA testing confirmed this fungus as Colletotrichum theobromicola.
“Since this is considered a new disease in boxwoods, we’re calling it stem canker of boxwood caused by Colletotrichum theobromicola,” Singh said.
“From the beginning, I was convinced that it is a new disease, and I informed diagnosticians in other states about it,” Singh said. “During 2012, Lorraine Graney with Bartlett Tree Company based in North Carolina sent me a boxwood sample from Virginia that exhibited similar symptoms.”
Since then, she has reported the fungus being associated with boxwoods in North Carolina, New York, Virginia and Indiana.
In 2013, Singh presented this issue at the Southern Plant Diagnostic Network meeting, and as a result, Margaret Williamson with the Plant Disease Clinic at Clemson University isolated the fungus from infected boxwoods and shared the material with him.
“We have discovered that the DNA in the fungus from South Carolina matches the DNA in the Louisiana fungus,” Singh said.
The next big step is to figure out how to manage this disease, Singh said. He believes systemic fungicides would be more effective than contact fungicides.
“The timing of fungicide application also will be a crucial factor in disease management,” he said.
In landscapes where the disease is already present, surface disinfection of pruning and cutting tools is important to reduce the spread of the disease. Removing dead and dying twigs and avoiding unnecessary injury also is important to avoid infection.
“Due to slow disease development, it will be a little while before we can find out which fungicides are effective in managing stem canker,” Singh said. “In the meantime, I would recommend that landscapers, nurserymen and homeowners follow good cultural practices and create an environment that will decrease the spread and development of the disease.”
Boxwood is an important perennial that is becoming increasingly popular in all landscapes, Singh said. Several varieties are available, and its vibrant green color and evergreen growth make it an excellent ornamental.
Boxwoods are used as stand-alone ornamentals at the entrances of homes and business. They are also grown as hedges around homes and commercial landscapes.
According to the 2010 USDA National Agricultural Statistics Service, the wholesale market value of boxwoods was estimated at $103 million annually.
If you notice any of the symptoms on boxwoods in your landscape, contact your AgCenter parish office.
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