Laurel Wilt Disease and the Endangered Pondberry Shrub

healthy-pondberry
Pondberry is also known as southern spicebush. Its current range includes Alabama, Arkansas, Georgia, Mississippi, Missouri, North Carolina, and South Carolina. USFS photo.

Pondberry (Lindera melissifolia) is a rare, federally endangered shrub that’s found scattered around bottomland forests of the southeastern U.S. In late summer, the shrub produces spicy, crimson-colored fruits. Like other native Lauraceae species, its leaves give off a sweet, citrusy scent when crushed.

And, like its Lauraceous brethren redbay and sassafras, pondberry is susceptible to the deadly, nonnative laurel wilt disease.

But pondberry reproduces vegetatively, mainly through rhizomes, or underground stems that produce new roots and shoots. Two USDA Forest Service scientists wondered if this different form of propagation might affect the shrub’s susceptibility to laurel wilt.

Plant pathologists Susan Best, who began the project while completing her master’s degree at Iowa State University, and Stephen Fraedrich conducted a series of experiments to determine how laurel wilt would affect pondberry colonies. Their results were published in Southeastern Naturalist.

The deciduous shrub has been threatened by habitat loss and previous insect and disease problems, but laurel wilt has rarely been documented in pondberry. Wilt diseases can spread through roots and infect other plants that develop from root sprouts. But would the disease travel through the underground, interconnected stems?

wilted-pondberry
The ramets of a pondberry plant can spread more than 12 feet from the original stem; laurel wilt disease covered that distance in less than two months. USFS photo.

Best and Fraedrich grew pondberry in pots and raised beds for several years. They inoculated single stems with the laurel wilt fungal pathogen, Raffaelea lauricola, or mock-inoculated a control group with sterilized water.

“Stems inoculated with R. lauricola began to show symptoms within two weeks. Leaves started to droop, turned brown, and plants wilted completely in about four weeks,” says Best. “The pot study demonstrated that the fungus could travel through the rhizomes, and the field study confirmed those results.”

In the pot studies, R. lauricola caused wilt in a majority of the ramets, or individual stems that grew from the original plant.

The wilt spread rapidly through runner rhizomes in the field experiments, killing up to 59 ramets at distances of more than 12 feet from the inoculated stems.

“We were amazed at the speed with which it moved through the rhizomes – how fast and how far,” adds Best.

“This study demonstrates that R. lauricola could impact an entire pondberry colony if it’s infected. Quick diagnosis and treatment are a must,” says Fraedrich.

laurel-wilt-pondberry
Humans often help laurel wilt spread to new areas by moving infested firewood. Don’t move firewood: buy it where you plan to burn it. USFS photo.

The scientists are now studying different treatment options. Pondberry’s rhizomes are found in the top five to eight inches of soil, which may help researchers to isolate and treat infected parts of the plant and stop the spread of the pathogen. More controlled experiments are essential, because pondberry’s federally endangered status limits management applications in its natural settings.

They’re also thinking about how to prevent infection. Laurel wilt’s carrier, the redbay ambrosia beetle, primarily attacks hosts with larger diameter stems – like sassafras and redbay. “Land managers should monitor their pondberry populations and keep an eye out for dead and dying redbay in the area,” says Best.

 Read the full text of the article.

For more information, email Susan Best at susan.best@usda.gov.

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