Pachysandra May Be Next Victim Of Boxwood Blight

By Nancy K. Crevier

The fungus Cylindrocladium buxicola, also known as Cylindrocladium pseudonaviculatum, and commonly known as boxwood blight, was identified in Fairfield County in October of 2011, according to Dr Sharon M. Douglas of the Connecticut Agricultural Experiment Station (CAES) in New Haven.

Boxwood samples submitted to the New Haven Experiment Station at that time were sent for further testing to the USDA Animal Plant Health Inspection Service, which positively identified the samples as the fungus that is deadly to nearly all varieties of boxwood used extensively in landscaping in this area.

According to a report posted by Dr Douglas on the CAES website last December, the disease was first identified in New Zealand in 2002, and has since been reported throughout Europe, where it is considered to be of great concern. It is not known how the blight was introduced into the United States, she said.

Last month, the CAES posted a new warning concerning Cylindrocladium pseudonaviculatum. Plant pathologists there have identified Pachysandra terminalis (common names: pachysandra, Japanese spurge) as a new host of Cylindrocladium pseudonaviculatum. Pachysandra is an evergreen groundcover popular in shade gardens. Its vigorous growing habit makes it a favorite for quickly covering large areas in residential and commercial gardens.

According to the CAES website www.ct.gov/caes, “Healthy pachysandra plants were inoculated with spores of C. pseudonaviculatum and lesions developed on the leaves ten days after inoculation. Three weeks after inoculation, many of the leaves with lesions yellowed and dropped. Heavy sporulation of the fungus was observed. This raises significant concerns about pachysandra as a potential source of inoculum for infection of boxwood and vice versa. More information will be forthcoming.”

Last December Dr Douglas noted that even if Cylindrocladium buxicola (psuedonaviculatum) is positively identified in a plant, it does not mean that nearby boxwoods must also be disposed of. It is possible to contain the disease by properly removing — and that includes digging up the entire plant — the infected plant. Bagged, diseased boxwoods can be thrown in municipal trash, or they can be buried.

Currently, there is no curative for Cylindrocladium buxicola, Dr Douglas said. Fungicides registered for use in Connecticut include chlorothalonil and mancozeb. They must be applied regularly to nonaffected plant stock and may be useful in preventing the spread of boxwood blight to healthy plants. Plants already infected will not be helped by fungicide applications.

“This is an experimental inoculation,” stressed Dr Douglas on Friday, March 23, referring to the new warning about pachysandra. “We have no evidence yet of it occurring in established or landscaped settings,” she said.

The concern is not so much for pachysandra, which in repeated experiments does not seem to develop stem cankers or completely die off when infected, but rather that it could spread the fungus to boxwoods in nearby settings, which cannot recover from the fungal infection.

“We don’t want people to be alarmed, but it is important for growers to know that they could be infecting their boxwood stock inadvertently, if they are also growing pachysandra that carries the fungus,” Dr Douglas said.

Because the experiments were carried out only on the Pachysandra terminalis, it is still unclear as to whether other varieties, such as the native Pachysandra procumbens, are susceptible.

If gardeners see symptoms of dark spots, leaf yellowing, and leaf drop in Pachysandra and are concerned that the plants may be exhibiting signs of Cylindrocladium pseudonaviculatum infection, Dr Douglas suggested they visit the www.ct.gov/caes for details on submitting samples to the lab, or call 203- 974-8500.

“Until we get a better handle on if this fungus is occurring in pachysandra in landscaping, we will continue to be learning more. Just the fact that [Pachysandra terminalis] is another host is of concern, though,” Dr Douglas said.