A new disease that attacks boxwood shrubs has been showing up in south Louisiana the past few years.

Beginning in fall 2011, plant pathologist Raj Singh, the LSU AgCenter’s “plant doctor,” began seeing an increase in calls about boxwood damage.

“It actually started when I received four samples of plants showing damage,” Singh says. “Two were from homeowners, and two came from commercial landscapes.”

The symptoms are random die-back of twigs with light tan-colored leaves. However, the roots and crowns of the affected plants appear normal. The infection also causes bright black discoloration of the stem immediately under the bark and extends all along the infected twigs.

Singh tentatively identified the fungus as Colletotrichum, which was later confirmed by DNA testing. “Since this is considered a new disease in boxwoods, we’re calling it stem canker of boxwood caused by Colletotrichum theobromicola,” he says.

In landscapes where the disease is already present, disinfecting the surfaces of pruning and cutting tools is important to reduce the spread of the disease. Removing dead and dying twigs and avoiding unnecessary injury to the plant also are 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 says. “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.”

If you notice any of the symptoms on boxwoods in your landscape, contact your AgCenter parish office.

You Asked

How and when should I cut back or prune my Meyer lemon tree? It produced green fruit last fall, but someone pulled all of the fruit off the tree. It has not produced fruit since then. Should I prune, and how far down? — Cathy

Many lemon and other types of citrus trees failed to set a crop this year in southeast Louisiana.

The severe freezes of January appear to have stressed many trees and prevented them from producing a crop. Expect a great crop next year. Pruning reduces production in citrus trees, so it is generally kept to a minimum. We generally prune citrus trees only if they have a problem that needs to be solved. — Dan Gill, AgCenter horticulturist

Got a gardening question? Write to GardenNews@agcenter.lsu.edu.