These two boxwood diseases must be handled differently

Get It Growing

Boxwoods are among the most common evergreen shrubs you’ll find in Louisiana landscapes, and that’s because these plants were once considered foolproof. Sadly, in recent years, many boxwoods have fallen victim to disease problems.

You may have heard of boxwood blight (Calonectria pseudonaviculata). While this disease is certainly troublesome for boxwoods, it has yet to be detected in Louisiana. Sometimes when people here refer to boxwood blight, they actually are talking about a couple of other diseases that are more prevalent in our state: boxwood dieback (Colletotrichum theobromicola) and Phytophthora crown and root rot (Phytophthora spp.).

Boxwood dieback and Phytophthora can cause nearly identical symptoms: browning and death of foliage and, eventually, entire plants.

“In order to distinguish between the two diseases, it’s always good to get this tested because the management for these two diseases is totally different,” said Raj Singh, a plant doctor who oversees the LSU AgCenter Plant Diagnostic Center.

Sending a sample to the center is the best way to find out whether your plants have boxwood dieback, Phytophthora crown and root rot or another disease. But you can try a simple at-home test, too: Scrape the bark off an affected boxwood stem using a knife or pruners, and if you see black discoloration, odds are that boxwood dieback is present.

Boxwood dieback is a fungal disease; Phytophthora is not. Fortunately, Phytophthora can be treated with fungicides, meaning boxwoods surrounding those infected with this disease stand a decent chance of not becoming infected.

“Fungicides do not cure the infection, but they inhibit the pathogen growth and stop it from spreading if done properly,” Singh said.

The outlook is much less rosy if you find yourself dealing with boxwood dieback.

There is no cure for boxwood dieback, which spreads easily with pruning. Some home gardeners and landscapers trim out dead sections of boxwoods in hopes of ridding plants of the disease. But this simply doesn’t work — and can in fact make matters worse.

“Cutting out the dead section is not going to solve the problem because the pathogen is systemic,” Singh said. “So, when you remove that, it’s going to go to the next healthy section, and you will end up having that section dead. We recommend that if you find this disease in early stages, remove the whole plant and discard it.”

In addition to spreading by pruning, recent greenhouse research has revealed that rootto- root transmission is possible. This finding makes it all the more imperative to remove plants that are confirmed to be infected, Singh said.

Visit www. LSUAgCenter. com/ PlantDiagnosticCenter for more information on boxwood diseases and how to submit samples to the lab for testing.