Boxwood Disease: What is Killing My Boxwoods?

This spring, we noticed that a lot of our boxwoods appeared to have a disease. They had been thriving for the three years that we have lived in this house. So I thought it was strange that they suddenly developed brown leaves and dead branches. I set out to figure out what could be causing this, and I learned that several boxwood diseases could be the cause. But I finally concluded that root rot had killed these particular boxwoods. In this article, I’m sharing what I learned to help you if your boxwoods aren’t looking so good.

Common Boxwood Diseases

Boxwood Blight

Boxwood blight is arguably the most destructive boxwood disease. Caused by the fungi Calonectria pseudonaviculata and C. henricotiae, this disease can lead to rapid defoliation and death of the shrub.

Symptoms include dark, circular leaf spots, light tan or dark brown stem cankers, rapid defoliation, and black, sticky spore masses under wet conditions. It often spreads through contaminated plants, tools, clothing, or even wind.

The best defense against boxwood blight is prevention. Purchase plants from reputable sources, regularly clean gardening tools, and isolate new plants before introducing them to the garden. If blight is detected, the infected plant should be removed and destroyed. Fungicide applications can help manage the disease but won’t completely eradicate it.

Volutella Blight (Pseudonectria Rousseliana)

Volutella Blight, also known as boxwood dieback, is a less severe but more common disease. Its symptoms include browning of leaves and stems, pinkish spore masses on the underside of leaves, and eventually, defoliation.

The disease spreads in wet conditions and is often associated with plant stress or physical damage. Proper cultural practices like watering at the base of the plant, avoiding excessive pruning, and promoting good air circulation can prevent the disease. If infected, pruning out and discarding diseased portions, combined with a systematic fungicide treatment, can help manage Volutella Blight.

Macrophoma Leaf Spot

Macrophoma Leaf Spot is a fungal boxwood disease that causes small, brown, circular spots with yellow halos on boxwood leaves. Over time, leaves may drop prematurely.

Although it rarely causes severe damage, it can make the plant unsightly. Good sanitation practices, such as cleaning up fallen leaves and debris, can help prevent the disease. Fungicide applications can also be effective if the disease is widespread.

Boxwood Leafminer

The Boxwood Leafminer is not a disease but a pest causing significant damage to boxwoods. The adult leafminer is a small fly that lays its eggs inside boxwood leaves. The larvae feed on the leaves from the inside, causing yellow or brown blisters on the leaf surfaces.

Preventing a Leafminer infestation involves regular monitoring, releasing beneficial insects that prey on leafminers, and pruning and destroying infested leaves. If necessary, systemic insecticides can be applied in early spring to control the pest.

Boxwood Psyllid

Another significant pest is the Boxwood Psyllid, an insect that feeds on boxwood sap, causing cup-shaped leaf curling, or “puffing”. While it generally doesn’t cause significant harm, heavy infestations can weaken the plant and make it more susceptible to diseases.

Keeping a clean garden, using insecticidal soaps or horticultural oils, and introducing natural predators like lady beetles can help control Psyllid populations.

Boxwood Root Rot

Root Rot is a deadly boxwood disease, caused primarily by two types of fungi – Phytophthora and Armillaria. These fungi invade the roots, causing them to decay and ultimately disrupting the plant’s ability to uptake water and nutrients.

Symptoms of Root Rot include yellowing or browning leaves, reduction in the growth rate, and overall wilting of the plant. As the disease progresses, the plant may exhibit branch dieback, and the root system becomes dark and mushy. Unfortunately, above-ground symptoms often appear only when the root damage is extensive.

The fungi causing Root Rot thrive in waterlogged soils. Therefore, to prevent this disease, boxwoods should be planted in well-draining soil and watered carefully to avoid waterlogging. Additionally, avoiding physical damage to roots and good sanitation practices can also help keep these pathogens at bay.

Once a plant is affected, treatment options are limited. The diseased plant, along with its root system, should be removed and destroyed to prevent the fungi from spreading. In the case of Phytophthora, soil can be treated with specific fungicides to kill the remaining spores.