Boxwood plants turning yellow is indicative of a problem that needs to be identified and remedied as soon as possible.
The most common causes of yellowing boxwood plants are boxwood blight, winter injury, salt damage, underwatering, boxwood leafminer, Volutella stem canker, Macrophoma leaf spot, and nematodes.
In this article, we will discuss the causes of yellowing boxwood.
How Can You Tell If Your Boxwood Has Boxwood Blight?
One of the first symptoms of boxwood blight is a lot of defoliation, or leaf drop. Plants that are not doing well often lose a lot of their leaves out of season.
Part of the plant may become chlorotic and even turn yellow and then brown. The leaves will fall off en masse, revealing bare branches.
Spots on your boxwood’s leaves are indicative of a possible infection. These spots can coalesce and grow larger. The black lesions may also be seen in the stem. If the weather is humid, there may also be a white substance on the underside of the leaf, caused by the formation of upright bundles of fungal spores.
Why is My Boxwood Turning Yellow?
Other causes of yellowing boxwood include:
Winter injury
Winter injury is one of the most common causes of yellowing boxwood. You can immediately tell something is wrong when the snow recedes and the uppermost and outermost leaves are yellow or brown. Most plants cannot tolerate temperatures of -10 degrees Fahrenheit or below, especially if they are close to the pavement or the side of the house. These locations can warm up the tissue of the plant too quickly.
Winter damage is most probable when the former snowline is still green. But as long as the damage to the boxwood is not too severe, you can simply prune out any of the damaged leaves and branches.
Salt damage
When salt is used to de-ice the sidewalk or walkway, it can damage your boxwood. The spray of the salt water can be forceful enough to dessicate the plant’s tissues and even kill the leaves. The salt from the spray that gets washed into the soil can also affect the water uptake of the plant, causing even further damage. This is also most noticeable when spring comes around.
It is easy to determine whether salt damage is the cause if there is a pattern of damaged boxwood only near walking surfaces.
Underwatering
Boxwoods can also suffer from drought stress if not given a sufficient amount of water every day. The leaves on the plant will turn yellow and then brown.
Drought stress is most severe in landscapes that have just been planted, because these plants are suffering from transplant shock. Boxwood that does not have great or effective irrigation, as well as those that have gone without rain or water for too long, will also suffer from drought stress. Boxwood that is grown in warm climates is also susceptible to drought stress.
Symptoms of drought stress include yellowing and browning of the center leaves and chlorotic foliage. Underwatered boxwood becomes chlorotic because the lack of water in the soil makes them unable to effectively absorb much-needed nutrients.
You can remedy an underwatered boxwood by making sure you water it when it needs to be watered. Check the soil by touching it. If the soil is dry, water the boxwood; if it is still moist, wait a day or two and check the soil again.
Boxwood leafminer
The boxwood leafminer can also cause the leaves of the boxwood to turn yellow and brown. This insect resembles a mosquito and will lay its eggs between the layers of leaves. The larvae will develop and feed on the tissue. Adult leafminers emerge from the leaves, leaving a hole where they exited. The affected leaves develop brown patches as the larvae develop inside them, and heavily infested leaves will eventually fall off.
Volutella stem canker
This fungal disease is caused by the fungus Pseudonectria buxi. It causes the leaves to become bronze or yellow. The fungus produces fruiting bodies when it sporulates on the undersides of the leaves. On the infected branches, the bark can become loose and may die back, leading to the death of parts of the plant.
This fungus is exacerbated by rainy weather. Unlike with boxwood blight, the leaves will not fall off in case of a Volutella infection.
Macrophoma leaf spot
Macrophoma leaf spot is caused by the pathogen Macrophoma candollei. This fungus causes yellow and reddish-brown lesions to appear on the leaves of the boxwood. When it sporulates, black fruiting bodies will appear on the undersides of the leaves.
Root and crown rot
Root and crown rot is caused by the fungus Phytophthora sp. They will appear as yellowing, browning and wilting on the foliage of boxwood plants. This fungal disease can also cause stunted growth, upward turning of leaves, root rot and discoloration of the stem near the soil line.
Fortunately, the leaves of plants infested with this fungus do not carry any fruiting bodies.
Nematodes
These microscopic pests are roundworms that feed on the roots of boxwood. The plant will turn yellow and wilt. Eventually, it will die back if the root has been damaged extensively.
You can try to prolong the life of your boxwood by watering and feeding it, but it will still succumb to the nematodes. You could try to grow nematode-resistant American boxwoods instead.
Conclusion
Boxwood turning yellow is indicative of a problem within the plant that needs to be identified in order to be remedied.
The most common causes of yellowing boxwood are boxwood blight, winter injury, salt damage, underwatering, boxwood leafminer, Volutella stem canker, Macrophoma leaf spot and nematodes.
Catching the problem early is key to minimizing the damage to your boxwood.
Image: istockphoto.com / MaYcaL