Although boxwoods (Buxus sp.) are not native to the U.S., their versatility has made them one of the most popular shrubs for edging, hedges, and topiaries in classic and modern landscapes. We at Iowa Plant and Insect Diagnostic Clinic see few foliar diseases of boxwood, but most commonly we receive photos and samples with symptoms of winter injury.
However, Boxwood blight, a disease that was first found on the east coast of the US in 2011, continues to be a problem to be on the look for in the landscape. Boxwood blight is caused by the fungus Cylindrocladium pseudonaviculatum that was first described in 2002, were severe outbreaks were reported in New Zealand and Europe. The pathogen has been confirmed in at least 15 U.S. states (including Virginia, Maryland, Rhode Island, Massachusetts, and Oregon), with the most recent report in the Midwest occurred on Christmas wreaths in Indiana (read more at this Clemson article).
Boxwood blight symptoms include foliar symptoms, stem cankers, and defoliation. Symptoms start with dark or light brown leaf spots, bronze or straw-colored foliage, sudden defoliation, and black streaks on affected branches. Infections can be severe in young plants and can kill seedlings. Several Buxus species can be affected, but English and common boxwood are highly susceptible to this disease. This pathogen can complete its life cycle in about a week under warm and humid weather conditions, and high moisture is required for infection. The fungus can be spread by water, animals, or human activities such as pruning.
When to monitor for Boxwood Blight?
Symptom and pathogen development are influenced by temperatures and available humidity (rain, dew, etc.). This disease may appear at different dates depending on the location. Nursery growers, landscape professionals, and gardeners can take advantage of a website and its smartphone application version, both designed to monitor the risk of Boxwood blight at http://uspest.org/risk/boxwood_app
This tool has four tabs: intro, inputs, graph, and table. Click on the input to learn more about this disease, see photos of the symptoms, and a succinct explanation of how to use and interpret the results the graph and table tabs. On the input tab, type a zip code, the search will display a list or you can choose the graphical representation of the weather stations nearby to that zip code. Select the closest to you. Then explore the graph and the table tabs for the results and learn the risk of infection and symptom development. It's a good idea to inspect plants in the days following high risk of boxwood blight (4-7 days). Avoid disturbing/ shaking boxwood branches when dew or excess moisture is present (for example early morning) to avoid dispersal of the seeds of the pathogen (spores).
If you are not sure or if you suspect your boxwood is suffering from box blight, don’t panic yet! We at Plant and Insect Diagnostic Clinic are here to help. This disease can be confused with several common boxwood problems. Leaf bronzing and straw-colored branches can also be caused by winter injury and by another fungal pathogen called Volutella. Similar symptoms can also occur from Phytophthora, Verticillium, or nematode infections.
To confirm a diagnosis, it is always best to send samples to a diagnostic laboratory. Look up your local NPDN plant clinic and make sure you follow sampling and shipping instructions to ensure a good quality sample. In Iowa, samples can be sent to us the Plant and Insect Diagnostic Clinic, please specify on the submission form “check for boxwood blight.” We will perform a yes or no assessment of the sample submitted free of charge.
Hi, my name is Lina Rodriguez Salamanca, I’m with the Plant and Insect Diagnostic Clinic. If you’re interested to learn more about boxwood blight and would like to track down this disease and learn what to look for on your boxwood, this is what we recommend you do.
In your browser, you can go to uspest.org/risk/boxwood_app. You will land on this page. And in the introduction here on the top left, you have an explanation of the pathogen that causes boxwood blight; the mane is Calonectria pseudonaviculata. And then it explains how this model works. More importantly, you can learn in here the different symptoms this fungus causes on boxwood. You can see the leaf spots with dark streaks on the stems and the defoliation. This particular model uses temperature and humidity to predict when the fungus is going to be active, and when the symptoms are going to start developing, and it will give you a risk.
Let’s go back to inputs here. You can enter your Zip code, or if you know the abbreviation of your station, you can use that. You can search for stations, or you can use the map to select the one station that is closest to you. Once you’ve selected your station, you can either look at the graphing representation of the risk or the table and in this case, we see that this week most of the week we’re very low risk here in central Iowa of boxwood blight development. However, on August 10th the day we did have a little bit of a bump in the risk. So this is good to know so that we can go and look again for the symptoms.
If you have some symptoms in your boxwood, or you’re not sure if you see maybe boxwood blight, get in touch with your local plant clinic. Here in Iowa the Plant and Insect Diagnostic Clinic can help you out diagnosing boxwood blight or any other boxwood problems. Look at our website at clinic.ipm.iastate.edu