An Emerging Plant Health Problem in West Virginia Landscapes
Boxwoods (Buxus spp.) are one of the most popular shrubs used in West Virginia landscapes. It is not unlikely to find diverse cultivars of boxwoods in landscapes across the state. With the increased population and high-density landscape settings, certain diseases like boxwood blight have become very destructive.
Boxwood blights are a fungal disease that can be fatal if no measures are taken to manage the disease at the early stage of infection and symptom appearance. There are two different fungal pathogens involved with blights – Volutella buxi and Calonectria pseudonaviculata .
A Volutella buxi infection turns leaves light green-yellow, which transition to bronze and finally become yellow-tan in color (Table 1). This is known as Volutella blight. This fungal pathogen also can infect stems, resulting in loose or peeling, yellowish bark (Table 1).
Another fungal pathogen known as Calonectria pseudonaviculata (previously called Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola) can cause boxwood blight that may be termed as Calonectria blight. Unlike Volutella blight, infection from Calonectria manifests as dark leaf spots that coalesce to form brown blotches (Table 1, right column). While Volutella-affected leaves can be attached to the stem for a longer time, Calonectria-affected leaves drop off very quickly, known as defoliation, giving the plant a barren appearance.
A key symptom that differentiates Calonectria blight from Volutella blight of boxwood is that narrow vertical black streaks (cankers) develop on green stems due to Calonectria infection, while Volutella-infected stems show brown to black girdling streaks, followed by loosening or peeling of the bark (Yang et al., 2021).
Diagnosis of the Disease
In moist conditions, orange or salmon colored fungal fruiting structures (sporodochia) are noticeable on stems or the underside of leaves (Table 1, left column) in case of Volutella blight. One can take a few suspected leaves and put them inside a zip-close plastic bag with a moist paper towel for three to four days to observe such growth. The undersides of infected leaves in case of Calonectria blight will mostly show white sporulation.
Survival and Spread of the Pathogen
Both fungal pathogens can survive on infected plants or fallen leaves on the ground from one year to another, known as overwintering. The spores produced on infected leaves and stems during the growing season can be splash-dispersed through irrigation, rainfall, wind or pruning tools. This can spread the disease within a plant or to nearby boxwood shrubs. Environmental stress, such as drought, excessive water or winter injury, can make boxwood plants vulnerable to infections.
Recent outbreaks of Volutella blight in West Virginia and neighboring states may have a connection with the cold stress from the arctic blast that occurred in December 2022. While Volutella blight was present in the state of West Virginia and is considered an opportunistic pathogen that infects only stressed plants to make it endemic, Calonectria blight is more aggressive, but occurrences in the state remain very low. However, there is a potential for Calonectria-infected plants to arrive to the area on new plants through nursery trade. Homeowners and landscapers should make sure that infected plants are not purchased and planted.
Prune out infected branches and destroy them by burning or hauling away in a trash bag. Pruning dense shrubs also can help in air movement, sunlight penetration and keeping canopy humidity low. Sanitize pruning tools by dipping in 10% bleach or 70% ethyl alcohol between each cut. Do not prune when foliage is wet. If possible, remove infected fallen leaves and litters.
Alleviate plant stress by providing irrigation and fertilization as needed. However, it is better to irrigate through drip irrigation and avoid overhead sprinkler to keep foliage dry. Do not use horticultural fabric as a mulch to suppress weed growth. It is better to use bark or compost mulch that may help retain moisture and promote root growth.
Alternative Hosts and Relative Susceptibility
Pathogens from these genera can also affect Pachysandra and Sarcococca. American, English and Korean boxwoods are all susceptible to blight, although disease severity varies among cultivars. For example, boxwood cultivars Green Velvet, Green Mound, Green Mountain and Pincushion have some resistance against Volutella blight compared to highly susceptible cultivar Green Gem. Similarly, cultivars Golden Dream, Green Beauty and National are resistant, but Suffruiticosa, Arborescens and Justin Brouwers cultivars are highly susceptible to Calonectria blight. It is interesting to note that cultivar Green Gem is tolerant to Calonectria blight but the most susceptible to Volutella blight.
Chemical control of these two diseases is also a little different. That’s why it is very critical to identify the disease accurately. If you suspect boxwood blight, send samples of the suspected plant to the WVU Plant Diagnostic Clinic for diagnosis.
A combination of chlorothalonil (trade names include Bravo, Daconil or Fung-onil) and thiophanate methyl (Cleary 3336) can provide good control of Volutella blight if used preventatively or at the very early stage of symptom expression. Removal of highly infected stems by pruning before fungicide application can improve the efficacy of the products. Other products that are registered for the control of Volutella blight include propiconazole, copper and mancozeb.
For preventing Calonectria blight, fungicides Heritage, Medallion, Mural or Palladium can be used in alternation with Daconil.
Table 1. Differentiating boxwood blight caused by Volutella buxi (Volutella blight) and Calonectria pseudonaviculata (Calonectria blight).
|Leaf symptom (lesion)
|Whole plant symptom
|Fungal growth after incubation
|Fungal spores (conidia)
Author: Mahfuz Rahman, WVU Extension Plant Pathology Specialist
Last Reviewed: August 2023