Boxwood Blight Symptoms and Causes

Boxwood Blight (also known as Cylindrocladium buxicola) is an aggressive fungal disease caused by the fungi Calonectria pseudonaviculata.  This disease strikes healthy Boxwood, quickly causing mass defoliation. You will typically see Boxwood Blight flair up during rainy or humid seasons, where the leaves stay wet for extended periods of time. If the fungi are present during ideal conditions, they will begin producing white spore masses on the underside of the leaves. Under magnification they look like white threads. Ideal temperature for growth is said to be 15-25 degrees Celsius (Daughtrey M., & Gilrein D., & Vescera M. (n.d.)).

Once the fungi begin to grow, they enter the stomata (pores found in the epidermis of leaves and young stems that are involved in the exchange of carbon dioxide and water between plants and the atmosphere). The disease then spreads systemically very quickly. Black, cloudy spots that lack a distinctive edge will begin to appear on the leaves. These spots often enlarge to encompass the whole leaf, and then the leaf usually drops from the plant (Daughtrey M., & Gilrein D., & Vescera M. (n.d.)). Dark coloured stem lesions that appear as black streaks may also be found on young green stems.

American and English boxwood are most susceptible. Asian species (like the Korean cultivars ‘wintergem’ and ‘wintergreen”)  are typically more resistant, but can still become infected. English/Korean crosses like ‘Green Mountain’, ‘Green Velvet’, ‘Green Mound’, ‘Green Gem’ offer more resistance than English or American, but not as much protection as Korean.

Treatment for Boxwood Blight

There is no cure for boxwood blight. Ideally the diseased boxwood should be removed or drastically cut back. Thoroughly cleanup all boxwood leaf litter, as the fungi can live in the dead leaves for a couple of years. Removal of Pachysandra and Sarcococca from the immediate area is also advised as they can also harbour the fungi. If you have other boxwood on the property, it is advisable to treat them regularly with a fungicide to help prevent infection. It is further advised to change up this fungicide regularly to prevent the fungi from growing resistant to it.

Other helpful strategies include the use of drip irrigation rather than overhead watering and locating your boxwood in full sun rather than shade. The application of a mulch helps to prevent spores in the soil from splashing up on the plant. Ensure your boxwood have adequate air circulation around them so that the leaves dry quickly. When introducing new boxwood to the landscape carefully examine them for any signs of the disease to avoid introducing the disease to existing boxwood. What to look for are whitish masses of spores on the underside of blighted leaves, often found near the base in container and field grown boxwood (Daughtrey M., & Gilrein D., & Vescera M. (n.d.)).

Sanitation plays a critical part in both the prevention and spread of boxwood blight. Try to avoid touching boxwood plants except to perform necessary maintenance. After handling boxwood disinfect tools and gloves, (along with anything else that may have come in contact with the boxwood, such as hoses and wheel barrel) in either a solution of 1 part bleach to 9 parts water or 8oz. liquid Lysol multipurpose cleaner per gallon of water. Alternately you can use Lysol aerosol spray or rubbing alcohol to disinfect tools, spray on and allow it to air dry. It is further advised to wash your clothing and to clean and disinfect the soles of your shoes.

Volutella Blight Symptoms and Causes

Volutella Blight is a less aggressive fungal disease caused by the fungi Pseudonectria buxi formerly called Volutella buxi. In contrast to Boxwood blight an injury is required in order for the fungi to gain entry into the plant. Environmental stressors like drought, winter injury, leaf miner, vole or rabbit damage, are often precursors for Volutella blight. Also in contrast to Boxwood Blight typically Volutella Blight affects select branches, the dieback is much slower, and the dead leaves tend to stay on the plant for months, rather than dropping.

To identify the fungi, examine the underside of the leaves with a magnifying glass. The fungi appear as peach coloured cushions. The ideal temperature for its growth is 20 to 25 degrees Celsius (Shi F.A., & Hsiang T. July 31, 2012). Moist humid conditions favour its growth. 

A little trick I use to test if the fungal spores are present is to take some samples of the boxwood and place them in a zip lock bag with a moistened paper towel. Keep the bag at room temperature for several days, then examine it with a magnifying glass. If the fungi is present, you will be able to see peachy coloured fungal growth on the backside of the leaves.  In reality the fungi only require 18 hours to begin sporulating but when these spore structures are young, they closely resemble those of Calonectria blight (white hair like structures).

Vulnerable  varieties; the University of Guelph found that ‘Green Gem’ is the most susceptible while ‘Pincushion’ is the least. They further found that ‘Green Velvet, ‘Green Mound’ and ‘Green Mountain’ had intermediate susceptibility (Shi F.A., & Hsiang T. July 31, 2012).

Treatment for Volutella Blight

As with Boxwood Blight there is no cure for Volutella blight that has gone systemic. However, if you catch the disease early you may be able to manage the symptoms and spread. Prune out affected branches, disinfecting your tool between cuts with bleach, rubbing alcohol or Lysol aerosol spray. Remove badly infected plants or severely cut them back. As a preventative treatment spray regularly with a copper-based fungicide, particularly spring and fall rainy seasons and after pruning, when wounds are present. Avoid pruning when the plants are drought stressed. Avoid overhead watering and ensure there is good air circulation around your boxwood. Protect your boxwood against winter injury or stressors. 

Summary 

With Boxwood Blight no wound or stressor is required and the entire plant is usually affected. The causal agent is the fungi Calonectria pseudonaviculata, which appears under magnification as white thread like structures. The fungi spreads quickly and black spots with a baize center appear on leaves and black streaking may appear on young stems followed by mass defoliation. With Volutella Blight a wound is necessary for infection to take place and thus only a few branches are usually affected. The causal agent is the fungi Pseudonectria buxi that appears under magnification as white thread like structures that mature to peach coloured cushions on the under surface of leaves and stems. The affected leaves die and turn brown but usually remain on the plant for months.

Photo credits: all photos have been taken by the author.

Updated on April 8, 2021

References

1. Daughtrey M. (Senior Extension Associate, Cornell University), & Gilrein D. (Extension Entomologist, Cornell Cooperative Extension of Suffolk County), & Vescera M. (Nursery & Landscape Specialist, Cornell Cooperative Extension of Suffolk County) (n.d.). Photographic guide of Boxwood Pests & Diseases on Long Island. Retrieved Dec.28th, 2020 from http://ccesuffolk.org/resources/photographic-guide-of-boxwood-pests-diseases-on-long-island

2. Shi F.A., & Hsiang T., July 31, 2012; School of Environmental Sciences, University of Guelph. Volutella Blight in Boxwood. Retrieved December29th,2020 from https://landscapeontario.com/volutella-blight-of-boxwood

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