Boxwood Blight Update
By José Fernández
If you have boxwood plants, please read the following!
Last week I read an article about a new research project that tested the hypothesis that use of mulch may reduce the incidence of boxwood blight at previously infected sites.
If you read my first installment on this relatively new disease, you will know the major difficulty of this pathogen (Calonectria pseudonaviculata) is that it infects otherwise healthy plants, and once it is at a site, removal and destruction of the infected plants is recommended, with no plant replacement for 3-5 years. This is because the spores remain in and on the soil to infect new plants. Existing asymptomatic plants were to be sprayed with fungicides to help prevent infection.
In this new study, two sites (field planting site and existing residential landscape site) were used to test the mulching hypothesis. The results at the residential site were particularly encouraging. At this site, infected plants were cut down, leaving stumps at around 15 cm in height (5.9 inches), similar to a rejuvenation or coppicing pruning. The pruned material was destroyed, but the stumps were freshly mulched to a depth of 8-10 cm (3-4 inches) in an area 2 x 2 m around the stump. Result? Complete protection (no infection) was observed the first season, and “excellent” protection the following season (low infection rate).
The authors of the study stated that the pathogen “primarily attacks leaves and stems”, and “there is no evidence to date of natural infection of boxwood roots” by this pathogen in the soil. Based on this information, recommendations for managing this disease will be summarized by the following steps, representing an integrated approach:
Above all else, do not allow new boxwood plants onto your property that are not guaranteed to be free of this pathogen by the supplier.
Any person/company working on your boxwoods must be able to guarantee that all equipment being used on the property has been cleaned and disinfected prior to entering your property to do work. Under no circumstances should debris from other properties be allowed to enter your property.
If a plant is deemed to have boxwood blight, remove the plant (bagged and dumped in trash or burned), clean away surface mulch and leaf litter, and add new mulch around the stump, periodically checking to maintain at about 3” in depth.
Remaining healthy plants should be put onto a fungicide spray program to provide topical protection for the next several seasons. Hopefully the coppiced plants will sprout vigorously and remain uninfected.
Surface irrigation (sprinklers) should be avoided near boxwood, since water splashing is one of the primary ways the spores are spread. Also, there is a positive correlation between inoculation rates and length of time foliage remains wet.
Exclusion of the pathogen will be the most important step in this program, and the more people become aware of this, the less the disease will be able to spread. Exclusion is maintained by the first two steps shown above. The mulching depth is rather more than normally recommended for plant health, but in this case the idea is to keep fungal spores from landing on stems and leaves.
Please pass this information along to anyone you know that has boxwood plants. The more people we have following exclusion principles, the less this disease will be able to spread.
Reference: Likins, T. M., Kong, P., Avenot, H. F., Marine, S. C., Baudoin, A., and Hong, C. X. 2018. Preventing soil inoculum of Calonectria pseudonaviculata from splashing onto healthy boxwood foliage by mulching. Plant Dis. 103: 357-363.
Your friendly neighborhood arborist,
José Fernández | Regional Manager, Russell Tree Experts
José became an ISA Certified Arborist® in 2004, and a Board-Certified Master Arborist® in 2015. Currently he is enrolled at The Ohio State University pursuing a Master’s Degree in Plant Health Management. José likes working around trees because he is still filled with wonder every time he walks in the woods. José has worked at Russell Tree Experts since 2012.