Preventing boxwood blight in nurseries
Boxwood blight was detected on plants sold at retailers during the spring sales season in Michigan. Nursery growers should be familiar with the symptoms and vigilant when inspecting their own stock and buying in product from other nurseries.
Boxwood blight was first detected in Michigan on planted and containerized boxwood in the fall of 2018 and in December 2018 it was also detected in boxwood wreaths. These detections were reported by MDARD (Michigan Department of Agriculture and Rural Development). Michigan State University Extension published an article on properly disposing infected wreaths and updated a downloadable fact sheet about the disease.
During the 2019 spring sales season, MDARD detected more infected boxwood at retail stores. Unfortunately, many boxwood plants had already been sold and this increases the likelihood that boxwood blight has been more widely spread in Michigan.
To prevent future outbreaks of boxwood blight, MDARD created the Boxwood Blight Cleanliness Program. It is a voluntary program that is highly effective in reducing the losses from the disease and the participants receive a special certification. Nursery growers desiring to grow any of the host plants and ship them to either Pennsylvania or Tennessee must comply with the Boxwood Blight Cleanliness Program. Those interested in the program should ask their MDARD nursery inspector.
The host plants of boxwood blight are: all boxwood (Buxus spp.), sweetbox (Sarcococca spp.) or pachysandra (Pachysandra terminalis). All nursery growers should follow these seven general recommendations to reduce the risk of boxwood blight in their crop:
- Know the symptoms of boxwood blight. Refer to the boxwood blight fact sheet published by MSU Extension.
- Isolate incoming host plants on rooted liners in the nursery for at least 30 days. Keep the host plants on a cleanable surface (landscape fabric or concrete), prevent water runoff in the area, do not spray fungicides effective for boxwood blight during the isolation time, and destroy all plant and soil debris.
- Avoid overhead irrigation of host plants prior to nightfall and minimize standing water in the production area. In addition, if at all possible, use drip irrigation on host plants to minimize risk.
- Sanitize host plant areas. Sanitize growing areas, containers and tools after every crop cycle and if infected plants are found. Dip tools in sanitizers for a minimum of 5 minutes—hydrogen dioxide, peroxyacetic acid or household bleach (1 part bleach: 14 part water, or Lysol concentrate disinfectant).
- Keep records of quantity, source, ship dates, fungicide applications and inspection records for a minimum of three years.
- If possible, grow your boxwood, sweetbox and pachysandra plants at your own operation and do not buy in more plants than necessary. Source your plants from reputable nursery suppliers and be aware of external quarantines in other states.
- Grow less susceptible cultivars. Consider avoiding the most susceptible cultivar such as Buxus sempervirens ‘Aurea Pendula,’ ‘Arborescens’ and ‘Latifolia Maculata.’ Less susceptible cultivars include: ‘Nana,’ ‘North Star,’ ‘Wedding Ring,’ ‘Northern Emerald’ and ‘Green Beauty.’ For a full list of cultivars, visit “Susceptibility of Commercial Boxwood Cultivars to Boxwood Blight.”
Keep in mind that there are numerous other problems that can cause similar symptoms in host plants. Similar to the winter of 2014-2015, the polar vortex that occurred this past winter caused many boxwood plants to exhibit winter injury. Other boxwood problems that also cause the browning of leaves and defoliation include salt damage, winter injury, boxwood leaf miner, drought stress and the fungal pathogen, Volutella.
For more information on look-alike boxwood problems, read “Browning of boxwood: Is it boxwood blight?”
If you suspect you have infected boxwood, it is extremely important to send in a sample for the problem to be diagnosed. Please send a sample to MSU Plant & Pest Diagnostics at firstname.lastname@example.org or 517-432-0988. Please double bag the suspected plants within the box shipped to Plant & Pest Diagnostics. They will evaluate the plant material and work to detect spores and distinctive structures produced by the boxwood blight pathogen. MDARD may be consulted in cases where boxwood blight is confirmed on material in commercial production.
For more information for landscapers and homeowners, check out, “Preventing the spread of boxwood blight in landscapes.”