Boxwood blight was totally unheard of until 2000 but it has spread through Europe and New Zealand with lighting speed. One thing that really bothers me is the rate at which it spread when it arrived in America. Consider this: in October 2011, authorities confirmed the disease in Connecticut, and North Carolina. By January 2012 it had arrived in Virginia, Maryland, Rhode Island, Massachusetts, Oregon and British Columbia. By March 2012, it was confrmed in Cleveland, Ohio. The discovery in Cleveland prompted the Ohio Agriculture Department to order a nursery to burn the affected plants and 10,000 more around them to prevent spread of the disease.
"I've never poured diesel fuel on boxwood, but if I did, that's what it would look like," says Lynn R. Batdorf, curator of the as-yet-uninfected boxwood collection at the National Arboretum in Washington, D.C.
Initially, Symptoms of boxwood blight include dark or light brown spots on leaves, often in a circular pattern, black streaks (cankers) on stems.
Straw to bronze‐colored blighted foliage, and defoliation.
Blighting and defoliation can occur suddenly, with complete leaf loss in severe cases.
Disease spreads rapidly throughout infected plants when conditions are warm and humid, and in shady areas.
Leaf spots can grow together to cover the entire leaf surface.
Lesions on stems
Infected stems can have multiple dark brown or black lesions either linear or diamond‐shaped. The black streaks found on stems progress from the bottom of the plant up. New growth continues to develop on healthy stems, and often the root systems remain healthy and intact .Spores of the pathogen can sometimes be seen on the underside of the infected leaves.
The disease quickly progresses to conspicuous leaf drop, and the bare twigs typically show black cankers.
The spores are spread by splashing during heavy rain or by wind or wind driven rain.
Spores on stem
All ornamental species of boxwoods are susceptible to boxwood blight.
It is difficult to predict the exact impact of this new boxwood disease, but experience in Europe and New Zealand makes it clear that both gardeners and the horticulture industry need to take this disease very seriously in order to prevent it from becoming a common, established problem everywhere boxwood is grown. Boxwoods, highly valued for their deer resistance and evergreen nature, could easily change from a ubiquitous landscape ornamental to a rare, unsustainable plant.
Accurate identification followed by prompt action can help slow the spread of this serious disease. Any horticultural business or homeowner with boxwoods currently on their property should search for symptoms.
I interviewed Jan Byrne, of the Michigan State Universities Department of Plant Pathology. At this point in time there are no fungicides available that will control this disease. Research done in the lab has shown promise. However no firm conclusions can be made until field trials have been conducted. These are being conducted now and hopefully we'll have an answer soon.
In the meantime, there are cultural steps that you can take to prevent the disease from affecting your plants.
- Adequate spacing between plants to allow for good air circulation.
- If possible avoid overhead watering; this helps to prevent spores from splashing up onto the plants.
- Rake up any leaf debris around your boxwoods, this type of fungus can survive in leaf debris for up to 5 years.
- Scout for the disease regularly. If you're not looking daily basis, don't let more than a week pass without inspection. If symptoms are found pull the entire plant and place into a plastic bag(s) and seal. This is the best method to keep the disease from spreading to your other plants. If you are unsure what to look for place a sample into a plastic bag and get it to your county extension office for confirmation.
I will be in contact with the researchers at Michigan State and as soon as a fungicide is available I will pass the information on.
This scenario reminds me of the Emerald Ash Borer invasion; however on the positive side researchers are working to find a solution ASAP.
Photos courtesy University of Virginia extension.