| JANUARY 25, 2012 -- A new threat faces landscapers and property owners in Connecticut with the eruption of a newly identified boxwood blight (Cylindrocladium pseudonaviculatum), which threatens to attack and kill thousands of boxwood plants in the region in coming months.
‘Exploded ... in October’
“This whole thing exploded onto the scene in Connecticut in October, so it is very new,” said Dr. Sharon Douglas, head of the Department of Plant Pathology and Ecology at The Connecticut Agricultural Experiment Station in New Haven. “That was about the same time it was detected in North Carolina and right over the border in Virginia. Since early December it has been found in Massachusetts, Maryland, Rhode Island, Oregon and New York State, but Connecticut appears to be the hardest hit. We have now identified it in five counties.”
As of yet, Litchfield County has not reported an incident, but Dr. Douglas said that may be because no one has yet sent in a sample. “It is just a question of time,” she said. “We rely on people self-reporting. It could be no one is thinking of boxwoods in Litchfield County at this time of year.”
The pathogen had been previously identified in the United Kingdom in the 1990s and has been in New Zealand since 2002. It was not formally identified in the UK, but when it appeared in New Zealand it was named as a new species, Cylindrocladium pseudonaviculatum. Later that same year, boxwood fungus from the UK was named Cylindrocladium buxicola. Both are now known to be the same fungus. It has now spread throughout Europe. While not regulated, it is considered a disease of great concern in the UK.
The geographic origin of the blight is not known. “How it arrived in the United States is unclear,” Dr. Douglas said. “This is a very serious disease and there is nothing that is curative—no fungicides have been effective.”
She said preventative measures applied before a plant is infected may stave off the infection, however. Connecticut homeowners can use the fungicides chlorothalonil and mancozeb before the symptoms appear and should repeat the process when conditions for infection are favorable. All visible surfaces of the plant should be treated, although this can be difficult because of the plant’s dense canopy. The fungicides should be applied until there is run-off.
Commercial nursery growers are encouraged to follow a preventative fungicide program that includes different products with different modes of action. Information about this is available on the Connecticut Agriculture Experiment Station Web site, www.ct.gov/caes.
“There are limitations on what homeowners can use,” said Dr. Douglas. “No one is going to find anything with boxwood blight listed on the label. We are encouraging people to send in samples if they are concerned. If you have any concern at all, get it diagnosed.”
That said, many people are already sending in samples. “We spent the fall handling hundreds of samples from growers, landscapers and nurseries,” she said. “Homeowners are just starting to send them in.”
The fungal infection, which causes rapid die-back of foliage, appears similar to other, more established—and less intractable—boxwood diseases such as Volutella blight, macrophoma lead spot, boxwood decline, winter injury and sunscald.
Infected plants initially have dark or light brown lesions on the leaves. These lesions often have dark borders. The spots enlarge and then coalesce and infected leaves turn brown or straw colored and defoliation occurs very quickly. The fungus also affects stems, resulting in dark brown or black lesions with an angular diamond-like pattern. Many lesions can be found along the stem, from the soil line to the shoot tips.
“Plants are defoliated very rapidly and every time the boxwood tries to push out new foliage, that foliage is infected. It eventually debilitates the plant to the point where it will die,” Dr. Douglas said.
The most susceptible species appear to be English (Buxus sempervirens “Suffruticosa”) and American or common boxwood (B. sempervirens), although many species of boxwood are susceptible to infection.
“We know how it is spread,” Dr. Douglas said, explaining that sporodochia can be found on the undersides of infected leaves and on the black lesions on stems. These sporodochia contain large numbers of sticky, cylindrical spores. These spores are dispersed by splashes of rain water and can be carried by the wind over short distances. They can also be transported on the clothing and shoes of humans or by birds and animals because the spores are sticky.
“It is very local,” Dr. Douglas said. “It is a sticky spore and not easily blown by the wind. Long distance spread occurs by moving an infected plant from point A to point B.”
Unfortunately, an asymptomatic but infected plant of resistant varieties—or those that have been treated with fungicides that suppress but do not kill the fungi—can introduce this pathogen to uninfected areas.
The disease has a rapid cycle that can be completed in one week in temperatures that range from 41 degrees to 86 degrees Fahrenheit. The optimum temperature for growth is 77 degrees and the fungi will die after seven days at 91 degrees. The disease does not require a wound to infect a new plant because it can penetrate through the plant cuticle and can enter the leaf through its stomata, but high humidity levels or water on plant tissues are necessary for infection.
The new blight is particularly worrisome to the landscaping community because boxwood is a plant traditionally used in formal gardens on estates and in landscaping. It is popular because it is deer resistant in a region where the large population of browsing ruminants already makes landscaping a challenge.
“We have a lot of very concerned people, especially in Fairfield and Litchfield counties,” said Dr. Douglas. “There are thousands of boxwoods on properties throughout Connecticut. It is an essential plant for local landscapes. We want to get the word out so people are aware.” |
