Tidewater Gardening - February 2012

Boxwood Blight

by

K. Marc Teffeau

One of the mainstay landscape shrubs on the Eastern Shore is the boxwood. Whether planted in a new home landscape, established for many years in an historical waterfront estate or in places like the Governor’s Palace in Colonial Williamsburg, boxwood species and varieties are usually one of those shrubs that you plant and then treat with neglect. The most common species of boxwood on the Eastern Shore are Buxus sempervirens ‘Suffruticosa’ (English boxwood) and Buxus sempervirens (common or American boxwood).
As I have written in past Tidewater Gardening columns, boxwood are not pest-free plants. Various diseases such as Volutella buxi, that causes Volutella blight, Macrophoma leaf spot, Phytophthora nicotianae and, in some situations, nematodes can also damage the plants.
Macrophoma leaf spot is considered a secondary invader and causes tiny raised black spots on the undersides of dying leaves. Phytophthora nicotianae can be found on all cultivars of B. sempervirens and causes wilting, dark brown discolored wood at the base of the stem, root rot and discoloring/dieback of the foliage. Nematodes feed on root tissues and cause root lesions, reduced root growth, wilting, stunting and yellowing of the foliage.
All of these diseases may contribute to a disease complex called ‘boxwood decline.’ This complex is poorly understood, but can lead to poor plant growth, small leaves and defoliation or dieback. The root systems of these plants are often smaller than expected and may be discolored or appear rotten.
English boxwood is typically more affected by boxwood decline. Over my horticultural career on the Eastern Shore I have seen numerous cases of what could be considered boxwood decline. You can’t identify a specific causal agent for the yellowing and death of boxwood foliage and branches, but it seems that there is synergistic impact from various causes.
Unfortunately, another disease has to be added to the list of boxwood pests. In the mid-1990s, plant disease specialists in the United Kingdom first identified a new fungal disease called Boxwood Blight, a.k.a. Cylindrocladium box blight. An outbreak of the disease occurred n the UK in 1998 and the pathogen is now considered widespread throughout most of Europe. By 2002, boxwood blight was present in New Zealand as well. The origin of this disease is unknown, but it is believed to have been introduced into the UK prior to being introduced into New Zealand.
How the fungus arrived in the United States is unclear, but within the last year, it has turned up in Virginia, North Carolina, Connecticut, Oregon and Maryland landscapes, garden centers and nurseries. The presence of the disease was officially confirmed by the USDA Animal and Plant Health Inspection Service (APHIS) last October. The disease organism which causes boxwood blight is known by its long scientific name of Cylindrocladium pseudonaviculatum or Cylindrocladium buxicola.
The most susceptible boxwood species appear to be English (Buxus Sempervirens ‘Suffruticosa’) and American or common boxwood (B. sempervirens), although Buxus microphylla (littleleaf boxwood), Buxus sinica var. insularis (Korean boxwood), and Buxus microphylla var. japonicas (Japanese boxwood)are also known hosts.
A major problem with the spread of this disease is that asymptomatic but infected plants of resistant varieties may introduce this pathogen to uninfected areas. This fungus colonizes all above-ground portions of the plant. Initial symptoms appear as dark or light brown circular leaf spots. Infected leaves then turn brown-tan, which is rapidly followed by defoliation, sometimes within a week of exposure to the disease. In addition, black lesions often develop on twigs and stems.
Plants are not killed by this disease, but become so defoliated as to be aesthetically unacceptable. Blighting and defoliation can occur suddenly, with complete leaf loss in severe cases.
Infected plants, introduced into older, well-established plantings, will rapidly spread the disease to healthy plants. In the nursery industry the disease can seriously impact the appearance and aesthetics of boxwood because the entire foliage typically becomes blighted, making the plant unsalable. Young seedlings can be killed by the pathogen.
This disease is spread primarily by water movement such as rain splash, overhead irrigation, runoff, in droplets carried by the wind, by animals in the landscape, on contaminated tools and equipment, or through movement of contaminated nursery stock. Disease spores are unlikely to travel long distances by wind. Human activities such as pruning may also spread the fungus. Some varieties of boxwood are more susceptible than others, possibly due to physical features of the plant such as cupped foliage that retains water easily.
Occurrence of this disease seems to be environmentally driven. Two of the areas where boxwood blight showed up in 2011, North Carolina and Connecticut, experienced extended rainy periods during last summer. The disease spreads rapidly throughout infected plants when conditions are warm and humid, and also in shady areas. This pathogen has a disease cycle that can be completed in one week. Infection can occur very quickly in warm ( 64 to 77◦), humid conditions. High humidity levels or free water are needed in order for infection to occur.
The boxwood blight organism can penetrate the leaf through the cuticle or enter through leaf stomata. C. Buxicola survives as mycelium on fallen leaves and can produce spores when environmental conditions become suitable. Reports in the UK have determined that the pathogen can survive at least 5 years by remaining on decomposing fallen leaves of Buxus sempervirens.
Often the stems of blighted boxwood will remain green under the outer bark until a secondary invader or opportunistic pathogen attacks this tissue, eventually killing the entire plant. C. buxicola is often associated with the secondary pathogen Volutella buxi, known to cause Volutella blight which is associated with plant wounds, but C. buxicola does not need a wound to infect, but can penetrate directly through the cuticle. Both fungi can also occur independently.
The appropriate ways to control this disease in the U.S. have not been studied in detail. Current recommendations are to limit spread and movement of the disease by destroying all infected plants. Infected plants should be burned to ash or sealed in heavy black plastic trash bags and taken to an approved landfill. Do not put infected plants n the compost pile.
Pruning of infected twigs and/or removal and destruction of fallen leaves and topsoil may help reduce the number of boxwood blight spores. Cleaning of the pruning shears with alcohol is recommended to prevent spread of the disease. Controlling water and humidity may also be helpful in controlling this fungus. Avoid overhead irrigation when possible by using drip irrigation instead. Thinning to open the canopy of large boxwood will allow air movement and keep humidity down, eliminating some conditions favorable to the development of the disease. Creation of topiaries should be avoided as it keeps humidity high inside the canopy.
In the UK, researchers have recommended that alternative ornamental plants, not in the boxwood (Buxus) family, may be used in place of susceptible host plants in the landscape to prevent re-infection by the disease. Japanese holly (Ilex crenata) may serve as an alternative plant, especially in landscapes that receive more sun exposure.
In general, Cylindrocladium diseases light boxwood blight are very difficult and costly to control with fungicides. Infected plants should be removed and destroyed immediately. Healthy plants my be protected by applying fungicides on both sides of the leaves to prevent both germination and penetration of the fungus. Due to the dense boxwood foliage, and its water-repelling nature, it may be difficult to get good coverage with a fungicide spray within the plant canopy.
If you think you might have boxwood blight in the landscape, contact your county extension office to submit plant samples to the diagnostic lab for further examination.
Happy Gardening!