Stay on top of boxwood blight

Boxwood blight symptoms in a propagation flat.

Boxwood blight, a new and lethal pathogen, has growers and landscape professionals on alert. The fungus Cylindrocladium buxicola (syn. C. pseudonaviculatum) was confirmed just a few short months ago in late October 2011 by the USDA.

In early October 2011, the Connecticut Agricultural Experiment Station (CAES) received samples of boxwoods with leaf spots and blights, rapid defoliation, distinctive black stem cankers and severe dieback. It’s not clear how the fungus arrived in Connecticut, but boxwood blight is found throughout Europe. Last year’s discovery in Connecticut was the first reported outbreak in the United States. Since October 2011, the disease has been confirmed by USDA in North Carolina, Virginia, Rhode Island, Maryland, Massachusetts, Oregon, New York and Pennsylvania. As these popular landscape plants are shipped during March and April, it’s likely that boxwood blight will show up in other states.

Boxwood blight has been reported to occur on all Buxus species, but some cultivars may be more susceptible than others, said Sharon Douglas, plant pathologist at CAES. (See Table 1) English boxwood and American or common boxwood appear to be highly susceptible, but the complete host range of this pathogen is not known, she said.

This is a costly pathogen where thousands of infected plants have been destroyed across several states.
 

1. Boxwood blight symptoms in container-grown plants. Note the amount of leaf debris in the pots and on the ground. Leaf debris must be removed to keep the disease from spreading. 2. Symptoms of boxwood blight from a landscape planting. 3. Lesions often have a concentric pattern or a zonate appearance.

Symptoms
Boxwood blight infects all aboveground portions of the plant, but does not appear to infect the roots.

Watch for dark or light-brown spots or lesions on the leaves. The lesions often have dark borders. Spots enlarge and then coalesce, often with a concentric pattern. Infected leaves turn brown or straw colored, which makes infected plants look “blighted,” Douglas said. Defoliation often occurs quickly after the foliar symptoms first develop.

The fungus also infects the stems – watch for distinctive dark-brown to black lesions. They’ll sometimes have an angular, diamond-like pattern. Many lesions can be found along a stem from the soil line to the shoot tips.

Infected plants attempt to regrow, but repeated infection and defoliation can weaken the root system and lead to plant death, especially in young plants or new transplants, Douglas said.

Transmission
Boxwood blight can spread rapidly under warm and humid conditions. In a production setting, it spreads quickly when plants are grown close together, where leaf debris is not removed and where overhead irrigation is applied. (See page 43 for Best Management Practices.)

Boxwood blight produces sticky spores that are splash-dispersed and can be carried by wind or wind-driven rain across short distances, Douglas said. Longer-distance spread may be caused through the activities of humans through contaminated clothes and tools. Some European literature suggests animals and insects may also disperse the spores, Douglas said.

The pathogen does not require a wound to infect the plant. It has a rapid disease cycle that can be completed in one week. The optimum temperature for growth is 77°F, but its temperature range is 41°F-86°F. The fungus is killed after seven days at 91°F.

Boxwood blight can persist in leaf debris for at least five years. Sanitation is imperative to keep this pathogen at bay.

The CAES is testing registered and experimental fungicides to determine if more efficacious products are available, Douglas said.

For more: Sharon Douglas, CAES, (203) 974-8496; www.ct.gov/caes.

Best management practices set for boxwood blight

The Connecticut Agricultural Experiment Station developed best management practices (BMPs) for nurseries and garden centers to help stop the spread of boxwood blight. These BMPs can be implemented by any green industry professional throughout North America. This is a summary of the BMPs. See the entire document at www.ct.gov/caes. As more information about this new pathogen becomes available, the station may alter the BMPs. Check the website for updates.

Exclusion

• Newly purchased plants or rooted cuttings should be isolated from existing boxwood in nurseries, garden centers or dealers for at least four weeks, but preferably longer.
• Keep plants labeled or barcoded to be able to track the vendor source.
• Carefully inspect plants or cuttings for symptoms at the time of purchase or when received from supplier. Personnel should be trained to detect boxwood blight. All symptomatic plants should be tested.
• Request a history of fungicide treatments with each shipment (fungicide name, application rate and time).
• Physically separate material by source – avoid comingling of plant material from different sources.
• In the holding area, all plant debris should be removed on a regular basis by vacuuming, sweeping or raking.
• Suspend the use of fungicides on new plants during the holding period.
• No returns of boxwood plants should be accepted onto the property to avoid possible entry of plants that may have been exposed to boxwood blight.

Water management

• Avoid overhead watering or working with plants when they are wet.
• Irrigation water from sources other than wells or municipal water sources should be tested annually for water quality and water-borne plant pathogens by a private testing lab.
• Increase spacing between plants.
• Avoid or minimize accumulation of standing water in boxwood blocks.

Sanitation

• Remove leaf debris by vacuuming or raking. Debris should be bagged for incineration or burial.
• Infected plant material should not be composted.
• A suggested sanitation protocol:
  • Pruning crews should focus on a single house to complete the pruning as rapidly as possible.
  • Pruning should not occur if plants are wet or if there is high humidity.
  • The day before pruning, spray plants thoroughly with ZeroTol.
  • Immediately after the last plant is pruned in a house, the crop should be sprayed again with ZeroTol. Routine fungicide programs can resume after pruning, as applicable.
  • Tools and equipment should be sanitized when moving between different Buxus blocks within a house.

• After every crop production cycle, remove all crop debris and disinfest propagation mist beds, sorting areas, cutting benches, machines and tools.
• Work in blocks with infected or exposed and potentially infected plants last – after completing work with healthy plants.
• Use new or clean and properly disinfested pots or flats for boxwood production. Use new, not reused, potting mixes. Sanitize all shipping containers, benches and equipment.

These symptomatic boxwoods are in various stages of decline.

Inspection

• Inspect all boxwood throughout the growing season. If boxwood blight symptoms are detected, immediately pull and remove whole plants and place them in plastic bags to avoid carrying infected material through the house or nursery.
• If you suspect boxwood blight, have the disease accurately identified by a specialist.

Record keeping/traceability

• Keep accurate, detailed records of:
  • Incoming and outgoing plants
  • Shipping records
  • Propagation of plant material
  • Mortality due to any cause
  • All chemical/fertilizer applications
  • Weather records

Training

• Educate and train personnel to recognize boxwood blight. Early detection is critical.
• Train personnel in BMPs, including sanitation.