Report from Europe - Dec. 11-18, 2011

"Summary of Findings from Western Europe Dec. 11-18, 2011"

Boxwood Blight (Cylindrocladium buxicola) Report from Europe

• Background

o Boxwood blight has been in Europe for almost 20 years. It was first found in 1994 and positively identified as a new species in 1998.
o Europe has found ways to live with and manage the disease. C. buxicola indeed can be a very serious disease for boxwood, but commercial growers in Europe continue to grow boxwood successfully. Nurseries manage boxwood blight through integrated methods including irrigation management, variety selection, and targeted chemical applications.

• Environmental Conditions Conducive to Disease Spread

o From laboratory research and field observations, we know that environmental conditions are the primary factor in symptom expression. This disease requires very wet conditions (not just relative humidity,but leaf wetness) and warm temperatures to show symptoms.
o We do not yet know how differences between the climate in the United States and Europe will impact the disease. We  experience colder winters and warmer summers than the United Kingdom and Belgium, where most ofthe recent research has been done. In Southern France, where the climate is warmer and drier, particularly in summer, they don't have the same problem with boxwood blight.
o Irrigation: Everyone we spoke with agreed that overhead sprinklers are problematic because they can spread spores from one plant to another and create more humid conditions. Drip irrigation is ideal.
o Temperature: Ideal for disease infection and progression is around 75°F.The disease can be active from 32-90°F,but is unlikely to infect at the extremes of that range. At prolonged cold or hot temperatures, the disease will not be active, but some structures could remain that could reinfect the plant when conditions become favorable again.
o Air circulation: Tightly pruning plants creates a microclimate within the plant canopy that is much more humid than the relative humidity of the air and creates a high risk situation for disease spread.
o Homeowners are at much less of a risk than commercial nurseries because they typically have little or no overhead irrigation, more isolated plants, and fewer sources of inoculum.

 Cultural Practices to Reduce Risk of Disease Spread

o Prune more loosely to improve air circulation. 
o Use a variety of plants to avoid monocultures.
o Increase spacing, both in nursery production and in landscape plantings. Field observations in Europe show that pathogen spread is much slower if plants are not touching.

• Chemical Controls to Prevent and Treat Disease Outbreaks

o In Europe, they have had success using chemicals both preventatively and to arrest the spread of the disease if caught early. No chemical will completely kill the pathogen, but if treatment is made immediately when symptoms are found, the plant can recover and flush new growth. Research in Europe has shown that the active ingredients chlorothalonil, azoxystrobin, and kresoxim-methyl are effective against the disease.
o The use of these chemicals may be restricted differently by state. All label instructions and state regulations must be observed when applying pesticides. Contact your local extension agency for more information.

• Cultivar Selection

o From laboratory tests and field observations, we know that Suffruticosa is the most susceptible variety. Symptoms are most likely to show up on these plants first.
o No variety has proven immune under high risk conditions, but the disease is unlikely to express at all in very low risk conditions. Consider the natural growth habit of different varieties as well as pruning practices to minimize high risk conditions.
o Control of environmental conditions and early detection are the keys to managing this disease.

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