Management of Boxwood Blight in the Landscape

MANAGEMENT OF BOXWOOD BLIGHT (caused by the fungal pathogen Cylindrocladium  pseudonaviculatum) will be important in our area over the next few years, after detections in MD, DE, and PA. Be cautious when bringing new boxwood plants into a site with established boxwood. A nursery quarantine or watch area may be a wise thing for landscapers to set up, putting plants on a watch for 4 to 5 weeks before installing in a landscape planting.

Black streaks on twigs of boxwood with boxwood blight
Black streaks on twigs of boxwood with boxwood blight

Dr. Karen Rane, diagnostician with University of MD, spoke with Dr. Jim LaMondia from the Connecticut Ag Experiment Station and Dr. Kelly Ivors from North Carolina State University recently, regarding management in a landscape with mature boxwoods. They emphasized that, if all infected boxwoods are not going to be completely removed and destroyed, then pruning out and destruction on-site of all infected branches is necessary to reduce inoculum.  Just a few fallen leaves will serve as an important inoculum source.  Dr. Ivors said that nurseries in NC use a flamer (used for weed management in organic systems) to destroy fallen leaves on the soil surface after plants have been removed.  Vacuuming hard surfaces using a shopvac can also help to remove fallen leaves.  Drip irrigation system instead of overhead can be important in management.  Wetting of foliage promotes splash dispersal of spores as well as providing the optimum environment for spore germination and infection. Sanitation and fungicide applications will be extremely important for a few years to control boxwood blight. One year of management will not be enough, because microsclerotia in fallen leaves can survive for several years, and serve as a source to cause infections if new growth is not protected.

Dr. LaMondia and Dr. Ivors emphasized the use of protectant and systemic fungicides in combination to help protect new growth from disease and to reduce sporulation.  A spray that includes chlorothalonil should be applied a day or two prior to pruning out diseased material in order to kill spores on the surface of the infected tissue, or soon after pruning.  Both researchers recommended using combination sprays containing both a protectant and a systemic fungicide either as a tank mix of two products with different modes of action or using a pre-mixed product that contains two active ingredients. Both suggested chlorothalonil as the best protectant product.  Fungicides sprays should be alternated every 2 weeks when conditions are favorable for disease, as one or two sprays in a growing season will not control boxwood blight. Dr. LaMondia suggested starting with a tank mix of thiophanate-methyl +chlorothalonil, followed two weeks later with a propiconazole + fludiozinil (Medallion) spray.  Dr. Ivors suggested alternating Palladium (fludioxinil and cyprodinil) and Spectro (thiophanate methyl and chlorothalonil).  The addition of a spreader sticker may be necessary to improve coverage but check individual product labels. Products should be applied by a certified professional with the proper equipment for coverage of large plants, according to the fungicide label. Please see the following web site for some chemical recommendations and rates: http://www.anla.org/docs/government%20relations/boxwood%20blight/7.22.13-BB.pdf

NFG 9/3/13

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