Feature
A Guide for
DETECTING BOXWOOD BLIGHT and
MANAGING ITS IMPACT
on Nursery Stock and in Landscape Plantings By Dr. Fulya Baysal-Gurel, Associate Professor of Ornamental Plant Pathology and Bhawana Ghimire, PhD Graduate Research Assistant, Tennessee State University, Otis L. Floyd Nursery Research Center
How and when were boxwood blight found and what are its plant hosts? Recently, a 150-year-old boxwood plantation, the focal point of the Carl Sandburg Home National Historic Site in North Carolina, was cut down because of a plant disease spreading like wildfire among boxwood plantings. Boxwood blight disease is caused by two fungal species, Calonectria pseudonaviculata and C. henricotiae. To date, only C. pseudonaviculata is present in the United States. This fungal pathogen was first reported in North Carolina and Connecticut in 2011, affecting boxwood shrubs in nurseries and landscape plantings. Since 2011, boxwood blight has been confirmed in 28 additional states. Boxwood, sweet box (Sarcococca spp.) and a few species of Pachysandra have been reported as host plants. Due to the potential for economic harm, the Tennessee Department of Agriculture established boxwood blight quarantine in February 2018. Proper precautionary actions should be taken to prevent the introduction of this threatening pathogen into healthy boxwood production areas and landscapes.
How do we monitor symptoms and signs of boxwood blight? Symptoms often first appear on the lower part of the plants, or within the plant interior. Initially, reddish-brown to blackish-brown concentric
22
tennessee greentimes WINTER 2021
FIGURE 1 — Black to brown necrotic lesions may be apparent on the stems and leaves
circular spots with tan to light brown centers are visible on infected leaves (Fig. 1). Angular dark brown to black lesions also may appear from the base of stems to the shoot tip. With disease progression, whole leaves will turn brown; defoliation usually begins from the lower branches and spreads to the upper part of the plant (Fig. 2). When temperature is warm (64–77ºF) and humidity is high, continuous leaf wetness may facilitate fungal sporulation. White spore masses that can be seen without magnification may be produced on leaf undersides (Fig. 3) and on the stem. Sudden and severe defoliation is one of the characteristic symptoms of boxwood blight disease.
What can be done to prevent, limit spread and severity boxwood blight disease? 1.1 Precautionary measures One of the most important precautionary measures that can be used to prevent the disease is excluding pathogen/infected plant materials from nurseries and landscapes. The fungus produces sticky conidia which can stick to the cutting tools, workers’ boots, and clothes. The accidental introduction of the disease from one site to another can be prevented by sanitizing tools, equipment, hard surfaces, vehicle tires and footwear using disinfectants such as Simple Green D Pro 5, KleenGrow (0.12%) and Greenshield. Boots and clothes can also be changed or cleaned before entering a new production area or client landscape.
