Boxwood blight
(Calonectria pseudonaviculata (Crous, J.Z. Groenew. & C.F. Hill) L. Lombard, M.J. Wingf. & Crous 2010)

Overview

Appearance

Calonectria pseudonaviculata is a member of the Ascomycota. Asexual spores are produced in slimy masses in sporodochia on branched, penicillate conidiophores, which consist of a conidiogenous phialide, a stipe, a sterile elongation and a vesicle. Primary phialides are uniseptate or aseptate; secondary phialides are aseptate, and tertiary phialides are rare. Vesicles are boat-shaped, broadly elliptical, and papillate. Conidia are straight, cylindrical, hyaline, and contain one septum. Conidia are produced in slimy clusters. Dark brown, thick-walled, multicellular microsclerotia are produced in infected plant tissue.

Symptoms

Symptoms of boxwood blight include circular, tan leaf spots with darker borders and linear, black streaks on stems. Whole leaves eventually turn brown and drop from the plant. Infected, susceptible plants may show sudden and severe defoliation. Defoliated stems may produce new shoots from axillary or terminal buds later in the season. Calonectria pseudonaviculata has been shown to infect other members of the Buxaceae, including Japanese spurge (Pachysandra terminalis) and Allegheny spurge (Pachysandra procumbens). Both species of spurge are commonly planted with boxwood in American landscapes; thus, infected spurge could serve as a potential source of inoculum for boxwood, and vice versa, in mixed landscape plantings.

Distribution

Calonectria pseudonaviculata is found in Europe and across the United States and Canada.

Ecological Threat

Calonectria pseudonaviculata Spores are spread over short distances by wind-driven rain or splashing water and are most infective during conditions of high humidity or free water. Fallen, infected leaves may also be moved by wind currents and serve as a source of inoculum. Long-distance spread of this disease occurs via movement of infected plants or plant debris, or infested soil or equipment. The sticky spores of the pathogen can also be spread on infested shoes, clothing, burlap bags, or animal fur. Resting propagules (i.e. microsclerotia and chlamydospores) have been observed in culture and within infected plant tissue. The pathogen has been found to survive in leaf debris placed either on the soil surface or buried in the soil for up to 5 years.

Control Efforts

An integrated management approach that includes avoiding introduction, sanitation, chemical control, and use of resistant cultivars is recommended to prevent spread of the disease.

Selected Images

Maps

Invasive Listing Sources

• Maryland Invasive Species Council - Invasive Species of Concern in Maryland