Calonectria pseudonaviculata (boxwood blight)

Taxonomy

Domain	Eukarya
Kingdom	Fungi
Phylum	Ascomycota
Subphylum	Pezizomycotina
Class	Sordariomycetes
Subclass	Hypocreomycetidae
Order	Hypocreales
Family	Nectriaceae
Genus	Calonectria

Scientific Name

Calonectria pseudonaviculata

Scientific Name Synonyms

Cylindrocladium buxicola

Cylindrocladium pseudonaviculatum

Common Name

Boxwood blight

Author: Mary Ann Hansen, Virginia Tech

Reviewed by: Anton Baudoin, Virginia Tech

Pathogen

Calonectria pseudonaviculata is a member of the Ascomycota. Its asexually reproducing stage is a member of the Hyphomycetes. 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, hence the species epithet "pseudonaviculata". Vesicles are 6.5-11µm, broadly elliptical, and papillate. Conidia are straight, cylindrical, hyaline, and contain one septum. Conidia are 42-68µm by 4-6µm and are produced in slimy clusters. Dark brown, thick-walled, multicellular microsclerotia are produced in infected plant tissue.

Symptoms and Signs

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.

Ecology and Spread

Spores of Calonectria pseudonaviculata are spread over short distances by wind-driven rain or splashing water and are most infective during conditions of high humidity or free water. The significance of spore dispersal by wind or air currents is not known, but, under most circumstances, it is likely limited to short distances, such as between plants, within hoop houses, or within a field. 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.

Calonectria pseudonaviculata has been shown to infect other members of the Buxaceae, including Japanese spurge (Pachysandra terminalis) and Allegheny spurge (Pachysandra procumbens). Natural infections have been observed in P. terminalis in the landscape in Connecticut. To date, infections of Allegheny spurge have only been demonstrated in laboratory inoculations. 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.

Geographic Distribution

Boxwood blight was first described in the United Kingdom in the mid-1990's; however, the pathogen remained unnamed until 2002, when the same disease was described in New Zealand and the pathogen was named Cylindrocladium pseudonaviculatum. Later that same year the pathogen was given the name Cylindrocladium buxicola in the UK. These two names are now known to be synonyms for the same fungus. The current accepted nomenclature for the boxwood blight pathogen (G1 genotype) is Calonectria pseudonaviculata. Two distinct genotypes of C. pseudonaviculata have been identified: G1 and G2. Due to significant differences between these genotypes, the G2 genotype was recently confirmed to be a distinct species of Calonectria. Current nomenclature for the G2 genotype is Calonectria henricotiae Gehesquière (Heungens and J.A. Crouch). Since the original reports, boxwood blight has been reported throughout Europe. The disease was detected for the first time in the United States in 2011 in Connecticut and, shortly thereafter, in North Carolina and Virginia. As of 2014, the disease has been found in the following states: AL, CT, DE, MA, MD, NC, NJ, NY, OH, OR, PA, RI, and VA, and in the Canadian provinces of BC, ON and QC.

Management

Management approaches to boxwood blight may differ, depending on whether the boxwood is planted in a landscape, commercial nursery, historic garden or boxwood greenery-producing site. Best management practices for each situation have been developed by several states, including Connecticut and Virginia. An integrated management approach that includes avoiding introduction, sanitation, chemical control, and use of resistant cultivars is recommended to prevent spread of the disease.

Avoiding Introduction

Because the spores of Calonectria pseudonaviculata are not well adapted to long-distance spread via air currents, the most likely method of introduction to a new site is on infected plants or plant material, or on tools, equipment or clothing that have been in contact with infected plants. Inspecting plants for symptoms of boxwood blight before introducing new plants to a landscape or nursery operation and sanitizing any equipment or clothing that has been used at infested sites are essential for avoiding introduction to new sites. Pay special attention to resistant cultivars that may have cryptic symptoms and act as a "Trojan horse" for pathogen introduction. Carefully inspect the inner canopy of plants for leaf spots or black streaks on stems. If plants have been treated with fungicides prior to purchase, symptoms may be masked for some time after treatment. Consider sequestering new plants away from existing boxwood for at least one month and monitoring for symptoms of boxwood blight before transplanting. Be aware that the boxwood blight pathogen can also be present on holiday greenery. Inspect holiday greenery containing boxwood for leaf and stem symptoms before purchase. Discarded greenery could serve as a source of inoculum to existing plantings.

Because spores can stick to clothing, always wear clean, laundered clothing when working in boxwood plantings. Make sure shoes are free of soil and plant debris. If shoes have been worn in areas that are suspect for boxwood blight, consider wearing disposable shoe covers before entering a new site. Disposable clothing covers and gloves can also be used at each new site. They should be bagged and disposed in the landfill after working at each site.

Avoid introduction on tools and equipment by making sure tools and equipment are free of soil and plant debris before leaving a site containing boxwood. Pruning tools should be dipped in a sanitizer for a minimum of 5 minutes between sites or between blocks of plants. Sanitizers available for home use that are effective for the boxwood blight pathogen include a solution of household bleach (1 part bleach: 14 parts water) or Lysol Concentrate Disinfectant (at label rates). Oil tools after use to prevent corrosion. Zerotol 2.0 (hydrogen dioxide + peroxyacetic acid) is an additional sanitizer that is available for use by commercial growers.

If landscape professionals are hired to manage property containing boxwood plants, ask them to explain what measures they have in place to prevent introduction of boxwood blight to a landscape. If they are unaware of boxwood blight or do not demonstrate that they clean their equipment between sites, it would be best to hire someone else.

Cultural Control

If boxwood blight is detected in a planting, removal of infected plants is the best way to prevent spread to other nearby boxwood. In addition to removing infected plants, it is also important to remove all remaining leaf debris, which can harbor the pathogen. Vacuum or rake and bag leaf debris, and dispose in the landfill. Alternatively, bury infected plant debris under two feet of soil. Do not compost infected boxwood plant material. Be careful to sanitize all tools and equipment that may have been used in infected plants before using them on healthy plants. Avoid working in boxwood plants when they are wet. Monitor remaining boxwood regularly for symptoms of disease so that new infections can be managed promptly.

Resistance

Boxwood species and cultivars that have demonstrated good resistance to boxwood blight in resistance trials in North Carolina as of 2014 are listed below. It is important to remember that even resistant cultivars can develop symptoms and the pathogen can produce spores on infected plant tissue; however, symptoms are harder to spot on resistant cultivars. Infected, resistant cultivars can serve as a source of inoculum for susceptible plants if they are introduced to existing plantings, especially if they are planted near highly susceptible boxwood, such as English and American boxwood. It is important to ensure that newly introduced plants are free of disease before planting them near susceptible boxwood plants.

Resistant Boxwood Species and Cultivars:

Buxus harlandii

Buxus microphylla 'Golden Dream'

Buxus microphylla 'Wedding Ring'

Buxus microphylla var. japonica 'Green Beauty'

Buxus sempervirens 'North Star'

Buxus sinica var. insularis 'Nana'

Buxus sp. 'Northern Emerald'

Chemical Control

Some fungicides have shown efficacy against boxwood blight when applied prophylactically; however, to date, no fungicides have shown curative activity against the pathogen and, thus, fungicides are not recommended for use on plants that have already been diagnosed with the disease. Fungicide active ingredients with good preventative efficacy that are labeled for both nursery and landscape use include: chlorothalonil, chlorothalonil + thiophanate methyl, tebuconazole, and fludioxonil. Follow label rates and timing of application. If fungicides are used, care should be taken to apply them at recommended intervals, especially when weather conditions are favorable for disease. Air temperatures >60^oF and a forecasted rain event are conducive to disease development.

Diagnostic procedures

Morphology

The fungus sporulates readily on infected plant tissue and can usually be detected within 5 days of moist chamber incubation. It can be cultured on a variety of media, including potato carrot agar, potato dextrose agar, carnation leaf agar and malt extract agar. Sporulation occurs 7-10 days after incubation on carnation leaf agar at 25^oC under near-ultraviolet light. When grown on malt extract agar (MEA), colonies as viewed through the bottom of a culture plate are fuscous black at the center, fading to sienna moving outward. The margin of the growing colony is white. The optimum temperature for growth of C. pseudonaviculata is 25^oC. The minimum temperature for growth is 5^oC and no growth occurs above 30^oC.

Microsclerotia can be visualized in infected leaf tissue by staining leaves as described below.

1. Prepare a solution of 1:1:1 (glycerol, lactic acid, water) with 0.05% cotton or trypan blue (aniline blue).

2. Place bits of infected tissue in the mixture and boil for 30 sec to 1 min. Allow to cool.

3. Destain in 1:1 (glycerol, water) for one hour or overnight. (Note: It can be difficult to see microsclerotia without destaining.)

DNA Sequencing

The ß-tubulin gene and the MAT2 gene can be used to differentiate species of Cylindrocladium (Calonectria). Primers T1 and Bt2b are used to amplify ß-tubulin; primers ColHMGa and ColHMG2 are used to amplify MAT2 using standard procedures (Henricot and Cullham, 2002).

Resources and References

• Crous, P.W., J.Z. Groenewald, and C.P. Hill. 2002. Cylindrocladium pseudonaviculatum sp. nov. from New Zealand, and new Cylindrocladium records from Vietnam. Sydowia 54:23-34.
• Gehesquière, B., Crouch, J.A., Marra, R.E., van Poucke, K., Rys, F., Maes, M., Gobin, B., Höfte, M. and K. Heungens. Characterization and taxonomic re-assessment of the box blight pathogen Calonectria pseudonaviculata, introducing Calonectria henricotiae sp. nov. In press.
• Henricot, B. and A. Culham. 2002. Cylindrocladium buxicola, a new species affecting Buxus spp., and its phylogenetic status. Mycologia 94: 980.
• Ivors, K.L., L. W. Lacey, D. C. Milks, S. M. Douglas, M. K. Inman, R. E. Marra, and J. A. LaMondia. 2012. First report of boxwood blight caused by Cylindrocladium pseudonaviculatum in the United States. Plant Disease 96(7): 1070.1.
• Malapi-Wight, M., Hebert, J.B., Buckley, R., Daughtrey, M., Gregory, N.F., Rane, K., Tirpak, F., and J.A. Crouch. 2014. First report of boxwood blight caused by Calonectria pseudonaviculata in Delaware, Maryland, New Jersey, and New York. Plant Disease 98 (5): 698.
• Malapi-Wight, M., Hébert, J.B., Rivera, Y., Ismaiel, E., Saied, N., Gehesquière, B., Heungens, K., and J. A. Crouch. Comparative Genomics in the Boxwood Blight System: Insights into the Global Diversity of the Mating-Type Locus. 2014 APS-CPS Joint Meeting, Minneapolis, MN.
• The Connecticut Agricultural Experiment Station. Boxwood Blight.
• Virginia Cooperative Extension. Boxwood Blight Task Force.

Acknowledgments

• National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2011-41530-30708 as part of "Diagnostic Image Series Development for Supporting IPM in the Southern Region" (USDA-NIFA-RIPM-003351)

Reviewed by: Anton Baudoin, Virginia Tech