Occurrence of boxwood blight caused by Calonectria pseudonaviculata in Ohio landscapes

Authors: EMANUEL, ISABEL - The Ohio State University; FARINAS, CORALIE - The Ohio State University; LIN, SHAN - The Ohio State University; PIERZYNSKI, JOY - The Ohio State University; Crouch, Jo Anne; PEDUTO-HAND, FRANCESCA - The Ohio State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2019
Publication Date: 6/12/2019
Citation: Emanuel, I.B., Farinas, C., Lin, S., Pierzynski, J., Crouch, J., Hand, F.P. 2019. Occurrence of boxwood blight caused by Calonectria pseudonaviculata in Ohio landscapes. Plant Disease. 103(10):2670. https://doi.org/10.1094/PDIS-03-19-0520-PDN.
DOI: https://doi.org/10.1094/PDIS-03-19-0520-PDN

Interpretive Summary: Boxwood blight disease is caused by a deadly fungus that leads to significant damage of boxwood plants worldwide. Boxwood blight has been found sporadically in the Ohio commercial nursery trade since 2012, but until recently the fungus has not become established in the landscape. This research describes for the first time multiple outbreaks of boxwood blight disease affecting mature boxwood grown in home gardens in Ohio during the summer and autumn of 2018. These outbreaks mark the first time that the disease has become established in Ohio gardens. Knowledge of the distribution and persistence of this disease will be useful to the horticultural industry, growers, plant pathologists, and plant regulatory officials working to minimize the spread of boxwood blight in the United States.

Technical Abstract: Boxwood (Buxus spp.) is a popular landscape plant in the U.S. with an annual wholesale value of approximately $126 million (USDA-NASS, 2012). Boxwood blight, caused by the fungus Calonectria pseudonaviculata, was first reported in North America in 2011 (Ivors et al. 2012) and continues to spread throughout the U.S. The pathogen has been sporadically detected in Ohio nurseries since 2012 (Ohio Department of Agriculture, 2012), however, to date there are no reports of the disease in the landscape. Between August and October 2018, four diseased boxwood samples from homeowner yards in Franklin, Knox, Cuyahoga, and Licking Co., OH were received by the C. Wayne Ellett Plant and Pest Diagnostic Clinic at The Ohio State University. Affected plants ranged in age from eight months to five years and showed symptoms of black elongated stem cankers, necrotic leaf spots, and signs of white sporulation on the underside of the leaves, consistent with the description of boxwood blight. Isolations were made by surface sterilizing infected leaves and plating onto potato dextrose agar (PDA) with 0.015% streptomycin sulfate and 0.01% tetracycline hydrochloride. One representative isolate from each of the four samples was subcultured on PDA at 28°C for 4 days without light, then moved to 24°C for 7 days (12h photoperiod). Colonies were light brown surrounded by dark brown concentric rings with a white border. Conidia were cylindrical, hyaline, 38 to 58 × 3 to 8 µm (n=50), with a single septum. To identify the fungi to species level, the ß tubulin, calmodulin (CaM), histone H3 (H3) and rDNA ITS (MK48054-5; MK503136) markers were PCR amplified and sequenced as described (Gehesquière et al. 2016). Sequences were identical from all four isolates. NCBI GenBank BLASTn comparisons showed all sequences shared 100% identity to C. pseudonaviculata. However, H3 and CaM sequences showed just 98.3% and 99.5% identity to C. henricotiae; these diagnostic nucleotide differences identified the species as C. pseudonaviculata (Gehesquière et al. 2016). Pathogenicity of two isolates was determined by inoculating two year-old B. sempervirens ‘American’ potted plants. For each isolate, the entire foliage of three plants was sprayed with a 105 conidia/mL sterile water-Tween 20 (0.05% v/v) solution until run-off. Three control plants were sprayed with sterile water-Tween 20 only. Following inoculation, plants were individually enclosed inside a plastic bag for 24 h to favor infection, then removed from the bags and arranged in a completely randomized design in a tray covered with a vented humidity dome and maintained inside a greenhouse (day=27°C, night=22°C). Light brown leaf spots were observed on the abaxial leaf surface two days post-inoculation (DPI). Leaf spots expanded, turned dark brown, and became visible on the upper side of the leaves 3 DPI. Dark brown to black cankers were observed on the stems 5 DPI. Plant defoliation occurred 7 DPI. No symptoms were observed on the control plants. Re-isolation and morphological identification of each isolate fulfilled Koch’s postulates. This report constitutes the first confirmed instance of boxwood blight in Ohio landscapes. While implementation of best management practices to prevent introduction and spread of the disease continues to be adopted by the State’s nursery industry, the confirmed presence of inoculum in the landscape has the potential to cause significant damage, not only to commercial establishments, but also residences, historical landmarks, national parks and conservatories.