Location: Horticultural Crops ResearchTitle: Confirmation of Calonectria pseudonaviculata causing boxwood blight of Buxus sempervirens and Buxus cultivars in Oregon
|SACHER, GABRIEL - Oregon State University|
|PUTNAM, MELODIE - Oregon State University|
|CASTROAGUDDIN, VANINA - Oak Ridge Institute For Science And Education (ORISE)|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2020
Publication Date: 5/5/2020
Citation: Sacher, G.O., Weiland, G.E., Putnam, M.L., Crouch, J., Castroaguddin, V.L. 2020. Confirmation of Calonectria pseudonaviculata causing boxwood blight of Buxus sempervirens and Buxus cultivars in Oregon. Plant Disease. 104(6):1862. https://doi.org/10.1094/PDIS-01-20-0078-PDN.
Interpretive Summary: Oregon is the number one boxwood producing state in the nation. The disease, boxwood blight, was discovered in the state in 2011. However, a formal report was never filed. This research reports that boxwood blight is still found in parks, private residences, and at boxwood nurseries in Oregon eight years after its introduction. Six boxwood nurseries, a park, and a home residence were surveyed and boxwood plant samples with leaf spots and stem lesions were collected. The pathogen, Calonectria pseudonaviculata, was isolated from these tissues and cultures of this pathogen were grown in Petri plates. Spores were produced from these cultures and used to inoculate healthy boxwood plants located in a greenhouse. Leaf spot and stem lesions developed within 6 days and the pathogen was reisolated, proving that it was capable of causing the same disease that was observed in the nurseries, parks, and residences. This report confirms that the boxwood blight pathogen continues to occur in Oregon where it can cause significant plant losses. This finding is important because it shows that nursery growers and landscape managers must be careful to disinfest equipment so that they do not spread the disease to other locations.
Technical Abstract: Oregon is the top boxwood (Buxus cultivars) producing state in the U.S.A. ($25 M/$125 M nationwide). Boxwood blight is a serious disease that was detected in Oregon in 2011, but this finding was never published. Consequently, researchers and others interested in the disease have been unable to reference a report indicating that the causal agent, Calonectria pseudonaviculata, occurs in the state. In 2019, black leaf spots, angular black stem lesions, and defoliation were observed on boxwood hedges in several public parks and private residences, as well as on boxwood stock plants at twelve nurseries throughout the Willamette Valley. The most commonly infected cultivars included Green Velvet, Green Gem, Common, Wintergreen, and Green Mountain. Symptoms were sometimes infrequent (<5% of =1000 plants) and mild (1 to 3 leaf spots or stem lesions per plant), but could also be widespread (50 to 100% incidence) and more severe (>75% canopy affected). Calonectria pseudonaviculata was consistently isolated from infected plants at six nurseries by placing symptomatic leaves and stems into moist chambers. Identity was confirmed with species-specific SSR markers (LeBlanc et al. 2019). Conidia were produced from two isolates as described (Guo et al. 2015) and sprayed onto = 1-year-old boxwood plants (10 to 30 cm in height, depending on cultivar). For each isolate, eight plants of 'Green Velvet' (Trial 1), six plants each of 'Green Beauty' and 'Common boxwood' (Trial 2), and six plants each of 'Green Gem' and 'Chicagoland Green' (Trial 3) were inoculated. An equal number of each cultivar were sprayed with distilled water as controls. Plants were incubated in plastic bins. After 24 h, plants were transferred to a greenhouse (25°C/18°C max/min). Within 6 days, leaf spots and stem lesions developed on inoculated plants, but not on controls. The pathogen was reisolated from 3 to 4 inoculated plants of each cultivar and identified using culture characteristics and conidial morphology. This report confirms that C. pseudonaviculata continues to occur in Oregon landscapes and nurseries.