First Report of Boxwood Blight Caused by Calonectria pseudonaviculata in California.

Abstract

Samples from 9-year-old boxwood (Buxus sempervirens ‘Suffruticosa’) plantings from a residential estate in San Mateo County were submitted to the California Department of Food and Agriculture Plant Pest Diagnostics Center Lab in December 2016. Partially to nearly completely defoliated plants were found in which many of the leaves had violet brown, water-soaked spots. Green stems that became gray and dry as the disease progressed had narrow, longitudinal black lesions. Within a few months, symptomatic plants were discovered throughout the estate, despite sanitation practices implemented to prevent the spread of disease. Hyaline conidia, which ranged from 47.5 to 62.5 × 4.0 to 6 μm (53.5 × 5.1 μm average), were zero to two septate and produced in slimy masses in the leaf spots and stem lesions. Pieces (4 mm³) of tissue were excised from the margins of the spots and lesions, surface sterilized in 0.6% NaOCl for 2 min, and placed onto one-half-strength acidified potato dextrose agar (APDA). Velvety brown colonies grew from the pieces, eventually producing a tan to reddish margin with a ring of white sporulating conidiophores. The internal transcribed spacer region of rDNA (White et al. 1990) and portions of the translation elongation factor 1-alpha (Hunter et al. 2006) and beta-tubulin genes (Crous et al. 1999) were amplified from extracted DNA from one isolate and the amplicons sequenced (GenBank accession nos. MH078193, MG930486, and MG905088). BLAST analysis of a portion of the beta-tubulin gene was 100% identical with a sequence of the Calonectria pseudonaviculata culture ex-type, isolate STE-U 3399 (KF815124). To confirm pathogenicity, conidia were collected by flooding 15-day-old sporulating cultures with sterile distilled water and inoculating three 3.79-liter potted B. sempervirens plants, approximately 30 cm tall, with 3 ml each of a spore suspension (7 × 10⁵ spores/ml). Plants were placed in a dew chamber at 20°C for 72 h and then in a growth chamber at 20°C with a 12-h photoperiod. After 6 days, gray to purple leaf spots with orange centers were seen on the plants, with a few branches beginning to defoliate. Plants sprayed with water as controls and kept under the same environmental conditions as the inoculated plants remained healthy. After 13 days, leafspots, black stem lesions, severe dieback, and defoliation were observed on all inoculated plants. C. pseudonaviculata was isolated on APDA from symptomatic tissue from all inoculated plants but was not recovered from the control plants. C. pseudonaviculata was first discovered in the United Kingdom in the mid-1990s and has been detected in New Zealand, Europe, Canada, and the United States on the eastern seaboard (Farr and Rossman 2018). Following this detection in San Mateo County, boxwood blight was found at three additional estates in two counties in northern California. The presence of boxwood blight has limited the planting options for affected estates and bordering properties. To date, the disease has not been found in commercial nurseries in California. The above average rainfall in northern California in the fall of 2016 through spring 2017 may have provided optimum conditions for spread of this disease.