Semester of Graduation

Fall 2020


Master of Science (MS)


Plant Pathology and Crop Physiology

Document Type



Boxwood (Buxus spp. L) is one of the most common and widely planted perennial ornamentals in both home gardens and commercial landscapes. Grown for its evergreen, dark green foliage, boxwood cultivation dates back to 4000 BC in Egypt. Although considered hardy, boxwood is susceptible to several plant pathogens. Recently reported boxwood dieback, a fungal disease caused by Colletotrichum theobromicola, has been spreading at an alarming rate within the United States. Boxwood dieback consists of symptoms that resemble to those caused by Phytophthora root rot, Volutella blight, and some abiotic disorders and can be easily misdiagnosed in nurseries and landscape plantings and may lead to ineffective management recommendations and practices. Additionally, previous studies have shown that it may take up to two to three months for the symptoms to appear under greenhouse conditions. Boxwood breeders and growers have shown great concerns regarding the disease due to its delayed onset of symptoms, non-availability of early, rapid, and accurate diagnostic tools, and lack of effective management practices. Therefore, the primary objectives of this study were to develop an accurate diagnostic method to detect the disease in boxwoods at early disease development stages and to devise effective disease management strategies including host range and screening of fungicide for their efficacy. A diagnostic TaqMan real-time PCR assay for in planta detection and quantification of C. theobromicola was developed. This species-specific assay targets calmodulin (CAL) gene and successfully detected the pathogen from symptomatic boxwood tissue at early stages of the disease development. Host range studies were conducted by screening a wide variety of boxwood cultivars under greenhouse conditions. Out of the 11 cultivars screened, ‘Little Missy’ was found to show latent infection to boxwood dieback. Finally, fungicide efficacy studies were conducted by screening nine chemical compounds to determine their effect on mycelial growth as well as spore germination inhibition of eight isolates of C. theobromicola collected from eight different states in the United States. Of the nine fungicides, difenoconazole+pydiflumetofen showed maximum mycelial growth and spore germination inhibition at 1 ppm active ingredient followed by fluxapyroxad+pyraclostrobin, and pyraclostrobin+boscalid at 5 ppm active ingredient. Azoxystrobin+benzovindiflupyr significantly inhibited mycelial growth at 1 ppm but reduced spore germination at 10 ppm active ingredient. This study provides the boxwood industry with much needed important and applied information regarding rapid and accurate detection, host susceptibility, and fungicide efficacy to effectively manage boxwood dieback and to reduce its further spread.

Committee Chair

Singh, Raghuwinder