Increasing Container Spacing and Decreasing Irrigation Frequency Reduces the Spread of Boxwood Blight Among Container Boxwood Plants

Boxwood blight, caused by Calonectria pseudonaviculata, is an introduced disease that causes substantial losses in Oregon's nursery industry. Growers make decisions about plant spacing and irrigation to produce boxwood in the most cost-effective manner. These decisions change the microenvironment in which the pathogen resides and affect how boxwood blight develops and spreads within nurseries. To understand this, we evaluated how plant spacing (0 and 15 cm apart) and irrigation frequency (1, 2, and 3×/day) affected disease severity and pathogen movement in plots of the susceptible boxwood cultivar ‘Green Velvet’. Spacing was the most important factor. Regardless of irrigation frequency, boxwood blight only spread to plants placed pot tight (0 cm apart) with canopies touching and did not spread to plants spaced 15 cm apart with separated canopies. Irrigation frequency only affected boxwood blight on plants at the 0-cm spacing, where disease severity and spread intensified with increasing irrigation frequency. Seasonally, boxwood blight spread slowly during the summer when the weather was warm and dry and in the winter when temperatures were too cold. Disease was more severe and spread more rapidly in the spring and fall when the weather was cool and rainy. These results show that growers can minimize disease losses by growing plants further apart so that their canopies do not touch and by irrigating less frequently so that the foliage dries quickly. In addition, growers should scout for and protect against boxwood blight in spring and fall, when the potential for disease spread is greatest. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 “No Rights Reserved” license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2026.