Effect of Mulching and Irrigation on Growth and Disease Incidence of Dwarf Boxwood

Buxus sempervirens L. ‘Suffruticosa’ (dwarf boxwood) is a popular ornamental for landscape use that is currently suffering from decline caused by plant pathogens such as Phytophthora species. High soil moisture is associated with this decline. Excessive irrigation and mulch may keep soil moist thus promoting diseases and inhibiting growth. In this study, effects of pine bark mulch, irrigation and inoculation with Phytophthora nicotianae Breda de Haan on dwarf boxwood were examined. Plants were treated with two mulch levels (no mulch and 5-cm pine bark) and three irrigation levels (no irrigation, 2.5 cm per week, and 5 cm per week), and half of plants were inoculated. Plants were rated for disease symptoms after inoculation. Only plants inoculated with P. nicotianae showed significant reduction in growth, and they gradually developed symptoms of Phytophthora root rot. Dwarf boxwood that showed pathogenic symptoms had low to zero photosynthetic capacity. P. nicotianae was recovered from crowns and roots of symptomatic plants. Additional irrigation and pine park mulch did not have significant effects on dwarf boxwood in the first year of experiment, which was possibly caused by unusually high precipitation in the growing season. Extra irrigation and pine bark had no significant effect on disease incidence or growth.