Rooting Response of Boxwood Cultivars to Hot Water Treatment and Thermal Sensitivity ofCalonectria henricotiaeandC. pseudonaviculatain Diseased Boxwood (Buxusspp.)1

Abstract

Boxwood blight is caused by Calonectria henricotiae (Che) and C. pseudonaviculata (Cps). Unrecognized symptoms on Buxus cuttings used for propagation could potentially serve as a source of inoculum and result in pathogen spread. In this study, cuttings of boxwood (Buxus spp.) cultivars ‘Justin Brouwers', ‘Nana', ‘Green Beauty', and ‘Green Velvet' were assessed for root production after exposure to 45 C (113 F) or 47.5 C (118 F) water for 0 to 60 minutes in 5 minute increments. The number of roots greater than 1 cm (0.4 in) in length produced by cuttings of all cultivars three months after treatment in 45 C water for up to 60 minutes was not statistically different from the non-immersed control. A similar response was observed for cuttings of all cultivars treated in 47.5 C water for up to 60 minutes, except for cv. ‘Nana,' which produced fewer roots than the non-immersed control after 35 minutes of exposure to heated water. Experiments conducted on diseased, detached boxwood leaves of susceptible cultivar ‘Justin Brouwers' at 47.5 C, 50 C (122 F) or 52.5 C (127 F) showed significantly reduced production of conidia and viability of Che and Cps after 25 to 30 minute exposure to 47.5 C water (44%) or 12 minute exposure to 50 C water (22%). After 8 minutes of exposure to 52.5 C water, little or no sporulation was observed for either pathogen.Index words: Boxwood blight, Calonectria pseudonaviculata, Calonectria henricotiae, Buxus, hot water treatment, rooting, cuttings, plant propagation.Species used in this study: Boxwood blight [Calonectria henricotiae Gehesquière, Heungens and J.A. Crouch and C. pseudonaviculata (Crous J.Z. Groenewald & C.F. Hill) L. Lombard, M.J. Wingf & Crous] Boxwood [Buxus sempervirens L. ‘Justin Brouwers'; Buxus sinica (Rehder & E. H. Wilson) M. Cheng var. insularis (Nakai) M. Cheng ‘Nana'; B. sempervirens ‘Suffruticosa'×B. sinica var. insularis ‘Green Beauty', and B. sempervirens ‘Suffruticosa' × B. sinica var. insularis ‘Green Velvet'].