Multi-year field plantings evaluating boxwood cultivars for susceptibility to the blight pathogens (Calonectria spp.) in Northern Germany

Authors: BRAND, THOMAS - Lower Saxony Chamber Of Agriculture; BELTZ, HEINRICH - Lower Saxony Chamber Of Agriculture; EHSEN, BJÖRN - Lower Saxony Chamber Of Agriculture; ADHIKARI, URMILA - Virginia Tech; DAUGHTREY, MARGERY - Cornell University; Luster, Douglas - Doug; KONG, PING - Virginia Tech; HONG, CHUANXUE - Virginia Tech

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2022
Publication Date: 7/21/2022
Citation: Brand, T., Beltz, H., Ehsen, B., Adhikari, U., Daughtrey, M., Luster, D.G., Kong, P., Hong, C. 2022. Multi-year field plantings evaluating boxwood cultivars for susceptibility to the blight pathogens (Calonectria spp.) in Northern Germany. Plant Disease. 107:713-719. https://doi.org/10.1094/PDIS-05-22-1102-RE.
DOI: https://doi.org/10.1094/PDIS-05-22-1102-RE

Interpretive Summary: Boxwood plants worldwide are threatened by a deadly fungal disease known as boxwood blight. Currently the disease is spreading rapidly across the U.S. and parts of Asia but has been present in Europe since the late 1990s. Information is needed on which boxwood cultivars are more resistant to the disease, to assist breeding efforts in the US and Europe. Although studies have been conducted in growth chambers and greenhouses, field data on cultivar susceptibility under natural climatic conditions is lacking. In this study, two multi-year field trials were conducted to evaluate boxwood cultivars for their susceptibility to the blight pathogens in northern Germany. Fifteen cultivars were included in the first trial from 2007 to 2012 and 46 cultivars in the second trial from 2014 to 2017. Both trials were done in a field infested with boxwood blight, supplemented with infected plant tissue added to the soil before planting. Boxwood variety ‘Herrenhausen’ was the most resistant cultivar, followed by B. microphylla var. japonica, B. microphylla var. koreana, ‘Green Mound’, ‘Winter Beauty’ and ‘Faulkner’. This study provided field data showing how different boxwood cultivars performed under different levels of disease pressure in an area where the pathogen was dominant, validating previous results obtained through inoculation of detached leaves, cuttings or containerized plants. This knowledge will help boxwood growers and gardeners to choose less susceptible cultivars and help plant breeders to select for disease resistance.

Technical Abstract: Two multi-year field trials were conducted to evaluate boxwood cultivars for their susceptibility to the blight pathogens Calonectria pseudonaviculata and C. henricotiae in northern Germany. Fifteen cultivars were included in the first trial from 2007 to 2012 and 46 cultivars in the second trial from 2014 to 2017. Both trials were done in an infested field, supplemented with infected plant tissue added to the soil before planting. Each hedge section with ten plants per section was assessed annually for blight severity expressed as proportion of leaves blighted and fallen. Blight severity varied significantly among cultivars (P < 0.05). In the first trial, mean severity ranged from 0.03 to 0.11 for the most resistant cultivars while 0.35 to 0.96 for the most susceptible ones. Similarly, in the second trial, mean severity ranged from 0.06 to 0.27 and 0.71 to 0.97 for the most resistant and susceptible cultivars, respectively. ‘Suffruticosa’ was consistently the most susceptible cultivar, followed by ‘Myosotidifolia’, ‘Marianne’, ‘Raket’, ‘Morris Midget’, and ‘Polar’. ‘Herrenhausen’ was the most resistant cultivar, followed by B. microphylla var. japonica, B. microphylla var. koreana, ‘Green Mound’, ‘Winter Beauty’ and ‘Faulkner’. This study provided field data showing how different boxwood cultivars performed under different levels of disease pressure in an area where C. henricotiae was dominant, validating previous results obtained through inoculation of detached leaves, cuttings or containerized plants. This knowledge will help boxwood growers and gardeners to choose less susceptible cultivars and help plant breeders to select for disease resistance.