Breeding and selection of Buxus for resistance to Calonectria pseudonaviculata

Abstract

Box blight is a widespread disease of Buxus caused by the pathogen Calonectria pseudonaviculata. It is responsible for significant losses in nurseries, gardens and wild boxwood populations. Our goal was to maximize the efficiency of a breeding programme towards increased disease resistance. The use of artificial inoculation of young F1 seedlings with C. pseudonaviculata spores under greenhouse conditions appeared to be a reliable tool for early selection of interesting prebreeding material. Overall, the four hybrid populations screened showed a segregating behaviour between their parents when determining the percentage of diseased leaves and lesion diameter. Genotypes were also found with an increased tolerance as compared to the parental species. Approximately 50% of the seedlings had the same score for both parameters after artificial inoculation in the greenhouse and in the field. Of the seedlings that showed severe symptoms in the greenhouse, <15% showed no disease symptoms in the field. Therefore, for larger breeding programmes, we propose a two‐step selection procedure: first artificial inoculation at seedling level to eliminate all genotypes with severe symptoms and then evaluation of the remaining seedlings in the field. Using this strategy, we were able to select several genotypes in our four hybrid populations with improved resistance to C. pseudonaviculata.

Multi-Year Field Plantings Evaluating Boxwood Cultivars for Susceptibility to the Blight Pathogens (Calonectria spp.) in Northern Germany

“…In Germany, the use of plant protection products in public green spaces is strictly regulated. Strategies currently used against boxwood blight that are based on fungicide application are not sustainable for public green spaces due to the paucity of suitable registered fungicides available for this special application at these sites (Van Laere et al 2018;BVL 2022). In many German municipalities, political decisions have been made to refrain from the use of pesticides (Verschwele 2016).…”

“…In an effort to breed for resistance against C. pseudonaviculata, Van Laere et al (2018) made interspecific crosses among populations of Buxus species (B. harlandii, B. microphylla, and cultivars of B. sempervirens) and selected several hybrids with reduced leaf drop and lesion diameter. Screening is key during the early pre-breeding and breeding steps to identify the most promising boxwood material.…”

“…However, examples of inconsistent ranking (Kramer et al 2020), differential response to the same pathogen (Guo et al 2016;LaMondia and Shishkoff 2017) and lack of reproducibility (Van Laere et al 2018) that may have arisen due to differences in inoculation techniques, pathogen isolates, or evaluation methods, have been noticed in recent years (Guo et al 2016;LaMondia and Shishkoff 2017). For example, 15% of the F1 hybrids identified as susceptible under artificial inoculation in the greenhouse were also resistant when screened under field conditions (Van Laere et al 2018). In general, detached leaves might react differently than whole plants (Orłowska et al 2013).…”

Combating an Invasive Boxwood Pathogen – Calonectria pseudonaviculata – in the United States by Shifting Production to Less-Susceptible Cultivars

“…The attempt to understand resistance of boxwood cultivars to BB has involved different groups across the U.S. (Ganci 2014;Ganci et al 2013;LaMondia 2015;LaMondia and Shishkoff 2017;Kramer et al 2020) and Belgium alike (Van Laere et al 2019;Luypaert et al 2016). Most of these efforts classified boxwood cultivars in terms of their susceptibility to the causal fungus after challenging different cultivars with sufficient pathogen inoculum to cause disease.…”