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

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 a naturally infested field, supplemented with infected plant tissue added to the soil before planting. Each cultivar had three replicate hedge sections with ten plants per section and they were assessed annually for blight severity expressed as proportion of leaves blighted and fallen. Blight severity varied significantly among years (P < 0.0001) and cultivars (P < 0.05) within each trial. In the first trial, mean severity ranged from 0.03 to 0.11 for the most resistant cultivars and 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 ‘Marianne’, ‘Myosotidifolia’, ‘Raket’ and ‘Morris Midget’. ‘Herrenhausen’ was the most resistant cultivar, followed by B. microphylla var. japonica, B. microphylla var. koreana, ‘Green Mound’, ‘Faulkner’, and ‘Winter Beauty’. This study provides field data showing the performance of boxwood cultivars under different levels of disease pressure in an area where C. henricotiae was dominant. This knowledge will help boxwood growers and gardeners to choose less susceptible cultivars and help plant breeders to select for disease resistance.

Evaluation of fungicides for management of boxwood blight caused by Calonectria spp. under field conditions in Northern Germany

“…Another active substance from the azoles (FRAC Group 3), myclobutanil (Pilzfrei Ectivo), also had limited effectiveness in our study. This might be due to the dominance of Che at the research site (Brand et al 2022). These results highlight the importance of monitoring for the presence or dominance of Che when selecting fungicides for blight mitigation.…”

“…All fungicide applications were made with a backpack sprayer with a water volume of 100 mL/m 2 at the application rates mentioned in Table 1. At the end of the trial, fallen leaves and infected plants were shredded and incorporated into the topsoil to acquire an area contaminated with the pathogen for succeeding trials (Brand et al 2022).…”

“…The 2012 trial was conducted in a nearby field, where plants were inoculated by scattering infested soil and fallen leaves collected from an infested field where blight trials had been performed since 2006 (Brand et al 2022). Inoculum was added to the trial field on August 20, 2012, immediately after the first treatment with fungicides, earlier the same day (Table 2).…”

“…The current management approach largely depends upon the use of tolerant cultivars and fungicide applications. Although a handful of cultivars are less susceptible to boxwood blight, no cultivar is completely resistant to the disease (Brand et al 2022;Kramer et al 2020;LaMondia and Shishkoff 2017;Yoder et al 2022). In order to maintain a broad spectrum of economically important Buxus species, varieties and cultivars, disease management is heavily dependent upon chemical control strategies.…”

“…The objective of this study was to fill this knowledge gap by evaluating fungicides with 19 different active ingredients from 9 different Fungicide Resistance Action Committee (FRAC) groups in fields where Cps and Che were both present (Brand et al 2022). Results from the study will enable growers and landscapers to better manage boxwood blight.…”