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

Fungicide protection is the current approach for management of boxwood blight caused by Calonectria pseudonaviculata (Cps) and C. henricotiae (Che). However, published studies evaluating fungicides under field conditions have been focused on Cps. The objective of this study was to evaluate fungicides in Northern Germany where both Cps and Che were present. Three trials were conducted between 2006 and 2016. In 2006, plants were artificially inoculated with a conidial suspension, while infested soil and plant debris were added to a different field as inoculum for the 2012 trial and this field was used again without further addition of inoculum in 2015. Fungicides were applied one to five times and assessments were done three to six times, depending upon the trial. The highest level of disease severity occurred in 2015 (0.91), while the lowest occurred in 2012 (0.01). Among the fungicides evaluated in 2006, preventive sprays of Cercobin FL, Switch, Harvesan, Pugil 75 WG, Dithane NeoTec and Euparen M WG were most effective, with blight control above 65%. In 2012, all treatments including Askon, Cabrio Top, Malvin WG, Dithane NeoTec and Osiris showed > 75% blight control. In 2015, Bayer Rosen-Pilzfrei Baymat and Switch were the most effective (> 82%). Extended in-season blight control was also observed with some fungicides. Additionally, a few fungicides that were evaluated in more than 1 year showed reduced effectiveness over time. This study filled several major knowledge gaps especially regarding fungicide efficacy against Che under field conditions and thus provides crucial information for developing chemical control strategies.