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

Authors: BRAND, THOMAS - Lower Saxony Chamber Of Agriculture; BELTZ, HEINRICH - 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: Journal of Plant Diseases and Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2022
Publication Date: 12/21/2022
Citation: Brand, T., Beltz, H., Adhikari, U., Daughtrey, M., Luster, D.G., Kong, P., Hong, C. 2022. Evaluation of fungicides for management of boxwood blight caused by Calonectria spp. under field conditions in northern Germany. Journal of Plant Diseases and Protection. 130:325–335. https://doi.org/10.1007/s41348-022-00691-8.
DOI: https://doi.org/10.1007/s41348-022-00691-8

Interpretive Summary: Fungicide protection is the current approach managing 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. The 2006 trial was done by artificially inoculating plants with a fungal spore suspension, the 2012 trial was conducted with plantings in infested soil and plant debris and the 2015 trial was conducted without additional inoculation. Fungicides were applied one to five times and assessments for disease severity were done three to six times, depending upon the trial. The highest level of disease severity occurred in 2015 while the lowest occurred in 2012. Several fungicides in different chemistry classes were shown to be effective within the trial periods. A few fungicides that were evaluated in more than one year showed reduced effectiveness over time. Some products tested in this study are not registered for use on boxwood in the U.S. by the Environmental Protection Agency. In the absence of effective registered fungicides, alternative products with relatively low toxicity will be needed to minimize blight development. This study filled several major knowledge gaps, especially regarding efficacy against Che under field conditions, and thus provides crucial information to develop chemical control strategies.

Technical Abstract: Fungicide protection is the current approach managing 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. The 2006 trial was done by artificially inoculating plants with a conidial suspension, the 2012 trial with infested soil and plant debris and the 2015 trial without additional inoculation. Fungicides were applied one to five times and assessments for disease severity 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, preventative sprays of Cercobin FL, Switch, Harvesan, Pugil 75 WG, Dithane NeoTec and Euparen M WG were most effective, with blight control above 65% when assessed 18 days post inoculation. 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 most effective with blight control above 82%. Also observed was extended in-season blight control of some fungicides. Additionally, a few fungicides that were evaluated in more than one year showed reduced effectiveness over time. Some products tested in this study are not registered for use on boxwood in the U.S. by the Environmental Protection Agency. This study filled several major knowledge gaps, especially regarding efficacy against Che under field conditions, and thus provides crucial information to develop chemical control strategies.