Impact of ammonia on Calonectria pseudonaviculata and C. henricotiae, causal agents of boxwood blight

Authors: HARVEY, ROBERT - Pennsylvania State University; DAVIS, DONALD - Pennsylvania State University; Shishkoff, Nina; PECCHIA, JOHN - Pennsylvania State University

Submitted to: Compost Science and Utilization
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/20/2019
Publication Date: 6/11/2019
Citation: Harvey, R.J., Davis, D.D., Shishkoff, N., Pecchia, J.A. 2019. Impact of ammonia on Calonectria pseudonaviculata and C. henricotiae, causal agents of boxwood blight. Compost Science and Utilization. 27:116-123. https://doi.org/10.1080/1065657X.2019.1586595.
DOI: https://doi.org/10.1080/1065657X.2019.1586595

Interpretive Summary: Boxwood blight is a new and emerging disease affecting boxwood, which is an economically important plant that also has great cultural significance, featuring in many historic Early American gardens such as those found at Mount Vernon and Monticello. Since 2011, the pathogen causing blight(Calonectria pseudonaviculata)has spread to 27 states and Canada; the wet conditions in 2018 caused a significant blight epidemic in many of these states. A second species of the pathogen is present in Europe and would pose an additional threat of unknown significance if it became established in the US. The pathogen is known to survive in infected leaf litter, so it was necessary to test its survival in composted yard waste similar to what might be produced as part of a municipal composting operation. We reported that the decrease in pathogen survival in compost was greater than could be accounted for by high temperature alone and suggested that this decrease might be due to ammonia, a toxic gas produced during composting. In this study, we determined that ammonia within agar in Petri plates reduced radial growth of both pathogen species and gaseous ammonia reduced survival of the pathogen's survival structures. Our findings suggest that composting dead or dying blighted boxwoods in the presence of ammonia could reduce dissemination of the pathogen to healthy plants.

Technical Abstract: Boxwood (Buxus spp.) blight is a devastating disease caused by the Ascomycete fungus Calonectria pseudonaviculata in the U.S. and Europe. A second Calonectria species, C. henricotiae, is also pathogenic on boxwood in Europe, but is not present in the U.S. where it is classified as a quarantine pathogen. Composting can eradicate various plant pathogens and high temperature is likely the most important factor influencing pathogen eradication. We previously reported that C. pseudonaviculata microsclerotia survived exposure to 40 oC in an incubator without compost, whereas exposure to the same temperature and time, but with compost added, greatly decreased survival. That is, the decrease in Calonectria growth and survival in compost was greater than could be accounted for by high temperature alone. We hypothesized that the enhanced decrease in Calonectria growth and survival might be due to ammonia, a fungitoxic gas produced during composting. In this laboratory study, we determined that ammonia within agar in Petri plates reduced radial growth of both C. pseudonaviculata and C. henricotiae. In studies with C. pseudonaviculata, gaseous ammonia reduced microsclerotia survival. Our findings suggest that composting dead or dying blighted boxwoods in the presence of ammonia could reduce dissemination of both Calonectria species from blighted to healthy boxwoods.