The effect of boxwood leaf volatiles on conidial germination of Calonectria pseudonaviculata, the causal agent of boxwood blight

Authors: OHKURA, MANA - Oregon State University; Beck, Bryan; Scagel, Carolyn; Weiland, Gerald - Jerry

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2024
Publication Date: 3/27/2024
Citation: Ohkura, M., Beck, B.R., Scagel, C.F., Weiland, G.E. 2024. The effect of boxwood leaf volatiles on conidial germination of Calonectria pseudonaviculata, the causal agent of boxwood blight. Phytopathology. https://doi.org/10.1094/PHYTO-12-23-0507-R.
DOI: https://doi.org/10.1094/PHYTO-12-23-0507-R

Interpretive Summary: Boxwood blight is an introduced fungal disease that affects the $141 million US boxwood nursery industry. Research on the disease is hampered because spore germination is poor when the fungus is grown in the laboratory. We therefore conducted experiments to identify ways for improving spore germination. Almost no germination occurred when spores were placed on glass coverslips, but increased to nearly 100% when detached boxwood leaves were nearby. This indicated that a gas compound was responsible for inducing germination. We also discovered that detached leaves produce ethylene gas, so we tested whether ethylene was the compound responsible for improved germination. However, ethylene did not improve spore germination. Further research is needed to identify the compounds that induce germination. Results will be important for improving research methods that require high germination rates and for developing boxwood blight disease control methods.

Technical Abstract: The fungal pathogen Calonectria pseudonaviculata causes boxwood blight and is a significant threat to the boxwood industry as well as historic boxwood gardens. The pathogen produces conidia in sticky masses that are splash dispersed, which germinate and infect through stomata on the leaves or stems causing leaf spots and stem lesions. Despite its ability to cause severe infections on boxwood plants, the pathogen often has a low germination rate on artificial media under lab conditions. To identify cues that stimulate germination, we explored whether host factors could induce high germination rates. In this study, we demonstrate that C. pseudonaviculata spores achieve high germination rates when they are placed on detached leaves of boxwood and other known hosts, compared to potato dextrose agar and glass coverslips. We also demonstrate that germination is induced by volatiles from detached leaves of boxwood as well as the non-host Berberis thunbergii. When C. pseudonaviculata spores were exposed to volatiles from boxwood leaves in the presence of ethylene scrubber packs that contain potassium permanganate, the stimulatory effect on spore germination was reduced. However, ethylene, a regulator of leaf senescence did not stimulate germination of C. pseudonaviculata spores. This suggests the pathogen may have evolved to recognize one or more host volatiles, other than ethylene, to induce germination and to limit its growth until it senses the presence of a host plant.