First report of the NA2 clonal lineage of Phytophthora ramorum in Indiana causing ramorum leaf blight on Rhododendron

Authors: Press, Caroline; FIELAND, V - Oregon State University; CRESWELL, T - Purdue University; BONKOWSKI, J - Purdue University; MILES, L - Michigan State University; Grunwald, Niklaus - Nik

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/14/2020
Publication Date: 2/19/2020
Citation: Press, C.M., Fieland, V.J., Creswell, T., Bonkowski, J., Miles, L., Grunwald, N.J. 2020. First report of the NA2 clonal lineage of Phytophthora ramorum in Indiana causing ramorum leaf blight on Rhododendron. Plant Disease. 104(6):1875. https://doi.org/10.1094/PDIS-12-19-2543-PDN.
DOI: https://doi.org/10.1094/PDIS-12-19-2543-PDN

Interpretive Summary: The oomycete plant pathogen Phytophthora ramorum is the causal agent of ramorum leaf blight and sudden oak death. P. ramorum is known to cause leaf blight and stem dieback on several nursery species including Rhododendron. The disease typically results in dark, brownish lesions on leaves or stems which can cause wilting and death of the plant. Since the emergence of the disease, a federal quarantine was established to prevent the spread of the pathogen and nurseries that export P. ramorum hosts are subject to a federally mandated certification program for interstate export. This disease has had a significant impact on the US nursery industry via quarantine regulations imposed on nurseries infested with the pathogen. The pathogen has been introduced at least 3 times into the Western US as distinct genetic clones called NA1, NA2, and EU1. In the spring and summer of 2019, USDA-APHIS reported that a shipment of potentially P. ramorum infested plants was delivered to several Eastern and Midwestern states. Rhododendron leaves from numerous counties in Indiana were collected and genetic analysis was conducted to identify the genetic clone. All 26 of the Indiana isolates were unambiguously classified as belonging to the NA2 genetic clone. This is the first report of the NA2 clonal lineage outside of British Columbia, Washington, and California.

Technical Abstract: The oomycete pathogen Phytophthora ramorum is the causal agent of ramorum leaf blight and sudden oak death which was first reported in the United States in the 1990s. P. ramorum is known to cause leaf blight and stem dieback on several nursery species including Rhododendron. The disease typically results in dark, brownish lesions on leaves or stems which can cause wilting and death of the plant. Since the emergence of the disease, a federal quarantine was established to prevent the spread of the pathogen and nurseries that export P. ramorum hosts are subject to a federally mandated certification program for interstate export. This disease has had a significant impact on the US nursery industry via quarantine regulations imposed on nurseries infested with the pathogen. The pathogen has been introduced at least 3 times into the Western US as clonal lineages NA1, NA2, and EU1. In the spring and summer of 2019, USDA-APHIS reported that a shipment of potentially P. ramorum infested plants was delivered to several Eastern and Midwestern states. Rhododendron leaves from numerous counties in Indiana were sampled between April 18 and June 3, 2019. DNA was extracted and part of the cellulose binding elicitor lectin gene (CBEL) was amplified and sequenced with primers CBEL5U and CBEL6L. Sequences were aligned with the CBEL reference sequences of EU1 (KF679685), EU2 (KF679716), NA1 (EU688908), and NA2 (KF679712). All 26 of the Indiana isolates were unambiguously classified as belonging to the NA2 clonal lineage. This is the first report of the NA2 clonal lineage outside of British Columbia, Washington, and California indicating that this pathogen was most likely moved West to East. Prior documented shipments from the West coast East harbored only the NA1 clonal lineage.