First report of Dickeya fangzhongdai causing onion soft rot in New York State

Authors: Swingle, Bryan; MA, XING - Cornell University; BONASERA, JEAN - Cornell University; ASSELIN, JO ANN - Cornell University; BEER, STEVE - Cornell University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2019
Publication Date: 2/12/2020
Citation: Swingle, B.M., Ma, X., Bonasera, J.M., Asselin, J., Beer, S.V. 2020. First report of Dickeya fangzhongdai causing onion soft rot in New York State. Plant Disease. 104(4):1251-1252. https://doi.org/10.1094/PDIS-09-19-1940-PDN.
DOI: https://doi.org/10.1094/PDIS-09-19-1940-PDN

Interpretive Summary: In 2014 a destructive form of soft rot disease on onion occurred on a farm in Orange County, NY. The disease caused the onion plants to completely collapse within a couple days and spread rapidly to an adjacent field. Our laboratories used several methods to determine the cause of this disease outbreak. We determined that the disease was caused by a type of bacterium, Dickeya fangzhongdai, which has never been reported in the United States before. It is not known if this is a new introduction or whether our laboratory methods have advanced to a point that we are now able to efficiently tell the difference between closely related bacterial species. This work is important because it establishes this particular strain of Dickeya as the causative agent of soft rot on onion and provides practical knowledge so that effective management approaches can be recommended for soft rot on onions.

Technical Abstract: In 2014, an outbreak of onion (Allium cepa) soft rot disease occurred in Orange County, NY. Onset of symptoms correlated with rainy, hot, humid weather. Symptoms included browning, chlorosis, maceration of leaves, and plants collapsed within a few days. Surfaces of bulbs from diseased plants appeared normal, but tissue inside was macerated. The disease spread from a field of red onions to an adjacent field of yellow onion. We isolated bacteria from diseased plants and found a predominant type of small white bacterial colony with irregular margins. We stored a representative isolate, herein referred to as strain AP6. We found that AP6 belonged to the genus Dickeya using PCR and determined the bacterium’s genome sequence using next generation DNA sequencing and used those data to determine that AP6 belongs to the species Dickeya fangzhongdai. We completed Koch’s postulates to associate D. fangzhongdai AP6 with the observed disease. D. fangzhongdai was first reported in China as causing bleeding canker necrosis on pear and recently found as the causal agent of soft rot on Welsh onions in Taiwan, diseased Phalaenopsis orchids in Slovenia, Aglaonema in St. Lucia, and in water samples from Scotland and Malaysia. Onion soft rot disease has been reported in the U.S., but its causal organism had not been investigated previously. The data presented here show that D. fangzhongdai caused an outbreak of onion soft rot in Orange County, NY.