First report of wildfire disease caused by Pseudomonas amygdali pv. tabaci on perilla in Korea

Abstract

Perilla (Perilla frutescens) is an important crop which produces edible leaves and oil from its seeds which is used as a seasoning in Korean food (Kim et al., 2009). Perilla is cultivated throughout the year in greenhouses in southern Korea. In November 2020, irregular brown spots with halos that led to necrosis on most upper leaves were observed in Miryang (35°32´04.6˝N; 128°46´01.9˝E) (Figure 1-3). The symptoms were very similar to wildfire disease in soybeans. The disease spread to up to 20–30% of the perilla plants cultivated in two greenhouses. Five diseased plants were collected and the leaves of each plant were removed. To isolate the causal pathogen, supernatants of surface-sterilised and homogenised leaves were plated on King's B medium (Sigma-Aldrich, USA) and incubated at 28°C for two days. Fluorescent colonies occupied approximately 60% of the area covered by bacterial colonies, indicating that a fluorescent bacterium was the predominant isolate from the diseased perilla. One isolate (designated strain MY1) was collected for analyses of 16S rRNA gene sequences and multilocus sequence analysis (MLSA) using sequences of the gltA, gyrB, and rpoD genes (Hwang et al., 2005). A BLAST search showed that the 16S rRNA gene sequence of strain MY1 (GenBank Accession No. ON810536) corresponded to Pseudomonas amygdali strain DEB with 99% identity. Moreover, strain MY1 was closest to P. amygdali pv. tabaci strain ATCC 11528 in a phylogenetic tree constructed using MLSA (Yun & Kim, 2021) (Figure 4). The physiological and biochemical characteristics of strain MY1 were determined based on levan production, oxidase activity, potato soft rot, arginine dihydrolase activity and tobacco hypersensitive response (LOPAT) tests. In the tests, strain MY1 was positive for levan production and tobacco hypersensitivity, but negative for oxidase activity, potato soft rot and arginine dihydrolase activity. Based on the LOPAT and MLSA analyses, strain MY1 belongs to P. amygdali pv. tabaci 1a (Sun et al., 2021). To substantiate the pathogenicity of strain MY1, a bacterial suspension of the strain (adjusted to 108 CFU/ml) was inoculated onto ten four-week-old perilla and incubated at 28°C. After seven days, the inoculated perilla leaves showed similar symptoms to those observed originally in the greenhouse. Completing Koch's postulates, bacteria with fluorescent colonies were re-isolated from symptomatic tissue and the 16S rRNA sequence of the isolates was identical to P. amygdali. To the best of our knowledge, this is the first report of wildfire disease caused by P. amygdali pv. tabaci on perilla in Korea and globally. Perilla wildfire disease reduces the quality of leaves, so early detection and prevention are essential to reduce the spread of disease. S.Y. Choi and J.I. Kim contributed equally to this work. This work was supported by “Cooperative Research Program for Agriculture Science & Technology Development project (PJ01425901)” Rural Development Administration, Korea.