Clinical Plant Pathology
Our lab mainly focuses on biological control of various phytopathogens under both in vitro and under field conditions using beneficial rhizobacteria isolated from various sources, and their characterizations to gain an in-depth understanding of the biocontrol ability of the strains for improved sustainable agriculture.
The application of plant growth-promoting rhizobacteria (PGPR) can help the plant to grow more effectively by developing resistance against various phytopathogens. Biological control by PGPR has emerged as an alternative to reduce the use of conventional agricultural inputs for improving the quantity and quality of crop production. PGPR localizes in the plant rhizosphere and plays an important role in controlling phytopathogens. In recent years, the yield and quality of many crops have been reduced by attacking various diseases caused by phytopathogens; this can be overcome using biological control agents (BCAs). Bacillus species are a well-known producer of a variety of antibacterial and antifungal antibiotics against various pathogens and in different plant species. The spore-forming ability of a Bacillus sp. represents a candidate for the development of an efficient biopesticide formulation in the aspect of biological control.
Dr. Younmi Lee (postdoctoral fellow) is studying the occurrence of phenotypic variation of bacterial species, used as biocontrol agents, and their control during mass production. She recently invented the molecular changes associated with spontaneous phenotypic variation of Paenibacillus polymyxa E681 (E681), a biocontrol agent, and temperature-dependent control of variation. Spontaneous phenotypic variation of plant growth-promoting rhizobacterium (PGPR), which causes the loss of traits crucial for biocontrol, presents a large obstacle in producing commercial biocontrol products. Here, we report molecular changes associated with phenotypic variation in E681, and a simple cultural change that can prevent the variation. Compared to B-type (non-variant) cells of E681, its phenotypic variant, termed as F-type, fails to form spores, does not confer plant growth-promoting effect, and displays altered colony and cell morphology, motility, antagonism against other microbes, and biofilm formation. This variation was observed in all tested strains of E681, but the frequency varied among them. RNA-seq analysis revealed differential regulation of many genes involved in sporulation, flagella synthesis, carbohydrate metabolism, and antimicrobial production in F-type cells, consistent with their pleiotropic phenotypic changes. F-type cells's sporulation was arrested at stage 0, and the key sporulation gene spo0A was upregulated only in B-type cells. The phenotypic variation could be prevented by altering the temperature for growth.
Dr. Kotnala Balaraju (postdoctoral fellow) Young Soo Kim and are studying the characterization of Bacillus velezensis AK-0 as a biocontrol agent against apple bitter rot caused by Colletotrichum gloeosporioides. The genetic mechanism of the biocontrol traits of AK-0 was studied using the complete genome sequence and compared it with complete genome sequences of closely related strains. Further, reported the biocontrol activity of AK-0 against apple bitter rot caused by Colletotrichum gloeosporioides, which could lead to the commercialization of this strain as a microbial biopesticide in Korea. To retain its biocontrol efficacy for a longer period, AK-0 has been formulated with ingredients for commercialization, named AK-0 product formulation (AK-0PF). AK-0PF played a role in the suppression of the mycelial growth of the fungicide-resistant pathogen C. gloeosporioides YCHH4 at a greater level than the non-treated control. AK-0PF exhibited greater disease suppression of bitter rot in matured under field conditions. The genome sequence of AK-0 provides a greater understanding of the Bacillus species, which displays biocontrol activity via secondary metabolites. The genome has eight potential secondary metabolite biosynthetic clusters, among which, ituD and bacD genes were expressed at a greater level than other genes. This work provides a better understanding of the strain AK-0, as an effective biocontrol agent (BCA) against phytopathogens, including bitter rot in apple.
Hyeok Tae Kwon (MS candidate) is studying on isolation and characterization of Bacillus tequilensis from mushroom and the suppression of anthracnose caused by Colletotrichum acutatum in red pepper. Anthracnose caused by Colletotrichum acutatum is one of the most devastating diseases in several crop plants, including red pepper (Capsicum annum L.) in Korea. T be used as a natural biocontrol agent (BCA), Bacillus strain (GYUN-300) was isolated from Phellinus linteus, a popular medicinal mushroom, locally known as “sangwhang” in Korea. Using 16s rRNA gene sequencing, GYUN-300 was identified as Bacillus tequilensis. This antagonistic strain GYUN-300 suppressed the growth of pathogenic mycelia of C. acutatum, causes anthracnose disease in red pepper, at a greater level using a dual culture plate assay. Also, the development of anthracnose symptoms was reduced on detached immature green pepper fruits by treatment with GYUN-300 cell suspensions. Furthermore, GYUN-300 bacterial suspensions treatment significantly promoted plant growth in terms of seedling germination at a greater level compared to water-treated control. Based on in vitro tests, the strain was tested under field conditions in red pepper against anthracnose disease. This was supported by the increased production of IAA levels in GYUN-300 cell suspensions. These results indicated that B. tequilensis GYUN-300 is one of the promising agents in biocontrol of anthracnose disease and growth promotion in red pepper.
Jungyeon Kim (MS candidate) is studying on monitoring of fungicide resistance development in Colletotrichum spp., isolated from various apple orchards at Chungsong area in Gyeongsanbuk province, Korea. Colletotrichum spp., causing apple bitter rot (anthracnose) is associated with fruits, causing to reduce the yield and fruit quality. The use of chemical fungicides is being increased every year to control apple bitter rot disease in Gyeongsangbuk province, Korea, resulting in the development of resistance to bitter rot against fungicides. We isolated a total of 39 pathogenic fungal strains from disease infected apple fruits, that were collected from seven various locations (Jinbo-myun, Pacheon-myun, BuDong-myun, Bunam-myeon, Andeok-myeon, Hyundong-myeon, and Hyunseo-myeon) in the Chungsong region in Gyeongsangbuk province, Korea during the year 2019. All the isolated fungal pathogenic strains were identified as Colletotrichum spp. and cultured onto PDA plates amended with various commercial chemical fungicides to select strains that are resistant to fungicides under in vitro conditions based on the growth of mycelia on PDA. The chemical fungicides used were as follows: Trifloxystrobin 50% WG, Trifloxystrobin 22% SC, Kresoxim-methyl 50% SG, Kresoxim-methyl 44.2% SC, Acibenzola-S-methyl 1.75%+Dithianonem 30% SC, Acibenzola-S-methyl 1.42%+Chlorothalonile 44.5% SC, Dithianone 75% WP, Captan 50% WP, Tebuconazole 25% WP, Tebuconazole 25% EC, Metconazole 20% SC, and Fluazinam 50% WP. This study would be helpful to find a suitable biocontrol method as an alternative to chemical fungicides for sustainable agriculture.
Heejin Kim (BS candidate) is studying on screening of freshwater bacterial antagonists for biological control of major plant diseases. Many microbes associated with freshwater organisms have antimicrobial activity. In this study, she screened bacterial antagonists against five major phytopathogens using freshwater bacteria obtained from Freshwater Bioresources Culture Collection (FBCC). A total of 856 freshwater bacterial isolates were screened by culturing on peptone potato dextrose agar (PPDA) using a dual culture plate assay against fungal pathogens, such as Colletotrichum acutatum and Fusarium oxysporum, which cause anthracnose and wilt diseases, respectively. The results evaluated by the diameter of the inhibition zone of fungal growth, indicate that 86 isolates showed significant antagonistic activity against C. acutatum, while eight isolates exhibited antagonistic activity against F. oxysporum. Similarly, a total isolates were also screened for antagonistic activity against bacterial pathogens, such as Erwinia pyrifoliae (black shoot blight), Pectobacterium carotovorum (soft rot), and Xanthomonas arboricola (brown rot). Based on the diameter of the inhibition zone, 9, 11, and 3 isolates that exhibit significant antagonistic activity against X. arboricola, P. carotovorum, and E. pyrifoliae, respectively, were selected. Among all the isolates, the most promising isolate was identified as Brevibacillus halotolerans, resulting in freshwater microbes that can play a role in the control of plant diseases for sustainable agriculture.
Seunghwan Kim (BS candidate) is studying Pseudomonas viridiflava that causes leaf in Adenophora triphylla var. Japonica. Severe disease with leaf spots and necrotic symptoms was observed in Adenophora triphylla var. japonica HARA in a field at Andong, Korea. Infected plants initially showed spots with halo lesion, at later stages, enlarged and spread to the leaves, which turn yellowing and chlorotic. The symptomatic samples were collected from HARA and cultured on Potato Dextrose Agar (PDA). After incubation, a predominant bacterium was isolated from all samples. Fifteen isolates from independent samples were subjected to biochemical and pathogenicity tests. The isolates induced a hypersensitive reaction in tobacco leaves, gave a reaction in an anaerobe respiratory test, and were negative for levan, oxidase, arginine dihydrolase, gelatin hydrolysis, aesculin hydrolysis, and starch hydrolysis. Besides, LOPAT and Biolog profiles were found to exhibit 93% similarity with the bacterium, Pseudomonas viridiflava. Likewise, FAME analysis also characterized the strain as P. viridiflava with 87% similarity, and further identified at molecular by 16S rRNA sequence analysis. To our knowledge, this is the first report of leaf blight disease on Adenophora triphylla var. japonica caused by P. viridiflava in Korea.
Rosa Jang (BS candidates) is studying on screening of various Bacillus species against fungal pathogens (Botrytis cinerea, Fusarium oxysporum, and Colletotrichum acutatum) under in vitro conditions using dual culture plate assay.
Yunjeong Heo and Hyeonjin Park (BS candidate) is studying on antagonistic acitivity of apple rhizosphere bacteria against various plant fungal pathogens .
Dong Hun Kang (BS candidate) is studying on the isolation and identification of strawberry blight caused by Phomopsis obscurans to find out suitable biocontrol agent as an alternative to chemical control.
